ICOM机器常见问题和解决办法（英文谨入） [复制链接]

以下是收集的常见问题和解决办法，没有发布的型号就是没找到资料，不要问。

symptom:
"rushing water" or "aurora" sound in fm receive audio. ssb sounds raspy and distorted.

probable causes:
1) bad plastic trimmer capacitors on hpl unit. 2) old, dried out electrolytic capacitor c2 on hpl unit is allowing regulator to oscillate.

cure:
replace faulty trimmer capacitors c16 and c78 on hpl unit with ceramic versions of identical values. replace c2 electrolytic capacitor.

remarks:
to allow proper alignment of ceramic trimmers, reverse values of c16 and c78: make c16=12 pf, and c78=6 pf.

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symptom:
frequency instability.

probable causes:
1) bad plastic trimmer capacitors in hpl unit. 2) cracked solder at ic2 regulator on hpl unit.

cure:
replace trimmer capacitors as stated in the previous paragraph. resolder cracked solder joints at regulator.

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symptom:
no rx/tx, or no rx/tx when unit warms up. unit may have dead spots in tuning range. inspection reveals that the hpl loop is unlocked. trimmer capacitors on hpl unit have already been replaced.
probable

causes:
bad c9 capacitor on hpl unit.

cure:
replace c9 (1 礔, 50-volt) bp capacitor on hpl unit.

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symptom:
no frequency display, only decimal points. problem may be intermittent or temperature related.
probable

cause:
display dc-dc converter has noisy output.

cure:
replace all of the following dried-out electrolytic capacitors on the display unit: c3, c6, c7, c8, c9, c10, c11, and c14. (these capacitors are blue when they are new. capacitors that have turned gray or brown are old and dried-out.)

remarks:
while servicing the dc-dc converter section, look for cracked solder joints as this section of the radio tends to run hot. also, we recommend reprogramming the ram card after servicing the dc-dc section.

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symptom:
erratic display i.e., frequency listed will be out of the normal operating range of the radio. rotating the vfo knob may cause strange characters to appear in the display. cycling the power switch a few times may restore operation temporarily.
probable

causes:
1) cracked solder on ram unit is causing logic errors. 2) program in ram unit is corrupted. 3) ic21 regulator chip on logic unit is producing noise or oscillating.

cure:
replace filter capacitors c11, c26, and c26 on logic unit if they appear overheated or dried out. resolder cracked solder joints at j1 and j2 connectors on ram unit, then reprogram. if this does not help, substitute ram unit with a known good one. if the ram unit does not cure the problem, there may be noise from the dc-dc converter causing the logic unit to glitch. follow instructions for the previous problem.

remarks:
while servicing the dc-dc converter section, look for cracked solder joints as this section of the radio tends to run hot. also, we recommend reprogramming the ram card after servicing the dc-dc section.

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symptom:
intermittent rx/tx. may have dead spots in tuning range.
probable

causes:
bad data to hpl unit.

cure:
resolder solder joints at j11 connector on logic unit. resolder other suspected poor joints on logic unit. if this does not help, there may be a problem with ram unit or associated circuitry. (see previous listed repair.)

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symptom:
cpu noise in audio.
probable

causes:
poor ground on display unit.

cure:
tighten the screws that hold down the display unit pcb. if this does not help, install a plastic-coated metal shield over the display unit. this type of shield is available through our parts department.

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symptom:
loud, high pitched-squeal emanates from inside radio.
probable

causes: dc-dc converter transformer has a loose core.

cure: replace t1 (t0-9) transformer on the display unit.

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symptom:
very low rx sensitivity.
probable

causes: bad q7 fet on ygr unit.

cure: replace bad q7 fet (2sk125).

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symptom:
no rx. investigation reveals that t/r relay (rl1) on filter unit does not release in receive mode.
probable

causes:
leaky q39 transistor on main unit.

cure:
replace leaky q39 transistor (2sc2785).

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symptom:
drastic change in audio frequency response between usb and lsb., i.e. usb audio has too much bass while lsb audio has too much treble. tx frequency response is also poor.
probable

causes:
misadjustment of the bfo or a faulty crystal filter.

cure:
realign the bfo. if this doesn't help, then the problem is most likely a bad fi-1 filter on the main unit (10m24d4).

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symptom:
vfo tuning is erratic. sometimes frequency doesn't increment steadily when vfo knob is turned.
probable

causes:
dirty rotary encoder.

cure:
remove white plastic cover from back of rotary encoder and spray-clean the carbon track with blue shower solvent. spin vfo knob vigorously while spraying.

remarks:
do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. stuff a paper towel between the rotary encoder and display unit to catch spray run-off.

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symptom:
in 271h, low tx power output with high current draw from the power supply.
probable

causes:
bad c28 smd capacitor on pa unit.

cure:
replace c28 capacitor.

symptom:
unit will not turn on.

probable causes:
1) cracked solder joints at battery connections. 2) cracked solder at jumpers to vr unit. 3) bad flex cable. 4) problems on logic unit.

cure:
check the resistance between the "e" and "-" terminals on the bottom of the radio. should be 0 ohms. check the jumpers on the vr unit for cracked solder joints and repair as needed. if ok, verify proper logic board voltages as per the service manual. failure here may mean a bad flex cable or faulty cpu crystal (check pin 29 of cpu for oscillation).

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symptom:
unit runs off internal battery, but not external dc.
probable

causes:
1) faulty d9 diode on prt unit. 2) faulty dc jack.

cure:
replace faulty d9 diode and/or faulty dc jack.

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symptom:
unit runs off external dc, but not from internal battery.

probable cause:
faulty d15 diode on the main unit.

cure:
replace faulty d15 diode.

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symptom:
internal or external battery charging problems.

probable causes:
1) failure on prt unit. 2) missing components.

cure:
check for proper voltages on prt unit. start at d2 or d8 diodes and troubleshoot from there. check for missing components under adhesive insulator.

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symptom:
unit has display but no rx/tx at all.

probable cause:
pll unlock.

cure:
remove the main unit and check the +5 v, +5s, pll stb, data, ck lines.

remarks:
look for damage from a poorly executed out-of-band modification.

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symptom:
no tx output.

probable causes:
1) loose antenna connector and/or center conductor jumper wire. 2) faulty driver and/or pa module.

cure:
check antenna connector for tightness and resolder cracked solder joints as needed. if ok, check the apc and pdi voltages to the power module. if ok check the gain across the pa and driver: driver is approx. 10 db, pa is approx. 40 db. there should be very little loss between the power module and antenna jack.

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symptom:
no deviation. tx output is ok.

probable causes:
1) bad mic element. 2) faulty flex cable. 3) broken chip resistor on main board.

cure:
if unit works ok with external microphone, check for bad flex cable or internal mic element. if unit does not work with external mic, check for 75 mv p-p audio to mic amp. if present, check for a broken chip resistor on the main board where the shield screws into the upper right corner of the main board (top view).

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symptom:
no rx, or low rx sensitivity.

probable causes:
1) misalignment. 2) low r+5v. 3) faulty d7 diode on main unit.

cure:
make sure unit has not been misaligned. check the r+5v for proper voltage in rx (5 volts). if ok, check d7 diode on main unit and replace if needed. if unit still does not operate, check the gain of the rf stages:
* rf amp has 18 db gain
* mixer 30.875 mhz lo level is very low, about equal to antenna input levels.
* first mixer has approx. 15 db of gain.
* no loss through the filters.
* some loss through r6 resistor.
if the above stages are ok, check the detector unit for proper operation.

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symptom:
no audio, but unit seems to be receiving signals.

probable causes:
1) bad speaker. 2) bad flex cable. 3) burned foil trace. 4) broken jumpers to vr unit.

cure:
disconnect speaker and check for presence of audio on speaker wires. if present, replace defective speaker. trace audio at flex cable. replace flex cable if needed. check for a burned foil trace at ground of external speaker and repair as needed. this is caused by user plugging in external power to speaker jack. check for cracked joints at jumpers to vr unit and repair as needed. if all else is ok, check the output through the af unit: should be 5 volts p-p at full volume level. if it's not there, check for an output signal on the det out line of the det unit.

remarks:
if the audio is very low, check for a faulty c20 tantalum capacitor on the main unit and replace as needed.

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symptom:
intermittent operation of some keypad functions.

probable cause:
cracked solder to the keypad board.

cure:
inspect keypad board solder connection for cracks and repair as needed.

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symptom:
display shifting or rolling.

probable cause:
faulty cpu clock oscillator.

cure:
check cpu clock oscillator for proper level. replace crystal if needed.

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symptom:
channel knob shift down or frequency change does not operate.

probable cause:
open ground to vr board.

cure:
check vr board for proper ground. also check for cracked solder joints at vr jumpers.

1.) replace all plastic trimmer capacitors in pll/vco and rf/ygr circuits with ceramic types: 6 pf, 10 pf, and 20 pf; as called for in the service manual.

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2.) monitor j4 test point with a spectrum analyzer for a signal in the 265 mhz range. adjust c16, c18, c25, c29, c35, and c38 for peak signal on analyzer. (don't use an ifr 1200 service monitor; the signal will overload its analyzer.) output level at j4 should be 0 db or greater. switch between 430 and 440 mhz on the radio and fine tune the previously listed caps for no more than a 2 db variance between the two signals.

