| 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. |
