Icom IC-740 Common Questions page 3

We do not recommend adjusting the PLL Lock trimmers C4, C15,
Remarks:
C24, and C33. They do not contribute to this particular problem and
disturbing them may cause them to fail.
Frequency instability.
Symptom:
Probable
Dirty RIT/XIT pot and switches
Cause:
Clean dirty RIT/XIT pots and switches with Blue Showers solvent.
Cure:
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.
Probable
Misadjustment of the BFO, PBT oscillator.
Cause:
Align the BFO and PBT oscillator as per page 9-4 of the service
Cure:
manual.
Intermittent low FM TX output power.
Symptom:
Probable
Poor contacts in relay RL1 on FM unit.
Cause:
Replace bad RL1 (221-D009-M).
Cure:
Distorted/raspy-sounding SSB TX. Complaints of RFI. Unit tests fine
Symptom:
on dummy load. Unit is being used with an external Astron power
supply.
1) In adequate station ground. 2) External Astron power supply needs
Probable
additional RF decoupling.
Causes:
Recheck station ground. Improve if necessary. Install additional RF
Cure:
decoupling capacitors on Astron power supply.
TX oscillations or low, spurious output, especially on lower bands at
Symptom:
lower power levels. SWR reading may be high when transmitting
into a dummy load.
1) Bad Q10 on RF unit. 2) Bad electrolytic capacitors on PA unit. 3)
Probable
Band switch has been mis-aligned. 4) Burned components on the
Causes:
ANT SW unit. 5) Bad drivers or PA transistors.
Disconnect J9 on the RF unit and see if the problem goes away. If it
Cure:
does, then Q10 is most likely bad (replace it with a 2SC2878B). If
disconnecting J9 has no effect, examine the PA unit. Replace C18
and/or C24 on PA unit if they appear swollen or overheated. Check
the ANT SW unit for burned components and replace as needed.
Check alignment of the bandswitch wafers and correct if necessary. If
none of the above are causing the problem, then most likely the
drivers or PA transistors are bad.
Erratic display i.e., frequency listed will be out of the normal
Symptom: