Peavey Pacer - what might the purpose of this mod be

[aon]

Hi,

I recently acquired a 1978 Peavey Pacer that had a fault where it oscillated wildly and squealed like a pig. I managed to fix it - either cleaning and tightening the pots and jacks helped, or then removing this mod hanging off the collector of the intermediate gain amp did the trick (see top of schematic)



I'm not that great with analog stuff, so I wonder what the purpose of this might've been? First it looked like a filter to me, but then i started thinking that is it actually some kind of additional input or preamp output? And in either case, if there is no ground via whatever the jack is supposed to connect to, it's just floating in free air? It wasn't grounded at the jack either.

[g1]

Maybe an input for something with a hotter signal like a keyboard. If the jack was not insulated from the chassis it would be grounded that way.

[aon]

Maybe an input for something with a hotter signal like a keyboard. If the jack was not insulated from the chassis it would be grounded that way.

It was a plastic jack (sort of like on boss pedals) with a separate grounding connection. The ring of the jack wasn't bridged to ground. Maybe this was an omission on the part of whoever made the mod - there also was no star washer for either the pot or the jack.

Guess it might eventually be a good place to put an FX loop, since there are two holes already drilled...

[g1]

Is the jack all plastic, or does it have a metal bushing? The ones I am familiar with from Boss pedals have a metal bushing. If it has a metal bushing, and the bushing is in contact with the chassis (no insulating washers), then it is connected to circuit ground via the chassis.

The schematic shows an existing FX loop, is yours different from the schematic?

[aon]

Is the jack all plastic, or does it have a metal bushing? The ones I am familiar with from Boss pedals have a metal bushing. If it has a metal bushing, and the bushing is in contact with the chassis (no insulating washers), then it is connected to circuit ground via the chassis.

The schematic shows an existing FX loop, is yours different from the schematic?

Ah, maybe I was a bit unclear - the jack has a metal bushing but the connection for the "ring" isn't connected to the jack bushing. So yeah - inserting a metal plug would ground the circuit, but without a plug inserted it's floating.

I can't see an FX loop in that schematic - neither is there one on the amp itself.

[g1]

I guess you mean a shorting contact which connects tip to ground when nothing is plugged in, like the regular input jacks? You can't have that here as it would kill the signal through the amp when that extra jack is not in use.

Sorry about the FX loop comment, I mixed this circuit up with something else I was looking at.
In this case, it looks like the extra jack could function either as Send or Return.