| Symptom: |
ANY intermittent
problems, including intermittent RX, TX, display, power up, and
noisy RX/TX. |
| Probable
Cause: |
Regulator unit
problems. Heat and age is causing the regulator unit to produce
unstable voltages to the radio. |
| Cure: |
Recondition regulator
unit: Lift reg. unit PCB and rework all overheated and
suspicious-looking solder joints (you may find a lot of bad joint to
solder on an old unit). Repair cracked or damaged foil trace at L7
choke, if needed, and apply RTV compound between L7 choke and
neighboring components to prevent it from coming loose again.
Resolder joints at current-sensing resistor bar R22. Replace these
commonly overheated components: L11 coil, C4, C28 capacitors, and
R35 resistor. (For L11 coil, a type LW-16 choke may be substituted
for the LW-15 for increased reliability.) Examine the top side of
the reg. unit carefully for more overheated components. Replace all
(formerly) blue electrolytic capacitors that have turned brown with
heat and age. Be thorough. Clean board once repairs are completed
and examine your work. Apply fresh thermal compound to
heatsink-mounted components. |
| Remarks: |
Because it runs hot,
the regulator is a common source of trouble in the IC-781,
especially in those units with many operating hours. We strongly
recommend inspecting the regulator unit on all IC-781 that are in
need of repair before any other repairs are commenced. |
|
|
| Symptom: |
Intermittent low or no
RX/TX output. Radio is vibration sensitive. Symptoms may disappear
when radio is turned upside-down or when bottom cover is removed. |
| Probable
Cause: |
Cracked solder joints
at crystal filters in IF unit. |
| Cure: |
Resolder filters. |
|
|
| Symptom: |
No TX output. PA is not
drawing any current in TX mode. |
| Probable
Cause: |
No 32 volts to PA unit
from reg. unit. |
| Cure: |
Resolder cracked solder
joint at L7 choke on reg. unit and secure it with RTV to prevent it
from shaking loose again. |
| Remarks: |
While reg. unit is
lifted, it would be a good idea to service it as outlined above. |
|
|
| Symptom: |
Unit shuts down when TX
power is turned up. |
| Probable
Cause: |
Premature over-current
shut-down. |
| Cure: |
Resolder cracked joints
at R22 current sensing resistor on reg. unit. |
| Remarks: |
While the reg. unit is
lifted, it would be a good idea to service it as outlined above. |
|
|
| Symptom: |
Birdies, whistles,
and/or hash noise in RX, especially on lower bands. |
| Probable
Cause: |
Reg. unit is running
noisy. |
| Cure: |
Resolder all suspicious
looking solder joints on reg. unit. Replace overheated C24 capacitor
and L11 coil in reg. unit. |
| Remarks: |
While the reg. unit is
lifted, it would be a good idea to service it as outlined above. |
|
|
| Symptom: |
R20 resistor emits a
small puff of smoke when radio is first powered-up, especially if
radio has been turned off for a while. Radio seems to work properly. |
| Cure: |
Do nothing. This is a
normal occurrence and does not mean there is a reliability problem
in the radio. |
|
|
| Symptom: |
Radio is dead, no
lights, no display, no sound. |
| Probable
Causes: |
1) Open timer relay on
PI unit. 2) Open R20 resistor (3 ohm, 2 watt) on PI unit. 3) Open
start-up relay on PI unit. 4) Unit plugged into 110v when it is
configured for 220v operation. 5) Bad timer IC on logic A unit. 6)
Cracked solder joints in reg. unit. 7) Shorted rectifier D1 on PI
unit. (This causes R20 resistor and main fuse to blow.) |
| Cure: |
Correct as needed. |
| Remarks: |
If R20 resistor on PI
unit is burned, there may be failure on the reg. unit. Also, mount
replacement R20 away from the PCB to prevent it from ruining the PI
unit should it ever burn again. |
|
|
| Symptom: |
Power output is too
high (around 250 watts). Front panel RF power control and internal
ALC adjustment have little effect. |
| Probable
Cause: |
Bad detector components
on filter unit. |
| Cure: |
Replace D8 & D9
diodes (both 1K60) on filter unit. Check L17 coil. |
| Remarks: |
Check filter unit for
overheated components. |
|
|
| Symptom: |
No TX power output.
Inspection reveals that electrolytic capacitor C11 on PS unit has
exploded. |
| Probable
Cause: |
Biasing problem at
driver transistors- base voltage is higher than .68 volts. |
| Cure: |
Replace all obviously
burned components on PA unit, including driver transistors. Make
sure VCC to PA unit is around 33 volts. If OK, disconnect VCC
drivers and measure base voltage. If over .68 volts, replace D1
& D3 diodes (MV5 & 1S953). Realign driver and PA idle
current. |
|
|
| Symptom: |
No or low TX power
output on one or more bands. PA-IC meter reading is high on faulty
bands. |
| Probable
Cause: |
Burned components on
filter unit, usually toroids or relays. |
| Cure: |
Replace faulty
components and check the automatic tuner for a possible problem.
