Princeton vibrato problem

[MC808]

Hey everyone! First post… I have a Princeton clone that’s making a really intense noise, like a drill, when I try to plug the footswitch or shorted RCA cables in to the RCA connector. Here’s a link to a video. Grateful for any insights you might have to what the problem could be.

https://youtu.be/fiFOXx6AZEE

[Helmholtz]

Schematic? (As usual).

[MC808]

Sorry! Here’s a link. I should add the vibrato works fine, it’s only when I plug in the sorted RCA cable or a fender rca footswitch that I get this crazy sound. So something to do with the short is creating the noise.

https://www.tubeampdoctor.com/media/...f-14-rev_3.pdf

[g1]

It seems likely to be a build issue, so photo's of the guts will also be very helpful.

[stokes]

Did it work correctly at any time? Since it seems to work properly when the trem is on, I would suspect the connections associated with that RCA jack, or possibly the shortng jack you made doesnt have a solid short. Assuming the one you have plugged into the rev is working correctly try using that one to short the trem.

[MC808]

stokes Tried both those suggestions and yes the cable short works fine to turn off the reverb. Same using a footswitch and btw when i plug an ordinary RCA cable in there’s no noise though obviously the vibrato stays on. So it’s something to do with the short bleeding into the vibrato circuit?

[MC808]

@g1 attaching a photo, the arrow shows the RCA vibrato input.

[MC808]

It seems likely to be a build issue, so photo's of the guts will also be very helpful.

image above

[Chuck H]

That image doesn't help much because we can't see the jack wiring. A different angle that shows how the jacks are connected is what we need.

[Pedro Vecino]

I don't know if I have interpreted the image correctly, but the wire to the RCA connector seems to come out of a pin in the socket. According to the schematic it must come from the union of the two 10n capacitors in the vibrato circuit.

[MC808]

Pedro Vecino Actually I think you’re onto something there… I’ll take some better photos tomorrow and see if that’s the case. Thank you!

[g1]

Looks to me like the jacks could be in the wrong spots. But can't think of a scenario where shorting one out would make that noise. Maybe the one from the reverb drive?

[TomCarlos]

The image from the Tube Amp Doctor shows where the Blue wire should be connected. It looks like you have a yellow wire from a tube going to the Vibrato jack?

[MC808]

TomCarlos agree it does look like that but I think the yellow cable actually comes from the reverb rca connection junction that is slightly offset and the dark cable lower down is actually the trem (it’s marked blue in the schematic) It’s late here I’ll report back tomorrow when I open the amp up again. Thanks everyone for the help!

[Delta362]

Maybe I'm really overlooking something, but the vibrato on the Fender amps that I've worked on require the shorting jack to turn on the vibrato effect. In this case, it looks like there is vibrato with the plug removed, and the high speed drill sound when the shorting plug is installed.

I'd double check the wiring for V4 as well as verify the shorting of the jack is occurring in the correct part of the circuit. Maybe pull V4 tube and check voltages on the tube sockets with and without the shorting jack installed.