Fender M-80 Chorus Teapot

[The Dude]

I'm stumped on a Fender M-80 Chorus. There's an audible oscillation at power up. The frequency slowly rises like a teapot. It eventually stops, but is quite annoying until then. I believe it's coming from the chorus circuit. I can plug a dummy jack into the Stereo Return and short it to mute the noise, so power amps are not the problem. It seems like a cap charging, but I've checked and/or changed all the electolytics.

M-80 Chorus.pdf

[g1]

Have you tried shorting the Q1 fet in chorus circuit to verify whether the chorus circuit is the source?

[The Dude]

I haven't, but I'll try it tomorrow. I'm just about out of here for the night.

[glebert]

Have any of the pots been changed out? I worked on one of these a while back and just used regular PCB pots, then found out the triangular metal brackets on the pots were necessary for grounding, but almost like they were grounding something else in the circuit other than the pot. I ended up just making a jumper wire to make the connections. Can't remember what the exact behavior was with mine but it was weird and unpleasant.

[dmeek]

Have any of the pots been changed out? I worked on one of these a while back and just used regular PCB pots, then found out the triangular metal brackets on the pots were necessary for grounding, but almost like they were grounding something else in the circuit other than the pot. I ended up just making a jumper wire to make the connections. Can't remember what the exact behavior was with mine but it was weird and unpleasant.

Many Fender models of that vintage ran a ground through all of the pot brackets in series, for what reason I don't know. One cracked solder connection on a pot bracket and the ground goes open.

[The Dude]

Pots are all original. I checked the grounds through the pot mounts from one end to the other- all good. Shorting the FET does not stop the squeel.

[DrGonz78]

I would keep grounding out the signal path working your way towards the input. Since grounding out the return jack muted the noise you might find a point in the signal path where it doesn't mute the noise. Perhaps then it might narrow down to a point in the circuit where to look.

[52 Bill]

Be sure that the output speaker wires are dressed away from the circuit board. I think that one has stamped loops in the rear panel to tie the wires to.

There are also a bunch of small green mylar caps that reduce the high frequency response of the different op amps in there. They will often break one or both leads and float around inside the chassis.

Did you replace all of the power supply filters?

[glebert]

Does fiddling with any knobs or switching channels change the noise (better or worse)?

[The Dude]

Ok. I think I've got this damn thing. When it came in, C73, C74, C62, & C63 were leaking electrolyte all over the board. I removed the caps, cleaned the board with alcohol, and installed new caps. The problem persisted, so I started looking elsewhere. I couldn't find any additional defective components. Over the weekend, I was stewing on this dog on and off. I hate being stumped by any box of electronics. At some point it, becomes more of a personal challenge than a repair. C73 & C74 were leaking far more than C62 &C63 and coincidentally the left side was oscillating more than the right, so I figured it had to have something to do with the bad caps. I wondered if the electrolyte had been absorbed by the PC board, so today I cleaned it with acetone even more thoroughly. I then heated up both areas with a heat gun. Whatdya know? Oscillation gone. Even though there wasn't any visible electrolyte, I believe there was enough absorbed by the board that the board itself had become conductive. Thanks for the ideas and help everyone! I'm gonna cycle the thing on and off periodically today, but I'm pretty sure it's good to go. I see no more oscillation on the scope.

[glebert]

This reminds me of when we used to get dendritic growth on RF circuits when flux was not properly cleaned of the substrates. Here is an interesting article, basically it is unintended electroplating. http://www.ipc.org/4.0_Knowledge/4.5...n-Foresite.pdf

[The Dude]

Interesting read and thanks for posting it. I've done repairs with leaky caps more times than I can remember and I've never had this happen before. Usually a thorough cleaning of the top of the PCB gets the job done. This particular circuit board material does seem to be more porous, which is another thing that got me wondering. I first tried alcohol- not much change. Then tried acetone- oscillation was cut in half, so I thought I must be on the right track. I then took a heat gun to the board- slowly and carefully so as not to damage anything. When I fired up the amp again, oscillation was completely gone. Fingers crossed it's fixed and doesn't start oscillating again after sitting a while.