The rca ground shield legs broke. I repaired them with wire. Now left rear channel when set on HP filter is choppy. On full and LP the channel is fine. Right rear channel is fine on every filter. C
Amp in 4 channel mode RCAs plugged into front the left rear channel still get a choppy signal and can be controlled by the front gain. This is only when set in HP. And only Left Rear channel, the right channel is not affected.
Generally, you need a contact cleaner to clean pots and switches. The contaminants typically need some sort of solvent/cleaner/lubricant to be removed.