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3.) monitor pll output with a spectrum analyzer (not ifr1200) and adjust c58, c61, c70, c76, and c79 for peak signal. this must be a minimum of 0 db. next, fine tune for no more than a 2 db variance between 430 and 449 mhz in 1 mhz steps.

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4.) align 10.25 mhz oscillator on pll unit.

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5.) align vco as per the service manual alignment procedure.

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6.) set the 430 band frequency and 440 band frequency as per the service manual.

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7.) realign rf/ygr unit for maximum performance. alignment at 440 mhz will usually yield good performance from 430 to 449.995 mhz.

symptom:
unit immediately goes into the protect mode when rf is applied. only happens on specific bands. other bands are ok.

probable cause:
defective filter-switching relay in the low-pass filter.

cure:
locate and replace the bad relay.

remarks:
generally, when the relays fail, the armature winding shorts. this sometimes causes damage to the relay driver ic9 chip on the main unit, align with the associated chokes for that band. ic8 may also be damaged.

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symptom:
automatic antenna tuner has one or more of the following symptoms: 1) tuner drifts out of tune unless a carrier is present. 2) tuner goes through retuning sequence each time ptt button is depressed, even after match had already been found. 3) tuner slows, but never settles on a 1:1 match.

probable causes:
1) tuner needs alignment. 2) tuner requires installation of factory update.

cure:
try adjusting r19 and r25 trimmer pots in the main unit as per the alignment procedure on page 5-10 of the service manual. if this doesn't help, contact technical support regarding updates to the det and main units.

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symptom:
unit will not turn on. no lights, no fan noise. a relay click can be heard from linear when power switch in control head is pressed.

probable causes:
1) the circuit breaker is tripped. 2) a cable is not connected. 3) no +8 volts is being sent from exciter. 4) power supply is blown.
cure: check to make sure circuit breaker has not been tripped by turning it off, then on again. check again to make sure all cables are connected. if they are, look for +8 volts dc at pin 1 of the din cable that plugs into the 4kl. (the ic-781 had an internal fuse on this line). if the +8 volts is present inside the 4kl, remove the cover to the reg. unit and look for obviously blown parts. if evidence of damage is present, the reg. unit should be sent to one of our factory service centers for repair.

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symptom:
unit immediately goes into protect mode when switched to the 24 mhz band and above, even in receive.

probable cause:
unit has not been out-of-banded for operation on 10 m and 12 m bands.

cure:
call the icom technical support department for oob mod info.

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symptom:
unit puts out normal power but does not pass receive signal to transceiver.

probable cause:
bad switching relay. possible bad relays are rl15 on filter unit, rl5 & rl6 in tuner unit, and rl1 relay on the spltr unit.

cure:
locate and replace bad relay.

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symptom:
unit produces noise or "birdies" in the receiver on 40 meters and below. this noise occurs as a "swishing" sound that moves up or down the band.

probable cause:
noise generated by the internal switching power supply.

cure:
a modification can be installed in the spltr unit in early production units to help minimize the noise. please contact technical support for a copy of this modification. also, we have found that wrapping the power cord through a large ferrite toroid has alleviated the problem in some units.

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symptom:
ic meter reading is high while power output is low in some bands. protector meter reading is ok. no components are overheating on the combinr unit.

probable cause:
unit incorrectly senses an over-current condition and is cutting the power back.

cure:
tighten loose screws at wire-wound resistor r11 on pa unit. if this does not help, try performing the ic meter and ic alc meter alignment on page 5-8 of the service manual.

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symptom:
low output power with a high ic reading. unit burns components on combinr unit.

probable cause:
output power from pa units are not equal. possibly, there is a faulty pair of final transistors.

cure:
replace burned components on the combinr unit. disconnect and measure power output from each pa unit individually. locate and replace defective components on pa unit(s) (finals, etc).

remarks:
make a level check to be sure that the four outputs from the spltr unit is the same for each of the four pa sections.

symptom:
low rx sensitivity on 2 meters.

probable cause:
burned d6 diode on pa unit.

cure:
replace d6 diode (ma862).

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symptom:
no tx power output on one or two hf bands.

probable cause:
bad filter relay.

cure:
locate and replace faulty filter relay (will be type ahy-103) on the filter circuit board.

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symptom:
low/distorted rx audio.

probable cause:
burned spe circuit board trace on pll unit.

cure:
repair spe (ground) circuit trace on pll unit.

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symptom:
low tx output on hf & 6-meters.

probable cause:
shorted d18 diode and q1 transistor in filter unit.

cure: replace d18 diode (dap222) and q1 transistor (2sc4081).

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symptom:
low mic gain.

cure:
install field modification on main unit: change value of r236 resistor from 27 k, to 68k-82 k. the higher the resistor value, the greater increase in mic gain.

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symptom:
thin-sounding mic audio.

cure:
install field mod inside hm-103 microphone: remove rubber gasket from beneath mic element.

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symptom:
off frequency in fm tx. other modes ok.

probable cause:
fm vco lock voltage needs adjustment.

cure:
adjust fm vco lock voltage for 1.5 volts as per service manual.

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symptom:
unit is stuck in tx mode.

probable cause:
shorted t/r switching transistor.

cure:
replace shorted q32 transistor in main unit.

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symptom:
radio won't power up.

probable cause:
burned ground circuit trace in flex cable.

cure:
replace flex cable ep2 at main board.

though the ic-730 is a good-performing, reliable radio, many of these units have been in service for a number years and are beginning to show their age. the following preventative maintenance procedures should be done to all ic-730's that are in need of repair.


1.) on the rf unit, resolder the j2 and j3 connectors. due to the relatively thick coax cables on the connectors, these solder joints tend to crack with age and cause intermittent transmit and receive problems.

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2.) make sure the receiver preamp works. if inoperative, replace relay rl1 on the rf unit. install the factory update to prevent relay failure.

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3.) check transistor q5 on the rf unit. if it is a 2sc945q, replace it with the improved 2sc2878b to prevent spurious output from the transmitter and failure of the driver transistors. if q5 is a type 1636, it doesn't need to be replaced.

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4.) clean the rit pot and switch with tuner cleaner. these controls are usually noisy and erratic, making the rit hard to use.

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5.) tighten the chrome-plated pc board mounting screws. the screws and spacers tend to come loose if the radio is used in a high-vibration, mobile environment. don't forget to tighten the screws on the logic and if units located beneath the main unit.

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6.) check the bandwidth coupler for proper tightness. tighten as needed.

symptom:
no rx/tx at all. display is ok.

probable cause:
open r1 resistor on reg. unit.

cure:
replace r1 resistor (4.7 ohm, 1/2 watt).

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symptom:
preamp inoperative or intermittent. rx may be noisy.

probable causes:
1) bad preamp rl1 relay on rf unit. 2) cracked solder on rf unit in preamp section.

cure:
replace bad relay rl1 (fbr221d012). carefully examine the preamp section of the rf unit for bad joints and resolder as needed.

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symptom:
intermittent low rx sensitivity. actuating ptt temporarily restores rx or causes it to drop out again.

probable cause:
intermittent preamp relay on rf unit, or intermittent t/r relay on filter unit.

cure:
replace rl1 relay (fbr221d012) on rf unit. if this doesn't help, replace rl1 relay (sr-202) on filter unit.

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symptom:
unit jumps frequency unexpectedly when turning vfo knob. digital display does not track with this jump. the frequency jump is consistent and happens at the same place every time. actual operating frequency does not match in some parts of the bands.

probable cause:
ldo is out of alignment.

cure:
adjust r22 trimmer pot on main unit for best performance.

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symptom:
frequency instability.

probable cause:
dirty rit/xit pot and switches.

cure:
clean dirty rit/xit pots and switches with blue shower solvent.

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symptom:
drastic change in audio frequency response between usb and lsb., i.e. usb audio has too much bass while lsb audio has too much treble. tx frequency response is also poor.

probable cause:
misadjustment of the bfo or if shift oscillator.

cure:
realign bfo and if shift oscillator as per page 8-4 and 8-5 of the service manual.

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symptom:
intermittent tx output.

probable cause:
cracked solder joints at j3 connector on rf unit.

cure:
resolder solder joints at j3 connector.

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symptom:
distorted/raspy-sounding ssb tx. carrier power present in ssb modes. complaints of rfi. unit tests fine on dummy load.

probable causes:
1.) inadequate station ground. 2) external power supply used with unit needs additional rf decoupling.

cure:
recheck station ground. improve if necessary. if a 3rd party power supply is being used with radio, install additional decoupling capacitors across output terminals.

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symptom:
tx oscillations, or low spurious output.

probable causes:
1) bad q5 transistor on rf unit. 2) bad electrolytic capacitors on pa unit. 3) band switch is misaligned. 4) bad drivers or pa transistors.

cure:
disconnect j2 connector on the rf unit and see if the problem goes away. if it does, then q5 transistor is most likely bad (replace it with a 2sc2878b). if disconnecting j3 has no effect, examine the pa unit. replace c18 and/or c23 capacitors if they appear swollen or overheated. check alignment of the front and rear band switch wafers. if none of the above are causing the problem, then most likely the drivers or pa final transistors are bad.
remarks: if q5 transistor on the rf unit is a 2sc945, it should be replaced with a 2sc2878b.

symptom:
erratic display, segments missing from frequency display. radio seems to work ok otherwise.

probable cause:
bad solder at ic1 display driver chip on display unit.

cure:
resolder all pins on ic1 chip.

remarks:
ic1 is a surface mount ic.

