fender Prosonic 22 hum problem

[Gerhard Grass]

Hi to all,

currently I have a Fender Prosonic 22 on the bench. It produces hum (with a little bit of buzz).
It is a constant noise, totally independent if something is plugged in or not,
Vol and Gain turned to zero or not. Turning up the volume adds a little bit of hum as one would expect.
If you flip the standby-switch the hum disappears immediately.
The capacitors are at a first glance not damaged. Output power seems o.k.

I would appreciate any help or advice which may safe me from totally disassembing the amp ;-)

Thanks for any help in advance and
greetings from Austria!


Gerhard

[Jazz P Bass]

I would suggest that you measure the Vac ripple on all of the power supply nodes.

Start there.

[Chuck H]

I don't have a schem for a Prosonic 22. In fact I've never heard of one. If it's arranged like the standard Prosonic it has five or six preamp tubes depending onwhether the amp is equipped with reverb.

First determine if there is any interaction between the noise and any of the other control panel functions. ie: Does the treble control change the noise? Report back about this.

If none of the control panel functions change the noise then start removing preamp tubes one at a time. You can skip the first two since if the noise were there the tone controls should have affected it. So pull the third preamp tube. If the noise is still there, pull the fourth, etc. If you pull a preamp tube and it stops the noise try a new tube there. If the noise persists report on which tube stopped the noise when pulled.

It sounds like you may have noisy tube, a bad ground or a dirty contact somewhere. These tests help narrow down where to look.

[Justin Thomas]

Supersonic 22?

Justin

[Gerhard Grass]

Sry, my fault ! Of course it is a Supersonic !

There is definitly no interaction between the noise and the control functions.

Did as you told me and pulled the tubes.
When I pulled the reverb follower the hum or buzz was reduced to an acceptable level.
When I pulled the PI, the amp wass completly silent.

Changed the reverb follower as well as the reverb driver, wich didn`t change a thing.

What I found is the fact that knocking on the reverb follwer tube or the socket makes a microphonic noise.
As well independent from the tube which is in the socket.

thx Gerhard

[Drewline]

Supersonic 22

supersonic reverb fix.pdf
Super-Sonic 22 Svc Man Rev A 2010.pdf

[Gerhard Grass]

thx for the manual and the fix.

Unfortunately the amp is not in the specified range of serial numbers.
And the values for R75 and R76 are already changed to 100k and 1M5.

Next thing I tried in my despair was to tie down the gate of V4 (7) to ground with a 680nF capacitor, which didn't make much difference.
Tieing down the anode of V4(6) eliminated the buzz immediately.

Any suggestions what I can do next ?

thx for help

[tubeswell]

How did you get on with this?