SOLVED Help with Dr Trebor (PCB Guitar Mania)

[neiltheseal]

Hi everyone. I am having trouble with DR Trebor from PCB Guitar mania which is the same as the Pharmacist overdrive from pedal PCB. I know I should have bought this from Pedal PCB but It wasn't available when I ordered.

If anyone can assist it would be greatly appreciated.

I get a signal when engaged and all of the pots and Mach Shau switch seem to work however the sound is quiet and very spluttery. It sounds like a transistor is not biased properly.

With an audio probe i can get a good signal right up to the base of q2 but the collector and emmitter aver very quiet and spluttery as is the signal from there on. If I strum my guitar very hard the signal goes from quiet to extremely loud.

I assume the problem is around q2 but not not sure how to fix it. I have swapped the transistors and the problem is still there

Attached are some pics. I have some resistors in series and caps in parallel to make up necessary values. However these don't seem to be where this issue is (happy to be corrected)

IC1 (LT1054) voltage:
1 - 8.9
2 - 8.8
3 - 0
4 - 0
5 - 0.18
6 - 2.65
7 - 0
8 - 8.95

q1
E - 0.18
B - 0.66
C - 2.89

q2
E - 0
B - 0.47
C - 3

q3
E - 2.32
B - 3
C - 2.43

The PCBGM build docs are below. As far as I can see the schematic is the same as the pedal PCB docs (also below) except the 12k resistor from VC to r6 is connected to r16 (the other VC input) in the pedal PCB version.

https://pcbguitarmania.com/wp-content/uploads/2022/02/Dr-Trebor-Building-Docs-1.pdf

https://docs.pedalpcb.com/project/Pharmacist-PedalPCB.pdf

 

[neiltheseal]

Here is a screenshot of the pharmacist from pedal PCB. In this R19 (12k) connects R5 (22k) and R18 (33k).

The equivalent in the PCBGM has the 12k connected to the 22k and not the 33k. This had me intrigued so i tried jumpering the 12k (R13) to the 33k (R16) and the volume seemed to increase immediately and the sound was less spluttery. I might try soldering these two together tomorrow as it was hard to hold the jumper in place.

Does this sound like a good idea?

 

[neiltheseal]

Here is a video.

As you can see it is very spluttering and the volume depends on how hard I strum.

Does anyone have an idea what the problem might be?

 

[neiltheseal]

If anyone is interested, I managed to solve this issue by putting a 470k resistor between collector and base of q2. Seems to work quite well now.

Not sure if I made a mistake somewhere or if the PCB was damaged, but it's all working now.

I replaced all resistors around Q2 (R13-R16) and that had no effect whatsoever. The 470k between collector and base of q2 seemed to do the trick.