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symptom:
intermittent rx/tx. intermittent high swr. seems like antenna is intermittent.

probable cause:
cracked solder joint at antenna connector.

cure:
resolder joint at antenna connector where it connects to the filter unit. we recommend replacing the thin wire between the filter unit and antenna connector with a thicker wire to prevent future failure.

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symptom:
intermittent rx. seems like antenna is disconnected in receive mode. tx is ok.

probable cause:
cracked solder at external rx phono jack on main unit.

cure:
resolder phono jack to main unit.

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symptom:
no tx, or low tx output power on one or more bands with high current draw from power supply. other bands are ok.

probable cause:
damaged components in low-pass filter.

cure:
replace all obviously burned/overheated components on filter unit. also inspect pa unit for overheating.

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symptom:
raspy-sounding mic audio when used on antenna. radio sounds good when transmitting into dummy load. power output seems normal. may be complaints of rfi.

probable causes:
1) station ground is inadequate. 2) external power supply used with unit needs additional rf decoupling. 3) rf feedback in mic circuit.

cure:
check station ground. if a 3rd party power supply is being used with radio, install additional decoupling capacitors across output terminals. short pins 6 & 7 in microphone connector to remove rf feedback.
remarks: if nothing helps, radio should be checked for spurious output (see below).

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symptom:
spurious tx on some bands. swr reading may be high when transmitting into a dummy load. power is low on some bands. ic may be high.

probable cause:
bad components on pa unit caused by over-voltage or overheating.

cure:
check and replace as needed: c25 & c29 capacitors (1000 礔, 16v) and r25 resistor (4.7 ohm, 2 watt) on pa unit. also look for overheated components and solder joints on the l4 assembly.

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symptom:
power drops off as unit warms up. alc meter deflection remains high.

probable cause:
apc out of alignment.

cure:
follow apc alignment procedures in the service manual. (r262 trimmer pot on main unit.)

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symptom:
distorted rx/tx, or no rx/tx on one or more bands. distorted ssb audio sounds raspy and rough. problem may be temperature related.

probable causes:
bad trimmer capacitors on pl unit, hpl vco section.

cure: replace plastic trimmer capacitors c37, c45, c52, and c59 with ceramic equivalents. also replace the reference oscillator trimmer capacitor c138.

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symptom:
frequency instability.

probable causes:
unstable plastic trimmer capacitor c138 on pl unit.

cure:
replace c138 trimmer capacitor with a ceramic equivalent. we also recommend replacing the hpl trimmer capacitors at this time.

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symptom:
intermittent mic audio.

probable causes:
dirty mic gain control.

cure:
spray-clean control with blue shower solvent.
remarks: using a cleaning solvent other than blue shower may cause damage by removing a substance inside the control that allows smooth movement of the slider.

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symptom:
vfo tuning is erratic, sometimes lags or skips when changing frequencies or bands. sometimes least-significant digit in frequency displays "flutters" back and forth instead of changing incrementally when the vfo knob is turned slowly.

probable causes:
bad rotary encoder.

cure:
substitute rotary encoder with a known good one. it is recommended that the rotary encoder be replaced as a complete assembly if it is bad.
remarks: if the frequency tunes in only one direction, i.e. either up or down, there may be a problem on the logic unit.

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symptom:
no display, no rx/tx. inspection reveals pll unlock.

probable causes:
1) low power supply voltage. 2) bad ic8 chip on pl unit.

cure:
measure power supply voltage at pin 7 of either accessory jack. should not be less than 12 volts for reliable operation. if ok, replace ic8 chip on pl unit.

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symptom:
no rx/tx when cold or warm. may have dead spots in tuning range of each band.

probable causes:
pll unlock.

cure:
align lpl lock voltage on pl unit.

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symptom:
frequency unstable in tx on 10 meters, especially after warm-up.

probable causes:
pl unit needs factory update.

cure:
on pl unit, change value of r163 from 3.3 k to 1.5 k, and spread coil l36.

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symptom:
mic gain is low when using heil microphone element.

cure:
change value of r235 resistor on pl unit to 200 k.

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symptom:
radio trips/shuts down power supply when power connector is plugged in, even when radio is turned off.

probable cause:
shorted pa transistors.

cure:
replace shorted pa transistors. the pre-driver and driver transistors are probably bad too. check electrolytic capacitors c25 and c29 on pa unit for swelling and replace if needed.

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symptom:
drastic change in audio frequency response between usb and lsb., i.e. usb audio has too much bass while lsb audio has too much treble. usb/lsb rx frequency response may be equalized, to a degree, by adjustment of the front panel pbt control. tx frequency response remains poor.

probable cause:
misadjustment of the bfo and/or pbt shift oscillator, or bad fl-30 filter on main unit.

cure:
realign the bfo and pbt shift oscillator. if this doesn't help, replace the fl-30 filter.

remarks:
some filters are better than others so some test selection may be needed to satisfactorily alleviate the problem. the better-performing (but more expensive) fl-80 filter can be substituted for the fl-30.

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symptom:
"chirp" in cw operation.

probable cause:
1) low power supply voltage in tx. 2) 3rd party power supply needs additional rf decoupling. 3) early production ic-735 needs factory update.

cure:
check power supply voltage at pin 7 of either accessory jack in tx mode. it should not vary excessively from rx standby voltage, or drop below 12 volts. if a 3rd party power supply is being used with radio, install additional rf decoupling capacitors to its output terminals. if the ic-735 is an early production unit, it may need a factory update to prevent chirp. call icom technical support for update information.

unless otherwise noted, the following service questions apply to both models.

symptom:
frequency jump in lsb mode after changing modes.

probable cause:
lsb bfo is not being switched properly by logic; unit may need factory update.

cure:
with radio turned off, measure resistance of pin 17 to ground on ic24 chip on main unit. should be 47 k. if it measures above one mega-ohm, install a 47 k, 1/8 w resistor over the top of ic24 between pins 10 and 17.

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symptom:
radio is off frequency.

probable causes:
reference oscillator needs alignment.

cure:
field: first reset cpu to clear all bfo presets, then tune in wwv and align l29 coil on pll unit for zero beat in usb mode. shop: reset cpu. align l29 coil for 60.0 mhz at p2 test point. check alignment of bfo.
remarks: before performing this alignment, make sure that update to prevent lsb frequency jump has already been installed in radio (see above cure).

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symptom:
in ic-736, tx ssb audio sounds harsh. average talk power seems low.

probable cause:
driver and pa idle current is set too low.

cure:
realign idle current as per procedure on page 5-8 of the service manual with a change in specs: adjust r8 for a 300 ma increase and r44 for a 500 ma increase. total idle current draw should be about 1000 ma (1 a).

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symptom:
in ic-736, unit will not turn on. unit powers up if vcc cable is disconnected from pa.

probable cause:
shorted driver and pa fets.

cure:
replace driver and pa fets. inspect pa unit for overheated components and replace as necessary. re-align idle current after repairs as per new specs (see above).

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symptom:
automatic antenna tuner cannot find tune on some bands, typically 40 and/or 80 meters.

probable cause:
burned capacitors on tuner unit.

cure: replace capacitors c2 & c6 (82pf & 100pf) on tuner unit with 500 volt (or higher) versions. look for other burned capacitors in same area and replace as needed.

-------------------------------------------------------------------------

symptom:
automatic antenna tuner hunts but cannot find tune on any band.

probable cause:
bad detector diodes on tuner ctrl unit.

cure:
check these diodes on tuner ctrl unit: d1, d2, d3, d4, d5, d6, d11, and d12. replace as needed.
remarks: while tuner is disassembled, we recommend replacement of c2 & c6 capacitors on the tuner unit with 500-volt equivalents to prevent future failure (see above).

ic-737 only
symptom:
frequency unstable.

probable cause:
unstable trimmer capacitor in bfo.

cure:
replace bad plastic trimmer capacitor c294 (30 pf) on main unit with ceramic equivalent.

-------------------------------------------------------------------------

symptom:
no rx, no tx. dead spots in tuning range. caused by pll unlock.

probable causes:
unstable trimmer capacitor is causing pll unlock in dds vco.

cure:
replace bad plastic trimmer capacitor c205 (20 pf) on pll unit with ceramic equivalent.

-------------------------------------------------------------------------

symptom:
no rx/tx, or distorted audio on one or more bands.

probable cause:
unstable trimmer capacitor is causing pll unlock in hpl vco.

cure:
replace four plastic trimmer capacitors, c35, c46, c59 and c70 on pll unit with ceramic equivalents.

remarks:
while the pll unit is lifted, it would be a good idea to replace trimmer capacitor c205 (20 pf) in the dds loop.

ic-737 & ic-737a

symptom:
rx/tx audio pitch change between usb and lsb, i.e. lsb has too many highs while usb has too many lows. rx audio may be "balanced" with the pbt control.

probable cause:
bfo or pbt oscillator is out of adjustment, or fl-30 filter is bad.

cure:
re-align bfo and pbt. if this does not correct the problem, replace the filter.

