Electro Harmonix Muff Fuzz

"Positive ground" with NPN silicon transistors - an unusual combination. Note 0V/"Ground" rail on top, and -9V rail on bottom.

Troubleshooting Case Study - Discrete Transistors

The Muff Fuzz exists in both transistor and op amp variations. Pictured above is a transistor version.

The ultra observant reader will notice that there is no battery snap in the photo. I was flipping it around when I took the photo. Someone had installed it backwards, which is understandable. These are confusing because the transistors are silicon NPN types, but the battery is connected "positive ground" style. So don't try to daisy chain this into your pedal board!

This is has the 2 Transistor English Fuzz topology. There was no output after I got the battery snap straightened out. So I took dc measurements. No need for the actual schematic, just use the generic 2TEF circuit, Q1 is first and Q2 is second:

Q1

E: 0.00V

B: 1.15V

C: 2.79V

Q2

E: 2.15V

B: 2.79V

C: 2.30V

Can you figure out what's wrong?

Q2 appears to be saturated, with a forward bias of about 600mV across the base-emitter diode. Q2 didn't really get my attention.

What about Q1? Q1 caught my attention immediately. Check out the drop across the base-emitter diode. 1.15V? If this was a darlington transistor, sure, but it's not. What does this indicate? We've got an open base-emitter diode. Sure enough, that was the problem.

Q1's base voltage should be 600mV +/-100mV or so. Q2's collector voltage hit roughly 4.5V, or 1/2 the supply, after the new Q1 was installed.

I still didn't have output. I could see a great big fuzz signal all the way until the output capacitor. A poke later and I saw the leg had broken off the cap. After that replacement, the Muff Fuzz was rocking again.