Germanium Transistor Confusion

[drdn0]

Howdy,

Recently came into possession of a Trueman 1966 with, what I was told, was just a busted footswitch. Replaced the footswitch and it now switches, but still does nothing on the fuzz front.

Having a quick squizz over voltages, there's some weirdness going on around the transistors (GT402B). Each transistor has a trimpot to set voltages, but no matter on the adjustment I can never get more than 1v (well, -1v) on any of the legs.

Popped them out and threw them on my trusty DSO-TC3, and both transistors just read as a dual diode package with around a .27 forward voltage. I've never had any issue testing transistors of all kinds and varieties before, but I think it's VERY odd that they would both read almost exactly the same if they were both dead.

Anybody ever seen anything like this, or know if something like leakage/very low hfe could give incorrect test results?

[Rob Strand]

Having a quick squizz over voltages, there's some weirdness going on around the transistors (GT402B). Each transistor has a trimpot to set voltages, but no matter on the adjustment I can never get more than 1v (well, -1v) on any of the legs.

Those voltages might be normal.

The GT402B is pnp.   Don't know if that pedal is negative or positive ground but it does make a difference in interpreting the voltages.   Also there was a thread not too long ago which highlighted how easy it was to get the pinouts wrong on those transistors.

[drdn0]

Having a quick squizz over voltages, there's some weirdness going on around the transistors (GT402B). Each transistor has a trimpot to set voltages, but no matter on the adjustment I can never get more than 1v (well, -1v) on any of the legs.

Those voltages might be normal.

The GT402B is pnp.   Don't know if that pedal is negative or positive ground but it does make a difference in interpreting the voltages.   Also there was a thread not too long ago which highlighted how easy it was to get the pinouts wrong on those transistors.

Yep, PNP and positive ground. The -1v is the most (well, least) I see. It also definitely doesn't fuzz - SUPER faint with everything dimed, but given these are known for a massive volume increase when on something is definitely up.

All caps have tested fine, all resistors are within spec, no dry joints or any other very noticeable issues I can see with the pedal except for the strange transistor voltages. I do have a couple of NPN versions on silicon, and they have somewhere between 3 and 7v for collector voltage so I thought this might be ballpark (but obviously minus)

[Rob Strand]

ep, PNP and positive ground. The -1v is the most (well, least) I see. It also definitely doesn't fuzz - SUPER faint with everything dimed, but given these are known for a massive volume increase when on something is definitely up

I may have discovered the problem.   I found a pic of the PCB.  It's hard to see but I could swear there are three tiny SMD trimpots on the solder side of the PCB - perhaps you already know that.  The middle one appears to be wired in parallel with the bias pot.  If that trimpot is set too low in resistance it would make the bias pot on the front panel seem like it's does nothing - and visa versa.

Maybe you can see things clearer on the real unit.

I could try to trace the circuit but the switch wiring is also hard to see.
The trimpot on the side same side of the PCB as switch is in series with that parallel pot/trimpot combination.  If that trimpot is set too high then it might contribute to limiting the range of adjustment.

So those two trimpots are setting the range of adjustment of the Bias pot on the front panel.

(I suspect the third trimpot sets the bias point of the other transistor.)

[Rob Strand]

I did a bit of a trace.

Here's the main points:

- It's essentially a Fuzz-Face

- The first two trimpots and the bias pot are the collector resistor of Q1

- The third trimpot is the part of the collector resistor for Q2.



Note the switch seems the modify the bias of Q2.