Fender Princeton Chorus- switch pop?

[drewl]

Repaired a Fender Princeton Chorus recently, owner was complaining about some noise when you turn on the chorus.

There is a static-y type crackle or pop which sounds like typical contact de-bounce, the channel switch does it also.

Think it's normal, or is the guy being picky? Sounds normal to me but I was wondering if there was a way to knock it down.

The chorus didn't work before, C35 had blown and the bias was out of adjustment.
So it didn't make the noise before because the chorus didn't work before.

[nickb]

The channel and chorus switches control several analog switches via the common Fender bi-polar switching signal method.

The switch contacts are probably dirty so give them a good clean. If that doesn't do it replacement is the right thing to do. However, you might be able to fudge around it by adding a 100nF cap across each of R123 and R117.

[drewl]

Yeah, I was thinking of something along those lines.
I had cleaned the switches already.

Watched a few YouTube videos of this amp and they all seem to make the same noise to varying degrees.

https://www.youtube.com/watch?v=1ZFOG8yTTKY


https://www.youtube.com/watch?v=kDHi5KBFSdo

https://www.youtube.com/watch?v=WjPHxUtF9go

[drewl]

Yeah, the easy fix caps across the resistors made no difference.
I suppose I could try swapping out the 4053 chip, but I have a feeling it won't make a difference.

[nickb]

The 4053's would not be high on my suspect list. Does it behave the same with the footswitch? If you scope the switch signals to the 4053's do they seem to bounce at all?

[drewl]

It's kind of hard to see the little noise spike, you have to crank the intensity up on the scope.
You can see it coming through U8, I assume it's being generated back at U11, the switching opamp.

[nickb]

With those caps in place the time constant should be long enough to wipe out any extended dirty contact bounce so I'm of the opinion that this is a characteristic of the design. I suppose that is supported by the anecdotal Youtube vids.

You'll have a tough time tracking it down without a digital scope but that is what I would use to see where it's coming from. I'm thinking the normal charge injection from the 4053 might be the culprit so I'd look at the output of U8 pin 1 with no signal and look for a glitch.

I doubt it's worth the effort unless you are convinced it really is a fault.

[drewl]

Not worth the effort, just a picky customer.

He also complained about the reverb tank picking up vibrations!

Of course, it's bolted to the cab with no bag because it's a cheap ass amp!