-------------------------------------------------------------------------

symptom:
unit will not switch between antenna 1 and 2. front panel switch and led seem to work properly.

probable cause:
stuck relay on ant-sw unit.

cure:
replace bad rl1 relay (ae5343) on ant-sw unit.

remarks:
there is a factory update to prevent this failure in older ic-737s. call icom technical support for details.

-------------------------------------------------------------------------

symptom:
automatic antenna tuner hunts but will not tune on any bands, or sometimes finds a tune on some bands.

probable causes:
1) bad detector diodes on tuner unit. 2) bad tuner cpu.

cure:
check and replace faulty detector diodes d6, d7, d8, and d9 on tuner ctrl unit. if diodes are good, replace ic10 chip (upd7533c) on tuner ctrl unit.

symptom:
distorted rx/tx, or no rx/tx on one or more bands. distorted ssb audio, sounds raspy and rough. problem may be temperature related.

probable cause:
1) bad trimmer capacitors on pll unit, hpl vco section.

cure:
replace plastic trimmer capacitors c4, c15, c24, and c33 with ceramic equivalents.

-------------------------------------------------------------------------

symptom:
any of the following: 1) no rx/tx at all. display normal. 2) no cw operation. 3) some front panel controls do not operate reliably.

probable causes:
burned or open r1 resistor on reg. unit.

cure:
replace r1 resistor (4.7 ohm, 1/2 watt).

-------------------------------------------------------------------------

symptom:
intermittent no/low tx output. jiggling or pressing the compressor switch causes the power to return to normal.

probable cause:
bad compressor switch. (this is a common failure in the ic-740).

cure:
replace (do not clean) the compressor switch.

-------------------------------------------------------------------------

symptom:
low, erratic tx output in all modes. (compressor switch has already been replaced.) alc meter reading is unsteady and may drop out altogether. removing the top cover may cause problem to disappear for a while. pressing on the wiring harness near the if unit may cause the power/alc meter reading to fluctuate. problem may be temperature related.

probable causes:
1) poor solder joints on if unit. 2) dirty rf power/mic gain pot. 3) poor connections at if unit is causing low tx drive. this is sometimes caused by oxidation at the point where the wires are crimped to the connector pins. 4) bad relay on fm unit (if installed).

cure:
clean dirty rf power/mic gain pot. resolder cracked and suspicious-looking solder joints on if unit. if these do not help, remove and resolder the wire crimps in the rf connector pins in j2, j5, j12, j9, j19 and j20 on the if unit. be sure to not add too much solder to the connector pins or they will need to be replaced. be sure all connectors are fully seated after repair. replace rl1 on the fm unit.

remarks:
although resoldering the connector pins is a delicate and time consuming procedure, it will save you a great deal of troubleshooting time and frustration.

-------------------------------------------------------------------------

symptom:
tx power output is marginal on some bands. (compressor switch has already been replaced.) alc meter reading is low and steady, not erratic. adjusting alc pot does not increase power. troubleshooting reveals that no specific stage is malfunctioning.

probable cause:
tx if needs to be aligned.

cure:
set unit to 14.100 mhz, tx in rtty mode, and adjust l8, l10, and l11 coils on if unit for peak alc meter reading.

remarks:
if the above steps have no effect, troubleshoot for a defective tx stage, or follow steps for curing erratic tx.

-------------------------------------------------------------------------

symptom:
power gradually drops off when warm. alc meter indication remains at mid-scale. adjusting alc does not help.

probable cause:
apc misalignment.

cure:
realign alc, ic meter, and apc as per pages 9-1 and 9-1 of the service manual.

-------------------------------------------------------------------------

symptom:
vfo tuning is erratic, sometimes skips when changing frequencies or bands. displayed frequency sometimes does not increment smoothly when vfo knob is turned slowly.

probable cause:
dirty rotary encoder.

cure:
remove white plastic cover from back of rotary encoder and spray clean the carbon track with blue shower solvent. spin vfo knob vigorously while spraying.

remarks:
when cleaning the rotary encoder, do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. stuff a paper towel between the rotary encoder and display unit to catch spray run-off.

-------------------------------------------------------------------------

symptom:
unit jumps frequency unexpectedly when turning vfo knob. digital display does not track with this jump. the frequency jump is consistent and happens in the same place every time. actual operating frequency and displayed frequency do not match in some parts of the bands.

probable cause:
pll is out of alignment.

cure:
be sure you don't have a rotary encoder problem, first. try realigning the ldo as per the second "9-1" page in the service manual. if this doesn't help, follow the complete alignment procedures for the pll on both "9-1" pages in the service manual with the exception of the pll lock adjustment.

remarks:
we do not recommend adjusting the pll lock trimmer capacitors c4, c15, c24, and c33. they do not contribute to this particular problem and disturbing them may cause them to fail.

-------------------------------------------------------------------------

symptom:
frequency instability.

probable cause:
dirty rit/xit pot and switches.

cure:
clean dirty rit/xit pots and switches with blue shower solvent.

-------------------------------------------------------------------------

symptom:
drastic change in audio frequency response between usb and lsb modes., i.e. usb audio has too much bass while lsb audio has too much treble. tx frequency response is also poor.

probable cause:
misadjustment of the bfo, pbt oscillator.

cure:
align the bfo and pbt oscillator as per page 9-4 of the service manual.

-------------------------------------------------------------------------

symptom:
intermittent low fm tx output power.

probable cause:
poor contacts in relay rl1 on fm unit.

cure:
replace bad rl1 relay (221-d009-m).

-------------------------------------------------------------------------

symptom:
distorted/raspy-sounding ssb tx. complaints of rfi. unit tests fine on dummy load. unit is being used with an external 3rd party power supply.

probable causes:
1) in adequate station ground. 2) external 3rd party power supply needs additional rf decoupling.

cure:
recheck station ground. improve if necessary. install additional rf decoupling capacitors on the 3rd party power supply.

-------------------------------------------------------------------------

symptom:
tx oscillations, or low and spurious output, especially on lower bands at lower power levels. swr reading may be high when transmitting into a dummy load.

probable causes:
1) bad q10 transistor on rf unit. 2) bad electrolytic capacitors on pa unit. 3) band switch has been misaligned. 4) burned components on the ant sw unit. 5) bad driver transistors or pa transistors.

cure:
disconnect j9 connector on the rf unit and see if the problem goes away. if it does, then q10 transistor is most likely bad (replace it with a 2sc2878b). if disconnecting j9 connector has no effect, examine the pa unit. replace c18 and/or c24 capacitors on pa unit if they appear swollen or overheated. check the ant sw unit for burned components and replace as needed. check alignment of the band switch wafers and correct if necessary. if none of the above are causing the problem, then most likely the drivers or pa transistors are bad.

-------------------------------------------------------------------------

symptom:
erratic display i.e., frequency listed will be out of the normal operating range of the radio. rotating the vfo knob may cause strange characters to appear in the display. radio does not work properly. cycling the power switch a few times may restore operation temporarily, or radio may be stuck in the failure mode.

probable causes:
1) cpu is glitched and must be reset. 2) loose connector on logic or matrix unit. 3) bad diode on matrix unit.

cure:
turn off memory backup switch on rear panel of radio and leave radio unplugged for a few minutes to reset cpu. it is recommended that the switch be left in the off position to prevent future failure. re-seat all connectors on logic and matrix unit. if none of these solve the problem, there may be a bad diode on the matrix unit.

-------------------------------------------------------------------------

symptom:
either of the following: 1) preamp does not work. sensitivity of radio drops when preamp is engaged. 2) rx noisy and intermittent. preamp intermittent.

probable causes:
1) bad preamp relay on rf unit. 2) burned components in preamp section on rf unit. 3) cracked solder on rf unit in preamp section.

cure:
replace bad rl1 relay (fbr221do12). replace bad/burned preamp components on rf unit, usually rl1 relay, q11 transistor, q12 transistor, r58 resistor, c86 capacitor, etc... also check q10 transistor for failure. if bad, replace it with a 2sc2878b. carefully examine the preamp section of the rf unit for bad solder joints and resolder as needed.

symptom:
distorted rx/tx, or no rx/tx on one or more bands. distorted ssb audio; sounds raspy and rough. problem may be temperature related.

probable
cause: bad trimmer capacitors on pll unit, hpl vco section.

cure: replace plastic trimmer capacitors c78, c88, c96, and c107 with ceramic equivalents. we suggest using a 12 pf trimmer capacitor for c96 instead of a 7 pf as listed in the service manual. this will allow you to properly adjust the hpl lock voltage for that band.

remarks:
while you have the pll unit lifted, it would be a good idea to inspect and rework the soldering in the regulator section. also, inspect electrolytic capacitors c128, c129, c132, and c137 in the regulator circuit for discoloration caused by overheating. replace all overheated capacitors to prevent future failures.

----------------------
symptom:
frequency instability.

probable causes:
1) unstable trimmer capacitor c5 on 2nd l.o. unit. 2) dirty rit/xit pot and switches.

cure:
replace plastic trimmer capacitor with a 30 pf ceramic trimmer and realign. clean dirty rit/xit pots and switches with blue showers solvent.

remarks:
unstable c5 capacitor is very common in the ic-745. the plastic version of this capacitor should be replaced on all units that are in need of repair as preventative maintenance.

