Icom IC-751 Common Questions page 2

Probable
PBT oscillator is not running.
Cause:
Verify absence of 9.4665 signal at W86 on main unit in 751A, or
Cure:
W134 in the IC-751. Replace faulty X7 on main unit in IC-751A, or
X5 in the IC-751.
Frequency instability in USB mode.
Symptom:
Probable
Bad trimmer capacitor in BFO section on main unit.
Causes:
Replace plastic trimmer capacitor with a 30pf ceramic version and
Cure:
realign.
Drastic change in audio frequency response between USB and LSB.,
Symptom:
i.e. USB audio has too much bass while LSB audio has too much
treble. TX frequency response is also poor.
Misadjustment of the BFO or PBT oscillator, or a faulty crystal filter
Probable
FL-30 (or FL-80 in 751A) on the main unit.
Cause:
Check adjustment of the BFO and PBT oscillator. If these are OK,
Cure:
then the problem is most likely in the filter.
If the problem is intermittent, and the radio is an IC-751 (not "A"
Remarks:
version), then the ground return path for the -5 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 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 transformer shield of T1 instead.
AC hum or buzz in transmit and receive audio when an internal
Symptom:
power supply is installed.
AC is being induced into the PLL from the AC wires and power
Probable
supply case.
Cause:
Twist the AC wires that run from the rear panel to the power switch
Cure:
and route them away from the PLL unit. Tape them up against the
chassis if necessary.
In some radios, especially older IC-751s, it is not possible to
Remarks:
completely eliminate all AC hum when using the internal power
supply.
No, or very low RX sensitivity.
Symptom:
Bad components on RF unit, probable result of RF overload. To
Probable
verify this, check DC voltages at D47 on RF unit. Correct voltages
Cause:
are: Cathode side- RX: 8.5, TX: 13.8. Anode side- RX: 9.0, TX: 9.5.