Icom IC-751 Common Questions page 3

Replace all of these components on the RF unit: D42, D44, D45,
Cure:
D46, D47, Q15. Check C174 and C179 for leakiness. There may be
other failures. Unit is not repaired until listed voltages at D47 are
correct.
Distorted RX on strong signals. BC band RX sensitivity may be low
Symptom:
or marginal.
Probable
Q15 on the RF unit has become leaky.
Cause:
Check and replace Q15. If Q15 is leaky, there may be other problems
Cure:
on the RF unit. See cure for RX problem listed above.
RX disappears when outer conductor of coax is connected to antenna
Symptom:
jack. Inspection reveals 6 volts DC at the center conductor of the
antenna jack.
Probable
Shorted DC-blocking capacitor C40 on the ANT SW unit.
Cause:
Replace the capacitor.
Cure:
There are probably other problems on the RF unit if this capacitor is
Remarks:
bad. Check the voltages at D47 on the RF unit. Radio may have been
struck by lightning or sustained a large RF overload through the
antenna jack.
RX cuts out randomly when turning VFO knob in IC-751A with
Symptom:
UX-14 installed.
Incompatibility between UX-14 and 751A is causing the PLL to
Probable
unlock.
Cause:
Turn off transceive mode on UX-14 (bank S1, switch #3 to OFF
Cure:
position).
Hash noise in RX, birdies.
Symptom:
1) Voltage regulators on PLL unit are oscillating, producing noise on
Probable
VCC line. 2) Overheated components and/or loose screws are
Causes:
causing the power supply to run noisy. 3) Noisy DC-DC converter on
display unit.
Replace overheated C129, C132, and C137 regulator filter capacitors
Cure:
on PLL unit. Inspect power supply for overheated components look
OK, 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.