-------------------------------------------------------------------------

symptom:
erratic display i.e., frequency listed will be out of the normal operating range of the radio. rotating the vfo knob may cause strange characters to appear in the display. radio does not work properly. cycling the power switch a few times may restore operation temporarily. problem may be temperature related.

probable causes:
1) cracked solder connections on ram unit is causing logic errors. 2) program in ram unit is corrupted. 3) burned/overheated r20 resistor on logic unit is causing low voltage in logic unit.

cure:
check for burned r20 resistor (10 ohm, 1/2 watt) on logic unit. if it looks ok, resolder cracked solder joints at j1 and j2 connectors on ram unit, then reprogram. if this does not help, substitute ram unit with a known good one. if the substitute ram unit works properly, then the ram unit must be replaced.

-------------------------------------------------------------------------

symptom:
rx noisy and intermittent. preamp intermittent.

probable causes:
1) bad preamp relay on rf unit. 2) cracked solder on rf unit in preamp section.

cure:
replace bad relay rl1 (fbr221do12). carefully examine the preamp section of the rf unit for bad joints and resolder as needed.

-------------------------------------------------------------------------

symptom:
intermittent low fm tx output power.

probable cause:
poor contacts in relay rl1 on fm unit.

cure:
replace bad rl1 relay (221b-d009-m).

-------------------------------------------------------------------------

symptom:
no preamp operation. sensitivity of radio drops when preamp is engaged.

probable causes:
1) bad preamp relay on rf unit. 2) burned components in preamp section on rf unit.

cure:
replace bad rl1 relay (fbr221do12). replace bad/burned preamp components on rf unit, usually rl1 relay, q11, q12 transistors, r58 resistor, c86 capacitor, etc... also check q10 transistor for failure. if bad, replace it with a 2sc2878b.

-------------------------------------------------------------------------

symptom:
low, erratic tx output in all modes. alc meter reading is unsteady and may drop out altogether. removing the top cover may cause problem to disappear for a while. pressing on the wiring harness near the if unit may cause the power/alc meter reading to fluctuate. problem may be temperature related.

probable causes:
1) cracked solder joints on if unit. 2) dirty rf power/mic gain pot. 3) poor connections at if unit is causing low tx drive. this is sometimes caused by oxidation at the point where the wires are crimped to the connector pins. 4) bad relay on fm unit (if installed).

cure:
clean dirty rf power/mic gain pot. resolder cracked and suspicious looking solder joints on if unit. if these do not help, remove and resolder the wire crimps in the rf connector pins in j2, j5, j12, j9, j19 and j20 connectors on the if unit. be sure to not add too much solder to the connector pins or they will need to be replaced. be sure all connectors are fully seated after repair. replace rl1 relay on the fm unit.

remarks:
although resoldering the connector pins is a delicate and time consuming procedure, it will save you a great deal of troubleshooting time and frustration.

-------------------------------------------------------------------------

symptom:
tx power marginal on some bands. alc meter reading is low but steady, not erratic. cannot use alc to adjust for more power. troubleshooting reveals that no specific stage is malfunctioning

probable causes:
1) tx if needs to be aligned. 2) modification to if unit needs to be removed.

cure:
set unit to 14.100 mhz, tx in rtty mode, and adjust l8, l10 and l11 coils on if unit for peak alc meter reading. if this does not yield enough drive, remove the 220-ohm resistor on the bottom side of the if unit directly beneath l8 coil and retune. be sure to readjust power, alc level and metering, if needed.

remarks:
if the above steps have no effect, troubleshoot for a defective tx stage, or follow steps for curing erratic tx.

-------------------------------------------------------------------------

symptom:
vfo tuning is erratic, sometimes skips when changing frequencies or bands. displayed frequency does not increment smoothly when vfo knob is turned slowly.

probable cause:
dirty rotary encoder.

cure:
remove white plastic cover from back of rotary encoder and spray clean the carbon track with blue shower solvent. spin vfo knob vigorously while spraying.

remarks:
do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. stuff a paper towel between the rotary encoder and display unit to catch spray run-off.

-------------------------------------------------------------------------

symptom:
tx power gradually drops off when warm. alc meter indication remains at mid-scale. adjusting alc does not help.

probable cause:
apc misalignment.

cure:
realign alc, ic meter, and apc as per pages 9-1 and 9-2 in the service manual.

-------------------------------------------------------------------------

symptom:
ac hum or buzz in transmit and receive audio when an internal power supply is installed.

probable cause:
ac in being induced into the pll from the ac wires and power supply case.
cure: twist the ac wires that run from the rear panel to the power switch and route them away from the pll unit. tape them up against the chassis if necessary.

remarks:
in some radios, especially older ic-745s, it is not possible to completely eliminate all ac hum when using the internal power supply.

-------------------------------------------------------------------------

symptom:
drastic change in audio frequency response between usb and lsb., i.e. usb audio has too much bass while lsb audio has too much treble. tx frequency response is also poor.

probable causes:
misadjustment of the bfo or pbt oscillator.

cure:
align the bfo and pbt oscillator as per page 9-4 of the service manual.

-------------------------------------------------------------------------

symptom:
distorted/raspy-sounding ssb tx. complaints of rfi. unit tests fine on dummy load. unit is being used with an external 3rd party power supply.

probable causes:
1) inadequate station ground. 2) external 3rd party power supply needs additional rf decoupling.

cure:
recheck station ground. improve if necessary. install additional rf decoupling capacitors on the dc terminals of the power supply.

-------------------------------------------------------------------------

symptom:
noise in rx, birdies.

probable cause:
voltage regulators on pll unit are oscillating.

cure:
replace overheated c128, c129, c132 and c137 regulator filter capacitors on pll unit.

-------------------------------------------------------------------------

symptom:
no receive through lb antenna jack. no bc band rx.

probable cause:
open ds1 fuse on rf unit.

cure: replace open fuse.

-------------------------------------------------------------------------

symptom:
1) no rx/tx at all. display normal. 2) no cw operation, or, 3) some front panel controls do not operate reliably.

probable cause:
burned or open r1 resistor on reg. unit.

cure:
replace r1 resistor (4.7 ohm, 1/2 watt).

-------------------------------------------------------------------------

symptom:
tx oscillations/spurious output.

probable causes:
1) bad q10 transistor on rf unit. 2) open parasitic-suppression resistor on pa unit. 3) bad electrolytic filter capacitors on pa unit. 4) bad drivers or finals.

cure:
disconnect j9 connector on the rf unit and see if the problem goes away. if it does, then q10 transistor is most likely bad (replace it with a 2sc2878b). if disconnecting j9 connector has no effect, check the value of r22 resistor (4.7 ohms) on pa unit. this resistor may look ok but still be open. replace c18 and/or c25 capacitors if they appear swollen or overheated. if none of the above are causing the problem and alc meter deflection is low, then most likely the drivers or pa transistors are bad.
remarks: spurious output may also be caused by incorrect band switch voltage to the low-pass filter unit, or even faulty/burned components on the filter unit itself. the above causes should be ruled out first, since low-pass filter failures are not common in the ic-745.

unless otherwise noted, the following applies to both models.

symptom:
distorted, or no rx/tx on one or more bands. problem may be temperature related.

probable cause:
1) bad trimmer capacitors on pll unit. 2) noisy variable resistor on pll unit.

cure:
replace plastic trimmer capacitors c78, c88, c97, and c107 with equivalent ceramic types. remove excessive wax from around replacement trimmers to prevent wax contamination. we suggest using a 12 pf trimmer capacitor for c97 instead of a 7 pf as listed in the service manual. this will allow you to properly adjust the hpl lock voltage for that band. modification: if variable resistor r43 on the pll unit is the 470 ohm type, replace it with a fixed-value 220 ohm version. if it's the 2.2 k type, replace it with a fixed-value 1.2 k resistor.

remarks:
while the pll unit is lifted, we recommend soldering all joints in the regulator section as this area runs hot. also, inspect electrolytic capacitors c128, c129, c132, and c137 in the regulator circuit for discoloration caused by overheating. replace all overheated capacitors to prevent future failures.

-------------------------------------------------------------------------

symptom:
erratic display i.e., frequency listed will be out of the normal operating range of the radio. rotating the vfo knob may cause strange characters to appear in the display. radio does not work properly. cycling the power switch a few times may restore operation temporarily. problem may be temperature related.

probable causes:
1) cracked solder on ram unit is causing logic errors. 2) program in ram unit is corrupted. 3) burned/overheated r20 resistor on logic unit is causing low voltage in logic unit.

cure:
check for burned r20 resistor (10 ohm, 1/2 watt) on logic unit. if it looks ok, resolder cracked solder joints at j1 and j2 connectors on ram unit, then reprogram. if this does not help, then the ram unit must be replaced. if the substitute ram unit does not cure the problem, there may be noise from the dc-dc converter causing the logic unit to glitch. follow instructions for "no frequency display" problem.

-------------------------------------------------------------------------

symptom:
no rx audio in any mode except fm. no tx. unit is not unlocked.

probable cause:
pbt oscillator is not running.

cure:
verify absence of 9.4665 mhz signal at w86 test point on main unit in 751a, or w134 test point in the ic-751. replace faulty x7 crystal on main unit in ic-751a, or x5 crystal in the ic-751.

-------------------------------------------------------------------------

symptom:
frequency instability in usb mode.

probable causes:
bad trimmer capacitor in bfo section on main unit.

cure:
replace plastic trimmer capacitor with a 30 pf ceramic trimmer capacitor and realign.

