G3VGR IC735 QRP Mod      
 

The IC735 is normally factory set for a low power output of around 10 watts. It is easy to set this to 4-5 watts by adjusting the ALC potentiometers. This is the procedure I used.

View of ALC potentiometers
  1. Remove IC735 bottom cover.
  2. Connect RF Power Meter and 50 ohm load to antenna socket.
  3. Turn IC735 upside-down with front panel facing you.
  4. Locate R267 and R268 at rear right-hand corner (see photo).
  5. Connect DC power and switch rig on, set to CW mode.
  6. Set PO Front Panel Preset to minimum and key Transceiver.
  7. Adjust R267 for 4-5 watts RF output.
  8. Set PO Front Panel Preset to maximum and key Transceiver.
  9. Adjust R268 for 100 watts RF output (or less if required).
  10. Recheck high and low power readings and readjust if needed.
  11. Switch off IC735, disconnect cables and replace bottom cover.
You now have a fine QRP Contest rig.  If QRPp operation is desired, then an external negative voltage should be applied to the ALC input socket. This can be provided by a battery and potentiometer.

I have also modified my IC735 to provide more microphone gain when using a Heil HC4 insert. The stock Icom rigs have quite low gain microphone preamplifiers, relying on a built-in preamplifier in the Icom microphones. Increasing the value of R235, the collector-base feed back resistor on Q57, from 100K to around 200K will provide sufficient gain for the Heil insert. This resistor is located at the rear of the PL board and rework needs to be done carefully, due to the close component spacing. I removed the resistor completely, using desoldering braid and substituted a 180k resistor. As the R235 is mounted vertically, it would be possible to cut one lead and add another 100k resistor in series instead of desoldering the resistor. Some AF instability was experienced after making this modification and was easily cured by connecting the two grounds (pins 6 & 7) together on the microphone socket.

The Squelch ptentiometer takes up a lot of real estate on the front panel. I never operate FM or have any inclination to do so. I always tinkered with the idea of using this front panel control for something more useful. I was pleasantly surprised to find a mod on G4ICV's website.
G4ICV transposed the Squelch pot & Power Slider pot by simply cutting and joining 2 wires.  This is an extremely worthwhile mod to make.

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