5150 scratchy rhythm channel pre gain pot.. when first turned on.

[Indyryder02]

Here’s another weird one.
Peavey 5150 amp.
had a scratchy pre gain pot on rhythm channel. Wouldn’t clean up with exercise, so a shot of deoxit took care of it.
The strange part. When the amp is first cycled on. It has a scratch to it that sounds like dc on the pot.
but as soon as the pot is turned back and forth it’s gone. Nice and clean again. You can shut then amp down, and start it again, and it does the scratch every time. But it goes away once turned back and forth. Normally I wouldn’t even care, seems insignificant . But this is a super particular client I know will complain.
Just curious what phenomenon could be causing this. Static or capacitor charging and passing dc? New one for me so hoping someone else has ran into this and can give me a clue before I go deep. Thanks as always hive!

[The Dude]

Do you measure any DC on the pot before rotating it? Also, there are different versions of this amp. It would help if you linked or uploaded a schematic that matches your amp..

[Indyryder02]

I haven’t gone any further, sorry. I was hoping for more of an explanation on what could even cause such a concern. Seems strange. I’ll have to take it apart again to measure for dc to see if that’s actually what’s happening, as well as verify version and post schematic. Thanks.

[The Dude]

There could be some DC on the pot at initial power up. Your meter will tell the story. If that's the case, a schematic might help provide remedies.

[Jon Snell]

It is probably just worn out as it is unlikely to have any DC on it. The controls are isolated through coupling capacitors of a low value that will absorb any DC within a millisecond or so. Depending upon the model, coupling capacitors as shown.
I would replace the pot and check the coupling capacitor for peace of mind and reliability. They are not expensive.

[Indyryder02]

This schematic matches mine the best of what I’ve found.
5 preamp tubes not the 6 I found in the other model.
rest seems to match as well.

I have a Peavey 6505 here as well that I tried the same experiment on.. and it also does it. That one does it on the post gain knob.. I’ve noticed I can get different pots to do it, only first turn, when amp is first switched on. No dc on the pot that I can detect, but I think whatever is there is going null once I hit it with the probe.. cause it doesn’t make the noise after probing it..

Seems like a very minor concern.. but this is a very particular client.. (which I think is just switching amps on and off and playing with controls trying to find issues smh!

so I’ve moved on from that at this point.. I was hoping to be done with it.. but it has this little pop every now and then that so far I’ve traced to before the phase inverter.

Additionally, only on the rhythm channel.. it’s microphonic enough that tapping the mid and low pots with my ring, or scratching the pre gain knob with my finger nail or pressing the bright crunch switches etc are all highly sensitive…after checking the preamp tubes and chop sticking around the board, it seems most sensitive around K2 on the board especially tapping K2. I wouldn’t imagine the component itself to be causing this, I’ve seen it with old disc caps etc.

anyone else run into this? I’ve fixed dozens of these amps but never ran into any of these issues.


Thanks