SOLVED Fizzy Fender Blender

[ch ra]

I'm not normally a bad builder but I went through a phase where I was doing my work late at night and probably exhausted from work. So I've had a pile of PCBs that don't quite work right. Current problem is the Fender Blender PCB. Info here: https://docs.pedalpcb.com/project/Blender.pdf

I'll add a picture of it, but first here is the problem in a nutshell. I get sound and power but, the sound is very fizzy. Sustain and Tone and level pots have minimal to no effect. Blending to clean works, though the clean sound is a bit bassy and not pretty.

I traced the signal path from input and it looks like I get strong signal up to D1 and D2. At these diodes, I have signal at the cathodes but not at the anodes. I checked the soldering at D1 and D2 and that's fine, as is continuity to the base of Q4. Since I am getting input to the diodes, but no output to Q4, I am thinking that the diodes themselves must be the problem.

The diodes are not N34A as suggested because I do not have those diodes on hand--at least genuine ones, I have silicon N34As from Tayda lying around. However, I went with these Russian Diodes that I bought on EBay. I checked the specific ones that I put on the board on my component tester and they checked in at about .3-.4 V. So the Diodes work. What am I missing here? Is there another Diode I should try?

Thanks for your help!

 

[ch ra]

addition: If the diodes are the problem, is there a silicon diode that should work?

 

[Robert]

Make sure R16 is 820 ohm and not 820K.

 

[ch ra]

Just checked and it is 820 ohm, on the DMM it measures 816R

 

[ch ra]

I noticed that in the schematic the diodes are reversed relative to the usual direction of current flow--reverse biased right? Could it be that these diodes have a very different reverse voltage drop relative to typical germaniums?

 

[Robert]

Just a random thought, you are aware that the stripe on some Russian germanium diodes is marked on the anode rather than the cathode?

 

[ch ra]

@PedalPCB you nailed it. I had no idea that the evil empire was capable of such a thing. Damn that Putin, or Stalin, or whoever...I removed and resoldered all the diodes in the correct orientation.

It turned out that was not the culprit though, but I think I got it. Somehow the trace between the Sustain pot and C4 input was cut (or somehow not connected). The capacitor wasn't the issue, nor the sustain pot. So I ended up jumpering a capacitor to Sustain 2 with the other end going to the C4 output. Now it sounds ferocious!

At least this one doesn't seem to be a stupid mistake. One more troubleshooting project finished.