Vincent SA-31 Tone Control Fail

[VTDave]

I'm hoping for some advice on where to start trouble shooting this Vincent.

It performs perfectly well when the tone controls are disabled.

When engaging the tone circuit the whole sound field changes even with controls at null. By running only a set of headphones I have determined -
1) Left channel runs perfect. When going from no-control to control the sound stays flat. Bass and treble controls respond.
2) Right channel has an issue. When engaging the tone control, the bass response drops significantly even when setting is at null. Using the bass control, the sound does change (more and less bass) but never reaches the level of when tone is disengaged.

Has anyone seen this? Or can point me down a path?

Thanks, Dave

 

[mhardy6647]

Dirty (oxidized) contacts on the tone control "on/off" switch?
That's where I'd start based on your description.

EDIT: Given that you're a tech guy, I should probably assume you've already tried fiddling with the tone in/out switch and or the tone controls themselves as a diagnostic for dirty contacts(?).

PS greetings from across the river in NH (not all that close to Essex Junction, but close to White River Junction!).

 

[VTDave]

Oh yes the fiddling has been done. They made it rather involved to get at the switch so I thought I'd poke the forum first. Time to dig in.

 

[mhardy6647]

"No user-serviceable parts inside. Refer servicing to qualified service personnel."

auf Deutsch of course