| 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. |
