The power control circuit in IC-x75 is an ALC circuit. ALC circuits suffers from several drawbacks:
- slow reaction time due to time constants
- insufficient regulation due to low feed-back factor
These factors gives the problem that if the average power has been set to a low value (e.g. for driving a HPA), peak power can still be in the order of 3-4 dB higher. This leads to either considerable side band distortion or a low average power output.
note: If you are always using your transceiver barefoot, without connecting any PA or transverter, this modification will offer no benefit.
The solution to these problems is to use a simple forward gain regulation without the feed back circuit of the ALC. The original ALC is still kept for SWR protection only.
The ALC is working on gate 1 of Q13, which is a IF amplifier (9 or 10.75 MHz). The new power regulation uses gate 2 of Q13. Therefore the two circuits can exist in parallel.
The modification should also give you a bigger dynamic range in the power regulation (lowest power < 5W on the IC-x75H).
- replace R87 (470k) with 100k
- cut "POCO" wire (pin 3, P1/J1, MAIN, orange) at the plug (P1)
- connect the "POCO" wire to gate 2 of Q13 (e.g. via top of R87)
- adjust R250 (SWR protection), R256 (low power level) & R259 (high power level) fully counter-clockwise. (this will give you full SWR protection, but cancels the ALC in normal operation).
- put the front plate RF PWR in full output position (clockwise) and adjust R82 (Q13 output attenuator) until you have the nominal output power (initially the output power will be higher because you removed the regulation, but the PA will be saturated).
- the balance between FM, CW & SSB output can be adjusted with R137 (CW carrier level) & R105 (FM carrier level).
- other interesting adjustments: R135 (mixer balance i.e. SSB carrier suppression), R133 (TX BFO output level).