SOLVED Twin Face - Si/NPN side, odd Q2 voltage?

[uranium_jones]

I never thought it would come to this. Posting on the Troubleshooting forum for a Fuzz Face of all things. I guess that's the price you pay when your EE professor never figured out how to explain transistors clearly.

Tried to set trimmers before plugging in this Twin Face for an audio test. Q2 on the PNP side was no sweat. -4.5V Easy PNP-sey.

Flip the switch on the front of the enclosure to enter Si mode. Measure Q2 collector on the NPN side. It reads 9V all day and night, no matter what the trimmer is dialed into.

Check all the BC108 voltages:
Q1 voltages: C: 1.1V; B: 0.6V; E: 0.0V
Q2 voltages: C: 9V; B:1.1V ; E 0.8V
It seems like Q2 is the only one that's very abnormal, based on my googling about Fuzz Faces.

Did a visual inspection, tried to find shorts via continuity beep-testing, and measured all the voltages I could access between Q2C and the +9V input. They all read pretty much the same, +9V. Any ideas what I should look for in the morning?

Photos:


Yeah, I didn't snap any photos of the other side of the board. Everything looked OK, but I will try and take that gut shot if the problem persists.

 

[Coda]

How does it sound?

 

[uranium_jones]

How does it sound?

No idea. I'll have to try that part later.

 

[uranium_jones]

OK @Coda I finally got a chance to plug a telecaster into it. The fuzz sounds pretty quiet compared to the germanium side, and it's very gated. It loses so much attack that it almost sounds like a fade/in reverse effect.

 

[uranium_jones]

Bump, because once this weekend is over I'm hoping to disassemble the enclosure and check the other side of the pcb. It looked OK last time but maybe I missed something.

 

[fig]

I think C1 & C100 are backwards.

Nope, they are correct.

 

[Coda]

I think C1 & C100 are backwards.

Nope, they are correct.

You caught it before I could rub it in your face.

 

[uranium_jones]

So I used my DMM and checked all the resistor values, the trim pot value, and continuity everywhere I could think of. All the sockets have continuity to the other side of the board as well. Here's some photos of the NPN side. I did hit it with contact cleaner right after this too, just because this looks embarrassingly sloppy thanks to the flux.

 

[spi]

did you try swapping the order of the transistors to see if is the device? Or if you have some 2n5088s pop those in and see if it persists with different transistors.

 

[uranium_jones]

did you try swapping the order of the transistors to see if is the device? Or if you have some 2n5088s pop those in and see if it persists with different transistors.

I can try that. I did measure them both with my Chinese measure-o-matic and they looked fine. The hFE were very similar too.

 

[uranium_jones]

Holy crap, I finally figured it out.

Silicon NPN: switch is pointing up.
Germanium PNP: switch is pointing down.



Not sure why the Ge side was giving me a good voltage in the wrong position. Maybe I accidentally put it in the correct position at first.