Verify that the power output from the radio is not more than 160
watts. |
|
|
| Symptom: |
No RX/TX on 20 kHz of
each 500 kHz tuning step, especially when radio is cold or first
powered up. |
| Probable
Cause: |
PLL unlock. |
| Cure: |
Adjust DDS-LPL in both
PLL A and B units for proper lock voltage. |
|
|
| Symptom: |
Constant tone in
speaker when in CW mode. Unit goes into TX automatically in CW mode
when VOX button is pressed. |
| Probable
Cause: |
Shorted CW key line. |
| Cure: |
Replaced shorted RF
filter FI-103 on Connector 2 unit, and/or shorted F11, F12 and F13
filters on phones unit. |
| Remarks: |
Use shielded cable to
CW key to prevent future occurrences of same problem. |
|
|
| Symptom: |
AC hum or buzz in TX
audio. |
| Probable
Cause: |
Poorly shielded or
improperly shielded cable is hooked up to accessory jack in back of
radio. |
| Cure: |
Advise customer to
replace cable and have accessory checked for faulty ground. |
|
|
| Symptom: |
Intermittent CRT
display problems such as loss of vertical hold, raster with picture,
or complete blacking out. |
| Probable
Causes: |
1) Unsteady 13.8 volts
or video signal to CRT unit. 2) Bad solder joints on CRT unit PCB. |
| Cure: |
Verify steady and clean
13.8-volt supply at pin 1 of DC power connector on CRT unit. If this
is OK, resolder cracked solder joints in CRT unit PCB. |
|
|
| Symptom: |
CRT displays garbage or
incorrect characters. Radio seems to work fine, otherwise. |
| Probable
Cause: |
Bad CRTC unit on logic
A unit. |
| Cure: |
Troubleshoot and
replace defective components on CRTC unit. Most common problem is
one of the RAM chips has gone bad: IC3, IC4, IC5 or IC6. |
|
|
| Symptom: |
Computer control does
not operate in "transceive" mode. |
| Probable
Cause: |
Unit is configured for
DEFT (default) in CI-V set mode. |
| Cure: |
In the CI-V set menu,
change setting from DEFT to USER mode. |
|
|
| Symptom: |
CI-V control-unit does
not communicate with computer. Inspection reveals that no TTL signal
is present at the REMOTE jack when the VFO knob is turned. |
| Probable
Cause: |
Bad I/O transistors on
logic unit. |
| Cure: |
Replace bad Q3 and Q4
transistors (both 2SC2458GR) on logic A unit. |
|
|
| Symptom: |
Antenna tuner can't
find resonance on some bands. Both motors turn when tuner hunts. |
| Probable
Causes: |
1) Presets far out of
adjustment. 2) Burned variable capacitors C303 or C304. 3) Burned
relays on AT tuner board. 4) Bad relay RL1 on AT controller board. |
| Cure: |
Connect radio to a
dummy load and try adjusting presets. If a low SWR cannot be reached
by manual tuning, then most likely there are burned relays or
capacitors on the tuner unit. If unit is able to find a tune in the
preset mode but de-tunes in the automatic mode, then either RL1
relay is bad on the controller unit, or there is a problem with the
tuner SWR detection circuitry. |
| Remarks: |
Check for overheated RF
switching components on the Connector 2 unit near the antenna
connector. |
|
|
| Symptom: |
Antenna tuner doesn't
tune on any band. Inspection reveals one of the motors is not
turning. |
| Probable
Causes: |
1) Jammed motor gear
assembly. 2) Tuning capacitor plate has warped from heat and is now
jamming motor. 3) Blown motor driver transistors. |
| Cure: |
Be sure that the motor
is not jammed into one end of its tuning range. The motor can be
unstuck by applying external DC directly to the motor itself. (15
volts is usually enough. Disconnect J1 connector before applying
voltage to prevent damage to the controller unit.) If the motor is
not stuck, then the problem is most likely bad motor transistors.
These overheat when they go bad, so damage to the board will be
obvious. |
| Remarks: |
If the motor drivers
are blown, the IC6 (5-volts) regulator on the tuner unit may also be
bad. |
|
|
| Symptom: |
Clock does not hold
time. |
| Probable
Cause: |
Dead or weak clock
battery. |
| Cure: |
Measure lithium battery
BT1 in circuit on logic B unit (top unit). If it measures less than
2.8 volts, replace it. |
|
|
| Symptom: |
Scope is inoperative,
or radio does something unusual (such as loses RX) when scope is
activated. Problem disappears when scope unit is lifted or bottom
shield is removed. |
| Probable
Cause: |
Excessively long leads
on bottom side of scope unit are shorting to shield. |
| Cure: |
Trim leads so they do
not short against shield. |
| Remarks: |
Pay special attention
to the leads around the outside edge of the board. These are outside
the insulator inside the bottom shield. |
|
|
| Symptom: |
No audio from headphone
jack. |
| Probable
Cause: |
Burned components on
phones unit, most likely caused by RF feedback. |
| Cure: |
Replace burned R1 and
R2 resistors (10 ohms) with 1/2 watt versions. Check L8 and L11
coils. |
|
|
| Symptom: |
Noisy fan; rattles at
some speeds. |
| Probable
Cause: |
Faulty bearings in fan. |
| Cure: |
Replace fan. Fan cannot
be lubricated or serviced. |
|
|
| Symptom: |
Intermittent static
noise in either A or B VFO channel, or no RX at all in either A or B
VFO channel. Check reveal that both A and B PLL's are locked, and
1st L.O. injection signal is present and clean at J4 and J10
connectors on RF unit. |
| Probable
Causes: |
Bad solder at mixer
FETs on either A or B channel. In the case of no RX, mixer FETs Q2
& Q3 may be bad. |
| Cure: |
Resolder cracked joints
in failed mixer unit mounted to RF unit. Replace faulty Q2 & Q3
FETs (both 2SK125), and burned L7 coil. |
| Remarks: |
Board runs hot around
mixer section. Look for poor solder joints on this area on this
FETs. |
