| 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. |