-------------------------------------------------------------------------

symptom:
drastic change in audio frequency response between usb and lsb., i.e. usb audio has too much bass while lsb audio has too much treble. tx frequency response is also poor.

probable cause:
misadjustment of the bfo or pbt oscillator, or a faulty crystal filter fl-30 (or fl-80 in 751a) on the main unit.

cure:
check adjustment of the bfo and pbt oscillator. if these are ok, then the problem is most likely in the filter.

remarks:
if the problem is intermittent, and the radio is an ic-751 (not "a" version), then the ground return path for the -5 v supply between the display and main units may be poor. to cure this, connect a short wire jumper between the unused pin 2 at j2 connector on the display unit and chassis ground at the closest mounting screw of the main unit pcb. if no spare pin is handy for the connector at j2, the jumper may be soldered to the shield of t1 transformer instead.

-------------------------------------------------------------------------

symptom:
ac hum or buzz in transmit and receive audio when an internal power supply is installed.

probable cause:
ac is being induced into the pll from the ac wires and power supply case.

cure:
twist the ac wires that run from the rear panel to the power switch and route them away from the pll unit. tape them up against the chassis if necessary.

remarks:
in some radios, especially older ic-751s, it is not possible to completely eliminate all ac hum when using the internal power supply.

-------------------------------------------------------------------------

symptom:
no, or very low rx sensitivity.

probable cause:
bad components on rf unit, probable result of rf overload. to verify this, check dc voltages at d47 diode on rf unit. correct voltages are: cathode side- rx: 8.5 v, tx: 13.8 v. anode side- rx: 9.0 v, tx: 9.5 v.

cure:
replace all of these components on the rf unit: d42, d44, d45, d46, d47 diodes, q15 transistor. check c174 and c179 capacitors for leakiness. there may be other failures. unit is not repaired until listed voltages at d47 diode are correct.

-------------------------------------------------------------------------

symptom:
distorted rx on strong signals. bc band rx sensitivity may be low or marginal.

probable cause:
q15 transistor on the rf unit has become leaky.

cure:
check and replace q15 transistor. if q15 transistor is leaky, there may be other problems on the rf unit. see cure for rx problem listed above.

-------------------------------------------------------------------------

symptom:
rx disappears when outer conductor of coax is connected to antenna jack. inspection reveals 6 volts dc at the center conductor of the antenna jack.

probable cause:
shorted dc-blocking capacitor c40 on the ant sw unit.

cure:
replace the capacitor.

remarks:
there are probably other problems on the rf unit if this capacitor is bad. check the voltages at d47 diode on the rf unit. radio may have been struck by lightning or sustained a large rf overload through the antenna jack.

-------------------------------------------------------------------------

symptom:
rx cuts out randomly when turning vfo knob in ic-751a with ux-14 installed.

probable cause:
incompatibility between ux-14 and 751a is causing the pll to unlock.

cure:
turn off transceive mode on ux-14 (bank s1, switch #3 to off position).

-------------------------------------------------------------------------

symptom:
hash noise in rx, birdies.

probable causes:
1) voltage regulators on pll unit are oscillating, producing noise on vcc line. 2) overheated components and/or loose screws are causing the power supply to run noisy. 3) noisy dc-dc converter on display unit.

cure:
replace overheated c129, c132, and c137 regulator filter capacitors on pll unit. inspect power supply for overheated components, tighten the screws that hold down the reg. unit. don't forget to tighten the screw hidden beneath the wire bundle where the dc output lines leave the case. if none of these help, and radio is a ic-751 (not a version), then the display dc-dc converter may need modification. contact icom technical support for update information.

-------------------------------------------------------------------------

symptom:
no receiver audio at all in ic-751a. signals can be seen on s-meter.

probable causes:
cracked solder joint on main unit is causing no vcc to the audio amp.

cure:
resolder cracked solder joint at l46 coil on main unit. repair damaged foil trace, if necessary. re-route ferrite beads so they will not press down on l46 coil (or any other components) when the top cover is reinstalled.

-------------------------------------------------------------------------

symptom:
rx audio seems low in ic-751.

probable causes:
unit needs update to audio amp circuitry on main unit.

cure:
contact icom technical support for update bulletin.

-------------------------------------------------------------------------

symptom:
no tx output in all modes. problem is traced to no output at j8 connector on the rf unit.

probable causes:
bad q14 transistor amplifier on rf unit.

cure:
replace q14 transistor. there may be other problems on the rf unit. check the dc voltages at d47 diode.

-------------------------------------------------------------------------

symptom:
distorted/raspy sounding ssb tx. average talk power is low, even at higher mic gain levels. cw, rtty and fm seem normal. rx in all modes is ok.
probable causes: 1) external 3rd party power supply needs additional rf decoupling. 2) no bias voltage to driver or pa transistors.

cure:
install rf decoupling capacitors on 3rd party power supply (if used). if this does not help, measure voltage at bases of driver and pa transistors in tx ssb mode. should be around 0.67 volts. 0 volts indicates trouble and probably a bad q6 transistor. if problem is intermittent, check for bad solder at the legs of q6 transistor, or d2 diode.

remarks:
the driver and pa idle current are set too high in some ic-751s (not a version). align the idle current as per the service manual after repairs are made.

-------------------------------------------------------------------------

symptom:
when using an internal power supply, the power to the radio flutters on and off when the tx power output is increased toward maximum. unit runs fine when the internal power supply is substituted with an external supply.

probable causes:
cracked solder in the current sensing circuit of the power supply is shutting it down prematurely.

cure:
resolder joints at the r26 resistor (0.0012 ohm) inside the power supply.
remarks: this resistor appears as a metal bar soldered to the bottom pc board. solder the edges of the bar before the holes at either end.

-------------------------------------------------------------------------

symptom:
tx oscillations/spurious output.

probable causes:
1) open resistor or faulty electrolytic capacitors on pa unit. 2) pa unit needs modification. 3) bad drivers or pa transistors. 4) burned components on the ant sw unit.

cure:
check the value of r22 resistor (4.7 ohms) on pa unit. (this resistor may look ok but still be open.) replace c18 and c25 capacitors on the pa unit if they appear swollen or overheated. modification for old ic-751: install a metal film, 220-ohm 1/2 watt resistor parallel to c6 capacitor on pa unit if there isn't one installed already. inspect the d4 area of the ant sw unit and replace any burned components. if none of these solutions solve the problem, disconnect the j10 connector on the rf unit and see if the problem goes away. if it does, then there is a problem with the tx/rx switching on the rf unit. if disconnecting j10 connector doesn't help, then most likely either the drivers or pa transistors are bad, especially if the alc meter deflection is low.

remarks:
the spurious output may also be caused by incorrect band switch voltage to the low-pass filter unit, or even faulty/burned components on the filter unit itself. the above causes should be ruled out first since low-pass filter failures are uncommon in the ic-751(a).

-------------------------------------------------------------------------

symptom:
intermittent low fm tx output power in ic-751.

probable causes:
bad relay rl1 on fm unit.

cure:
replace bad rl1 relay (221-d009-m).

-------------------------------------------------------------------------

symptom:
no tx output at all in rtty or cw modes. ssb and am are ok.

probable causes:
bad carrier generator crystal on main unit.

cure:
replace bad x1 crystal on main unit.

-------------------------------------------------------------------------

symptom:
no frequency display, only decimal points. problem may be intermittent or temperature related.

probable causes:
display dc-dc converter has noisy output.

cure:
replace all of the following dried-out electrolytic capacitors on the display unit: c17, c18, c20, c21, c22, c23, c24, and c25. (these capacitors are light yellow when they are new. capacitors that have turned brown are old and dried-out).

remarks:
while servicing the dc-dc converter section, be on the lookout for cracked solder joints, as this section of the radio tends to run hot. also, it would be a good idea to reprogram the ram card after servicing the dc-dc section.

-------------------------------------------------------------------------

symptom:
poor carrier suppression in ssb mode in ic-751. will not adjust properly.

probable causes:
main unit needs factory update.

cure:
contact icom technical support for update bulletin.

-------------------------------------------------------------------------

symptom:
low garbled audio sound in speaker/headphones in ssb tx, even with monitor turned off.

probable causes:
rf feedback. 1) inadequate station ground. 2) external 3rd party power supply needs rf decoupling. 3) main unit needs factory update (ic-751 only).

cure:
check station ground. if a 3rd party power supply is being used, add rf decoupling capacitors to output terminals to prevent regulation problems. if the problem occurs in an ic-751, a factory update may need to be installed to the main unit. contact icom tech support for this information.

-------------------------------------------------------------------------

symptom:
radio seems to be stuck in "scan" mode.

probable causes:
bad ic2 chip on the logic unit.

cure:
replace ic2 integrated circuit (rp5g01-007).

-------------------------------------------------------------------------

symptom:
loud, high pitched squeal emanates from inside radio.

probable causes:
dc-dc converter transformer has a loose core.

cure:
replace t1 transformer (to-9) on display unit.

-------------------------------------------------------------------------

symptom:
vfo tuning is erratic, sometimes skips when changing frequencies or bands. display does not increment smoothly when vfo knob is turned slowly.

probable causes:
dirty rotary encoder.

cure:
remove white plastic cover from back of rotary encoder and spray clean the carbon track with blue shower solvent. spin vfo knob vigorously while spraying.

remarks:
do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. stuff a paper towel between the rotary encoder and display unit to catch spray run-off.

