Sony 7035 Volume Control Replacement

I have been asked to look at a crackly volume control on a Sony 7035 tuner amplifier. This is a nice unit from the 80s or so. The volume control is a ganged potentiometer along with balancecontrols. There's no way to get a replacement after all these years, so the only option is to attempt a repair or replace some parts with something else.

Looking at the circuit diagram, the volume pots are centre tap versions as well, which is used for the LOUDNESS function. Not having managed to find any four gang pots, one option was to use a processor and digital pots. Unfortunately the voltage on the pots is +/- 27V which is too much for the digital pots I coukld find. So, using a dual gabg pot for volume I decided to fix the banalnce to mid position and remove the LOUDNESS function.

I made a PCB on the CNC router

then removed the unwanted copper due to the voltages

Once fitted into the Sony, the volume control works with no crackling.

The original pots had an extended stem which the volume knob attaches to, so I need to machine an extender for the shorter pot I fitted. I hope the extender is solid enough once the knob is attached and that it won't wobble about too much.

The balance is mid position, the LOUDNESS and MUTE don't work, and I fitted a linear pot as the log pot I fitted didn't feel very good. I'll see how this solution works, but a second version may be in order.

I'm wondering if I can put a processor on the PCB and use FETs to effectively create a high voltage set of digital pots. One problem with this is that the processor may cause noise due to switching which the radio may pick up, as it is quite sensitive and will be in the same box.