Noisy Big Muff V2-4

Here's a case study in a noisy transistor and a method of troubleshooting it.

Symptom:

This vintage Big Muff had a noise problem. There was a really bad rumbling/popping sound that was present even with the guitar volume at zero (input shorted to ground).

Some noise is to be expected from any high gain distortion device, but high gain should be a somewhat evenly distributed "white" noise, not rumbling, and not popping.

Method:

1) Divide and conquer

First I checked the controls. VOLUME and TONE controls affected the rumble/pop sound, but SUSTAIN did not. So I can assume the noise generator is after SUSTAIN and before TONE. That leaves us with just Q3 and Q2 and the associated passive components.

2) Force transistor cutoff

How do we check the Q3 and Q2 stages without going crazy removing components and swapping? I chose to cutoff Q3 by alligator clipping the base to the emitter. This effectively silences Q3. The noise went away. Since I'm still listening to Q2, which is running normally still, I can eliminate Q2 as the culprit.

Cure:

New Q3 transistor

Popping noises are usually a bad silicon element, but a rotten resistor could make some noise too. I went ahead at this point and assumed Q3 was the noise generator, swapped it, and sure enough the noise went away.