-------------------------------------------------------------------------

symptom:
missing segments or mode indicators on display. problem may be intermittent.

probable causes:
bad connection between logic and display units.

cure:
check the connectors j5 and j13 on the logic unit where the flex cables to the display unit are attached. if they look ok, and none of their connector spring contacts flattened, replace the flex cables.

-------------------------------------------------------------------------

symptom:
low mic gain when using heil mic element.

cure:
install modification: in 751, change value of r264 resistor on main unit to 200 k. in 751a, change value of r21 resistor on af-vr unit to 47 k.

symptom:
garbled rx/tx, or no rx/tx on one or more bands. problem may be temperature related.

probable causes:
1) bad trimmer capacitors on pll unit. 2) noisy variable resistor on pll unit.

cure:
replace plastic trimmer capacitors c78, c88, c97, and c107 with ceramic equivalents. remove excess wax from around replacement trimmer capacitors to prevent wax contamination. we suggest using a 12 pf trimmer for c97 capacitor instead of a 7 pf as listed in the service manual. this will allow you to properly adjust the hpl lock voltage for that band. also, replace variable resistor r43 on the pll unit with a fixed-value, 220 ohm, 1/2 watt version. (use a 1.2 k resistor if the value of r43 variable resistor is 2.2 k.)

remarks:
while the pll unit is lifted, we recommend resoldering the solder joints in the regulator section as this area runs hot. also, inspect electrolytic capacitors c128, c129, c132, and c137 in the regulator circuit for discoloration caused by overheating. replace all overheated capacitors.

-------------------------------------------------------------------------

symptom:
frequency unstable in usb mode.

probable causes:
bad trimmer capacitor c202 in bfo section on the main unit.

cure:
replace plastic trimmer capacitor with a 30 pf ceramic trimmer capacitor and realign.

-------------------------------------------------------------------------

symptom:
no or very low rx sensitivity.

probable cause:
bad components on the rf unit, probable result of rf overload. to verify this, check dc voltages at d47 diode on rf unit. correct voltages are: cathode side- rx: 8.5 v, tx: 13.8 v. anode side- rx: 9.0 v, tx: 9.5 v.

cure:
replace all of these components on the rf unit: d42, d44, d45, d46, d47 diodes, q15 transistor. check c174 and c179 capacitors for leakiness. there may be other failures. unit is not repaired until listed voltages at d47 are corrected.

-------------------------------------------------------------------------

symptom:
distorted rx on strong signals. bc band rx sensitivity may low or marginal.

probable cause:
q15 transistor on the rf unit has become leaky.

cure:
replace faulty q15 transistor (2sc2878b). if q15 is leaky, there may be other problems on the rf unit. see cure for rx problem listed above.

-------------------------------------------------------------------------

symptom:
intermittent rx sensitivity. problem seems mechanical. tapping on case or switching between rx & tx may temporarily restore sensitivity.

probable cause:
intermittent contacts in relay on tuner relay unit.

cure:
replace rl13 relay (ly2-0-dc12v) on tuner relay unit.

-------------------------------------------------------------------------

symptom:
noise in rx, birdies.

probable cause:
voltage regulators on pll unit are oscillating.

cure:
replace overheated c128, c129, c132 and c137 regulator filter capacitors on pll unit.

-------------------------------------------------------------------------

symptom:
rx disappears when outer shield of antenna coax is connected. inspection reveals 6 volts dc at the center conductor of the antenna jack.

probable causes:
shorted dc-blocking capacitor c40 on the ant aw unit.

cure:
replace the capacitor.

remarks:
there are probably other problems on the rf unit if this capacitor is bad. check the voltages at d47 diode on the rf unit. radio may have been stuck by lightning or sustained a large rf overload through the antenna jack.

-------------------------------------------------------------------------

symptom:
no tx output in all modes. problem is traced to no output at j8 connector on the rf unit.

probable cause:
bad q14 amplifier on rf unit.

cure:
replace q14 transistor (2sc2053). there may be other problems on the rf unit. check the dc voltages at d47 diode.

-------------------------------------------------------------------------

symptom:
the power to the radio flutters on and off when the tx power output is increased toward maximum. inspection reveals that the internal power supply is unable to handle the current. unit runs fine when the internal power supply is substituted with an external supply.

probable cause:
cracked solder in the current-sensing circuit of the power supply is shutting it down prematurely.

cure:
resolder joints at the r26 resistor (.0012 ohms) inside the power supply.

remarks:
this resistor appears as a metal bar soldered to the bottom pc board. solder only the edges of the bar before the holes at either end.

-------------------------------------------------------------------------

symptom:
distorted/raspy-sounding tx in ssb modes. average talk power is low, even at higher mic gain levels. cw, rtty and fm seems normal.
probable cause: no base bias voltage at driver or pa transistors. measure voltage at bases of driver and pa transistors in tx ssb mode. should be around 0.67 volts. 0 volts indicates trouble.

cure:
bad q6 transistor (2sd880y) on pa unit. if problem is intermittent, check for bad solder at the legs of q6 transistor and d2 diode.

--------------------------------------------------------------------------------

symptom:
tuner won't tune on one or more bands.

probable cause:
presets are out of adjustment.

cure:
adjust presets.

-------------------------------------------------------------------------

symptom:
tuner does not tune on any band. inspection reveals that only one motor is turning.

probable cause:
bad motor driver transistors.

cure:
check motor driver transistors q1 & q2, or q3 & q4 (depending on the motor that is not turning). if the motor transistors overheated before they failed, also check related components for failure before replacing the bad motor driver transistor (or the new drivers will fail too).

-------------------------------------------------------------------------

symptom:
tx oscillation/spurious output, especially on lower bands at lower power levels.

probable causes:
1) open parasitic-suppression resistor on pa unit. 2) burned components on the ant sw unit. 3) bad drivers or finals.

cure:
check the value of r22 resistor (4.7 ohms) on pa unit. this resistor may look ok but still be open. inspect the d4 diode area of the ant sw unit and replace any burned components. if these solutions don't help, disconnect the cable at j10 connector on the rf unit and see if the problem goes away. if it does, the there is a problem with the tx/rx switching on the rf unit. if disconnecting j10 connector does not alleviate the problem, then most likely either the drivers or pa transistors are bad, especially if the alc meter deflection is low.

remarks:
spurious output may be caused by incorrect band switch voltage to the low-pass filter unit, or even faulty/burned components on the filter unit itself. the above causes should be ruled out first, however, since low-pass filter failures are uncommon in the ic-761.

-------------------------------------------------------------------------

symptom:
radio seems to be stuck in "scan" mode.

probable cause:
bad ic2 chip on the logic unit.

cure:
replace ic2 (rp5g01-007).

-------------------------------------------------------------------------

symptom:
intermittent display. s-meter may peg and rx/tx may disappear when the unit is in the failure mode.

probable cause:
cracked solder beneath the dp-6 dc-dc converter ic (ic1) on the display unit.

cure:
carefully remove the dp-6 converter ic from the display unit and resolder the pins of the ic to its circuit board. then reinstall the dp-6 back on the display unit.

-------------------------------------------------------------------------

symptom:
vfo tuning is erratic, sometimes skips frequencies in a certain area of vfo knob travel. least significant digit in frequency display sometimes "flutters" back and forth instead of changing incrementally when the vfo knob is turned slowly.

probable cause:
bad rotary encoder.

cure:
substitute rotary encoder with a known good one. it is recommended that the rotary encoder be replaced as a complete assembly if it is bad.
remarks: if the unit tunes in only one direction, i.e. either up or down, there may be a problem on the logic unit.

-------------------------------------------------------------------------

symptom:
loud, high pitched squeal emanates from inside radio. may be intermittent.

probable cause:
dc-dc converter transformer has a loose core.

cure:
replace dp-6 on the display unit.

-------------------------------------------------------------------------

symptom:
a whine emanates from inside radio in tx only at certain power levels. transmitter works ok.

probable cause:
loose core in dc-dc converter transformer inside power supply.

cure:
replace t2 transformer on reg. unit inside power supply.

symptom:
unit will not transmit or receive on all bands. very low noise from speaker. display and controls all seem to work properly.

probable cause:
pll unlock. usually this is caused by cracked/overheated solder connections on the pll unit, particularly in the lpl vco. check dc voltages at r6 and r201 resistors to verify the pll unlock condition.

cure:
resolder cracked solder joints on pll unit. pay particular attention to the solder joints around the regulators and inside beneath the lpl vco shield.

-------------------------------------------------------------------------

symptom:
dead spots in the bottom 250 khz (or so) range of each band when radio is cold. dead spots may eventually get smaller then disappear as radio warms up.

probable cause:
dds lpl trimmer capacitor c226 on pll unit has drifted out of adjustment.

cure:
readjust c226 trimmer capacitor to specs. if c226 is difficult to adjust or won't hold its adjustment, it has probably become unstable and should be replaced. when replacing c226 trimmer capacitor, inspect solder in lpl vco section and remove excessive wax from around replacement capacitor to prevent contamination of the new part.

remarks:
on older units, there is no address hole in the lpl shield cover to facilitate adjustment of c226 trimmer capacitor. on these units, you will have to unsolder and remove the shield cover to make adjustments. while you have the cover off, you may wish to drill a small access hole in the cover to allow easier (and more accurate) adjustment of c226 trimmer capacitor.

