Dead fuzz face build - Mullard OC42 test question

[bruceffect]

All, I have a dual fuzz face with a Silicon build on one side and Germanium on the other that I picked up used. The Si side works perfectly, but the GE side was DOA. Very low voltage on Q2 on the GE side.

I've troubleshot it down to an issue on the board. I have continuity from the pedal input to the board and from the pedal output to the board.

The Q2 is a mullard OC42, it's getting -.40v to it's base from Q1, but I'm only seeing .21v at the collector and the emitter.

I pulled it out and tested with diode mode on my multimeter. From B to C, I get .260. From B to E I get .265.

Based on what I've read on testing PNP transistors, I should get .45v to .9v.

Any thoughts on other things I should test or check, or is this transistor bad?

Summary of tests on the board before I pulled the transistor.

Power to board: -9.47v
Connections from switch and in/out jacks to board: all good
Mullard Red:  -.21v (emitter)
Mullard Green:  -.21v (collector)
Mullard Purple: -.40v (base)
Top hat Red:  0v (emitter)
Top hat Green:  -.40v (collector)
Top hat Purple: -.14v (base)
Trim pot makes almost no change in Q2 voltage (-.21 vs -.22)

[antonis]

Hi & Welcome.. 

260 & 265mV should be Ok for Ge BJTS (0.45 to 0.9 refer on Si ones) but your measurements should be reversed ..
(for p-n-p, junctions forward voltage drop is from Emitter & Collector to Base..)

A rough guide for FF bias voltages..

[bruceffect]

Thanks very much for the guidance! Sounds like I measured/tested properly, but wrote them down wrong in terms of the direction. : )

I've been using that schematic as a reference.

At Q1 on that diagram I have -.4v instead of -.7v. On Q2, I get -.21v instead of -4.5, but I did confirm that the -.4v on the collector of Q1 was aligned with the base on Q2. R3 is the trimpot, and it's in the 8k range.

If the transistor is good, what else would cause the failure of it to amplify the voltage as expected. Could C2 be the culprit?

Appreciate any advice on what to test next. I've got the board unglued and out so I can desolder and remove items to test.

[Electric Warrior]

looks like Q2 C and E could be shorting out.

[bruceffect]

Thanks! In a little while I'm going to solder Q2 back onto the board. I'll check all the traces then to look for a potential short. Maybe I'll get lucky and taking it out and putting it back in solves the problem.

[bruceffect]

Soldered the mullard OC42 back into the board. Still no output. Voltages stayed the same. Can't see any potential shorts.

Any ideas on other components to check?

[antonis]

Check voltage on Q1 Base and Q2 Emitter..

[bruceffect]

Here's what I'm getting now across both transistors. No gain out of Q2. Thoughts?

Q1 Base: -.22v
Q1 Collector: -.63v
Q1 Emit: 0

Q2 Base: -.63v
Q2 Collector: -.32v
Q2 Emitter: -.32v

[antonis]

Plz also check voltage on R2/R3 junction 'cause it seems like R3 is out of the game..
(maybe shunted by R2 or, most possibly, R2 & R3 are interchanged..)

[iainpunk]

what Electric Warrior probably meant was that Q2 IS the short.
germaniums don't like to be soldered to much, long leads can lessen the chance of problems, but if you have soldered them too slowly, they'll die on you. i recommend trying a new germanium transistor, and using a socket instead of soldering them directly on to the board!

cheers, Iain

[bruceffect]

Thanks guys. I have some new transistors on order. I think that Q2 is most likely the culprit. The Ge side of the pedal was working at some point in the past, so it seems like one thing failed, and all signs are currently pointing to Q2, but I will also confirm R2 and R3 are good while I'm waiting on the new transistors. I really appreciate the help!

[iainpunk]

get sockets as well!!!! instead of soldering the transistors, using sockets and inserting the transistor after the soldering process saves it from heat.

for the time being, you could try a Si transistor to test if the rest of the circuit is working correctly. (it might sound a bit different, but its mainly a tool to check if you aren't burning the transistor due to a short somewhere)

cheers, Iain