Squealing RAT

[Guybrush]

Hi all

I recently dug out an old Rat pedal I built some time ago. When switched on everything works great but when bypassed it's outputting a very audible whine. The whine changes pitch when I turn the tone knob and gets louder when (you guessed it) I turn the volume knob. This short video should hopefully demonstrate things.

As it's only affecting the bypass signal I'm thinking that the 3pdt may have gone bad so will try replacing it. Just wondering if anyone has any other suggestions before I start taking it apart.

Thanks as always!

[jimilee]

Audio probe it first.


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[Guybrush]

Audio probe it first.

Probe the circuit? As the problem disappears when the circuit is engaged surely the issue has to be with the switch or off board wiring?



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[WormBoy]

Is this circuit true-bypass switching? If it is, I would check the wiring first. Posting some gut shots might help someone spot the problem.

[jimilee]

Audio probe it first.

Probe the circuit? As the problem disappears when the circuit is engaged surely the issue has to be with the switch or off board wiring?



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I guess I should had been more specific. Listen to the audio across the switch, to see where the noise is introduced.  I would hate to see you unsolder a switch and resolver one and have the same issue.


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[mauman]

It sounds like the op amp is oscillating while the input is not terminated.   The oscillation is loud enough that it's bleeding into the bypassed signal path, maybe by radiation or maybe you have a bridge from ckt output to the bypassed signal path. (try unplugging the input jack and it might oscillate when it's active...)   IF it's true bypass, try grounding the circuit input lead during bypass, there's a way to wire the 3PDT to do to do that.  Also check the the in/out bypass wiring routes and move them away from the board if possible.  If not possible, shielded leads from in/out jacks to bypass switch might help.

[gtr2]

Make sure the 30pF compensation cap is connected to pins 1 and 8 of the LM308.

[mwelch]

I built a Rat pedal and installed a 12 position switch to switch between different clipping options.  One of the positions makes a squeal exactly like yours.