-------------------------------------------------------------------------

symptom:
no rx/tx when unit is first turned on, but will return if vfo knob is spun or if power is cycled on and off a few times.

probable cause:
lpl problem, probably c226 trimmer capacitor has drifted out of adjustment. see cure for previous listed problem.

-------------------------------------------------------------------------

symptom:
no rx/tx or distorted rx/tx on one or more bands. distorted ssb audio sounds raspy and rough. some bands may sound ok. problem may be temperature related.

probable cause:
bad trimmer capacitors on pll unit, hpl vco section.

cure:
replace plastic trimmer capacitors c11, c20, c29, and c37 with equivalent ceramic versions. remove excessive wax from around replacement trimmers to prevent wax contamination. we suggest using a 12 pf trimmer capacitor for c29 instead of a 7 pf as listed in the service manual. this will allow you to properly adjust the hpl lock voltage for that band.

remarks:
while you have the pll unit lifted, it would be a good idea to inspect the solder connections beneath the shield in the lpl vco section.

-------------------------------------------------------------------------

symptom:
frequency unstable in ssb modes.

probable cause:
bad trimmer capacitors in bfo.

cure:
replace plastic trimmer capacitors c42, c45 and c307 (all 30 pf) in main unit with ceramic equivalents.

-------------------------------------------------------------------------

symptom:
drastic change in audio frequency response between usb and lsb, i.e. usb audio has too much bass while lsb audio has too much treble. usb/lsb rx frequency response may be equalized, to a degree, by adjustment if the front panel if shift control. tx frequency response remains poor.

probable cause:
misadjustment of the bfo and/or if shift oscillator, or a faulty crystal filter, probably fl-30 located on the main unit.

cure:
check adjustment of the bfo and if shift oscillator. if these are ok, then the problem is most likely in the fl-30 filter.

remarks:
some filters are better than others so some test selection may be needed to satisfactorily alleviate the problem. the better-performing (but more expensive) fl-80 filter can be substituted for the fl-30 filter.

-------------------------------------------------------------------------

symptom: no or very low rx sensitivity.

probable cause:
bad components on rf unit, probable result of rf overload or lack of necessary update. check dc voltages at d47 diode on rf unit to verify problem. correct voltages are: cathode side- rx: 8.5 v, tx: 13.8 v. anode side- rx: 9.0 v, tx: 9.5 v.

cure:
replace these components on the rf unit: d42, d44, d45, d46, d47 diodes, q15 transistor. check c174 and c179 capacitors for leakiness. there may be other failures. unit is not repaired until listed voltages at d47 diode are correct.

remarks:
if the serial number of the unit is below 01800, the unit probably needs a factory update to the main unit to prevent future failure of these components. call technical support for details on this update.

-------------------------------------------------------------------------

symptom:
distorted rx on strong signals. bc band rx sensitivity may be low or marginal.

probable cause:
q15 transistor on the rf unit has become leaky.

cure:
replace faulty q15 transistor (2sc2878b).

remarks: if q15 transistor is leaky, there may be other problems on the rf unit. see cure for rx problems listed above.

-------------------------------------------------------------------------

symptom:
intermittent rx sensitivity. problem seems mechanical. tapping on case or switching between rx & tx may temporarily restore sensitivity.

probable cause:
bad relay rl13 on tuner relay unit.

cure:
replace rl13 relay (ly2-0-dc12v).

-------------------------------------------------------------------------

symptom:
rx disappears when outer shield of coax is connected to the antenna. inspection reveals 6 volt dc at the center conductor of the antenna jack.

probable cause:
shorted dc-blocking capacitor c15 on the ant sw unit.

cure:
replace c15 capacitor.
remarks: there are probably other problems on the rf unit if this capacitor is bad. check the voltages at d47 diode on the rf unit.

-------------------------------------------------------------------------

symptom:
ptt does not operate. when the send switch is flipped, the green rx led stays lit and unit does not go in the transmit mode. problem may be intermittent.

probable cause:
most common: cracked solder connection at tx reg q7 transistor on main unit. this transistor is mounted to the aluminum heat sink and is easy to find. less common: poor solder joint at ground jumper at mic jack. the ptt ground is located here.

cure:
resolder poor solder joints.

-------------------------------------------------------------------------

symptom:
no tx output in all modes. problem is traced to no output at j8 connector on the rf unit.

probable cause:
bad q14 transistor amplifier on rf unit.

cure:
replace faulty q14 transistor (2sc2053).
remarks: there may be other problems on the rf unit. check the dc voltages at d47 diode.

-------------------------------------------------------------------------

symptom:
distorted/raspy-sounding tx in ssb modes. average power is low, even at higher mic gain levels. turning on the speech processor seems to help a little. cw, rtty and fm seems normal.

probable cause:
no bias voltage to driver or pa transistors. measure voltages at bases of driver and pa transistors in tx ssb mode. should be around .67 volts. 0 volts indicates trouble.

cure:
replace bad q6 transistor (2sd880y) on pa unit. if problem is intermittent, check for bad solder at the legs of q6 transistor, or d2 diode.

-------------------------------------------------------------------------

symptom:
no or low output on all bands with high icc. inspection reveals burned d1 diode, l1 coil, l2 coil, r1, and/or r2 resistors on the ant sw unit.

probable cause:
these components burned while trying to tune a high swr load.

cure:
replace burned components. update: install a 0.0047 礔, 500v ceramic capacitor on the back of the ant sw board between a ground point and the foil trace junction between r1 resistor and l1 coil.

-------------------------------------------------------------------------

symptom:
the power clicks off or flutters on and off when the tx power output is increased toward maximum. inspection reveals that the internal power supply is unable to handle the current. unit runs fine when the internal power supply is substituted with an external power supply.

probable cause:
cracked solder in the current-sensing circuit of the power supply is shutting it down prematurely.

cure:
resolder joints at the r26 resistor (0.0012 ohm) inside the power supply.
remarks: this resistor appears as a metal bar soldered to the bottom pc board. solder only the edges of he bar before the holes at either end.

-------------------------------------------------------------------------

symptom:
tuner operates properly when a carrier is present, but drifts out of tune in ssb mode.

probable cause:
misadjustment of r105 and r110 trimmer pots on tuner unit.

cure:
follow alignment procedure for these pots on page 6-16 in the service manual.

-------------------------------------------------------------------------

symptom:
tuner "chatters" or makes a rattling sound while speaking into the microphone in ssb mode, mostly on upper bands.

probable cause:
tuner instability.

cure:
try aligning r105 and r110 trimmer pots on the tuner unit. if this doesn't help, and the unit has an older serial number, the tuner unit may need an update. call our technical support department for information.

-------------------------------------------------------------------------

symptom:
tuner does not tune on any band. inspection reveals that only one motor is turning.

probable cause:
motor gear assembly is jammed at one end of its tuning range. this failure is usually caused by an attempt to tune an excessively high-swr load.

cure:
disconnect j6 connector on tuner unit and momentarily apply a dc voltage (around 15 volts) to the stuck motor to un-jam it. reverse the polarity of the voltage to the motor if it doesn't unstick the first time. reconnect j6 connector and test the tuner. it should now work fine. if it does not, the motor driver chip (ic2) has probably failed.

-------------------------------------------------------------------------

symptom:
intermittent display. s-meter may peg and rx/tx may disappear when the unit is in the failure mode.

probable cause:
cracked solder joint beneath the dp-6a dc-dc converter ic (ic1) on the display unit.

cure:
carefully remove the dp-6a converter ic from the display unit and resolder the pins of the ic to its circuit board. then reinstall the dp6a back on the display unit.

-------------------------------------------------------------------------

symptom:
vfo tuning is erratic: sometimes skips frequencies in a certain area of vfo knob travel. 10 hz display digit "flutters" instead of changing incrementally when the vfo knob is turned slowly.

probable cause:
bad rotary encoder.

cure:
substitute rotary encoder with a known good one. it is recommended that the rotary encoder be replaced as a complete assembly if it is bad.

remarks:
if the frequency tunes in only one direction, i.e. either up or down, there may be a problem on the logic unit.

-------------------------------------------------------------------------

symptom:
loud, high pitched squeal emanates from inside radio. may be intermittent.

probable cause:
dc-dc converter transformer has a loose core.

cure:
replace dp-6a on the display unit.

-------------------------------------------------------------------------

symptom:
tx monitor gain seems low, even at maximum volume settings. radio has good mic gain, and the transmitter works ok.

probable cause:
low monitor output to af amp in early production run.
cure: install an update to increase output from the monitor circuit: on the main unit, reduce the value of r85 resistor from 10 k to 4.7 k.

-------------------------------------------------------------------------

symptom:
a mechanical whine emanates from inside radio in tx only at certain power levels. transmitter works ok.

probable cause:
loose core in dc-dc converter transformer inside power supply.

cure:
replace t2 transformer on reg. unit inside power supply.

-------------------------------------------------------------------------

symptom:
ci-v interface does not work. unit will not communicate with computer.

probable cause:
bad q5 and q6 transistors on logic unit.

cure:
replace q5 and q6 transistors (both 2sc2458-gr), then reset the cpu by holding down the clear button and powering up the radio.

未完，待续。请勿回贴。