Bob Benedict, KD8CGH
More info on the mod. I added wires:
from the volume wiper to the rotary center.
from rotary position 1 to Audio connector (original wiper destination)
rotary 2 & 3 to AF filter in
AF filter out to tie point at rotary rotary 1 (back to Audio connector)
I tried replacing the wires with shielded mono audio cable with one end of shields tied together and grounded but it didn't help.
The filter itself is on a perfboad mounted to the right edge of the speaker so it is also very near the Raduino.
Any suggestions on next thing to try?