SOLVED: Calamity fuzz - only working with onset on full

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sclemmer

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Calamity fuzz only works with onset control on full. Only substitutes in the build are 2m2 resistors instead of 2m. If I strum very hard I can get sound with the onset control half way up. Could this be a bad pot?
Also, no combination of the switch settings helps the issue.
 
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I’ve got 4.6v on Q1 and .5v on Q2. Is that not where I need to be? I socketed the drain resistor on Q1 but not Q2. The 201’s are legit, bought them from Mammoth probably 10 plus years ago.
 
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Oops, 4got to insert link in previous message. It's fixed now. Your JFET bias is in the ballpark. Is your circuitboard super clean? Flux reside could mess up the IC's operation. Or maybe one of the diodes is backwards. Post some pix of both sides of the board.
 
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Which part of the board could be cleaner? The spots between the switches? I took the pic right after cleaning it and couldn’t get between the switches. For R13, I didn’t have a 270k resistor so I connected a 240k and 30k together. Nothing else different from the build doc.
 
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Can you take pix that shows the bands on those two resistors clearly? From what I can see, looks like one of them is neither 240K or 30K. It's not a great viewing angle, so I could be wrong.

The entire board should be cleaner. That shiny stuff on the board is residual flux. All of it needs to be scrubbed off with IPA and a toothbrush. When flux absorbs moisture, it makes a conductive path. Given that this circuit has 10Meg resistors on it, even a tiny bit of stray current can upset the opamps.
 
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That resistor pair is at least close, I pulled and tested it, see below. I’ll try to get the residual flux between the switches cleaned up tomorrow and re-test.
Thanks for the help so far. 13D41024-8F54-4C7D-A1E4-B8721E2C7F1C.jpeg
 
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You don't need to remove the onset pot to test it. Unplug Q2 and you can test the ONSET pot in-circuit.

Let's check a couple of DC voltages, power on, ONSET to zero:
  • D2 cathode
  • IC1-3 (try both G switch positions)
The other switches don't matter.
 
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The voltages you measured on IC1-3 are good. You should be getting close to 5.1V on D2's cathode. Maybe you're not measuring on D2?

Check the voltage at IC1-1, it should be a little higher than IC1-3. I think IC1-7 should either be close to 0V or 9V, depending on the switch positions. This is a crazy circuit and trying to figure it out is giving me a headache. Maybe someone who has a working unit can measure IC1-1 and -7 for comparison purposes. I had one, but gave it back to the owner after I fixed it.

I should have mentioned before that the voltage on Q2-D depends on the position of the G switch. We care about the higher voltage reading.
 
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