Pepper Spray Troubleshooting

[misterpants]

I am building the 2019 version of the Pepper Spray, and I'm presently getting almost no output from the pedal. All I get are short, quiet staccato bursts of very distorted signal.

There are no part substitutions in the build. I am using socketed 1N914 diodes for D3 and D4. Q1 is a 2n404a from Small Bear, and Q2 is a 2N3565 from Small Bear.

Here is what I have done thus far:

• Verified the orientation of all diodes
• Verified the orientation and values of all polarized caps
• Verified the values of all non-polarized caps
• Verified the values of all resistors
• Used an audio probe to trace the signal
• Measured the DC voltages on Q1 and Q2 with a multimeter

The audio probe found a viable signal at the 100K harmonic pot, but almost nothing at the 50K balance pot.

The voltages are screwy.

Q1-C: 3.0mV
Q1-B: 16.2mV
Q1-E: 7.41v

Q2-C: 9.16v
Q2-B: 7.3v
Q2-E: 7.3v

I've attached a picture of the top of the board. It's hard to tell from the photo, but the flat part of Q2 (2N3565) is pointing toward the trim pot.

Any suggestions for next steps in debugging this are welcome.

[danfrank]

It sounds like your transistor(s) are barely biased (in cutoff) so only highest peaks of signal turn them on. Did you test both transistors for gain before installing them? With germanium circuits this is important because germanium transistors are all over the place spec wise. Your voltages are way off. Also, check C3 and C4 to make sure they are good. It's fairly common for tantalum caps to go out. ( I can't see the pic to see if you used tantalum caps )

[misterpants]

Thanks for your input. C3 and C4 are garden variety electrolytic capacitors (non-tantalum), 47uf and 2u2 respectively. I just verified with my multimeter that both capacitors are functioning correctly.

The voltage values are so far out of line that adjusting the bias via my trim pot has minimal effect.

I've attached an image of my testing rig for reference. Nothing fancy, just an old enclosure with a pair of input jacks and a 9v battery snap.

[misterpants]

I had the potentiometers wired backward. I fixed that, but the issue described above remains.

[misterpants]

I pulled the transistors, and can confirm that the 2N404A is fried. I'm going to try again in a few days with a new transistor.

[danfrank]

If you have a way to test hfe of the 2N404A, try to get one with an hfe between 50-100.