SOLVED Help me understand my audio probing

Grubb

Well-known member
I built a crazy bass pedal for a work colleague's band earlier this year and it has been working for them until recently. It has a fuzz, boost and filter inside it, all living on separate circuit boards. You can see more about it if you are interested on this page: Bass fuzz/filter

It has been returned to me for a repair as it has recently stopped filtering properly. Fuzz, boost and switching all work as expected. The filter circuit passes audio but the filter effect is not present at the output. I used my audio probe to trace the circuit and think I have located possible issues, but I just need some advice on what the problem is.

**EDIT: I altered the PCB file on my computer after I had this PCB fabricated, and subsequently I probed a couple of places incorrectly. The schematic has been updated. After checking, the fault seems to be at IC2 Pin 2 and the associated network. When I touch the probe to that net, all signal disappears, including all hum.

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I could be a bit wrong about my understanding, but my understanding of an op amp is that it works by sending a signal out the output that will make both the inputs the same. That is why you need a feedback loop back to the input. In this case one of the inputs is grounded, so the output will tend to make the other input ground potential. You may not hear anything at this pin at all if it is working properly, but have signal magically reappear at the output.
 
That is absolutely correct. The red network should have no signal.

The fact that you have signal at the input and output of the board says that the board is ok. That leaves the outboard wiring, jacks and stomp switch as suspects.
 
Ok thanks @Chuck D. Bones and @PJS, this is helpful. I can hear filtered signal at various pins along the LM13700N but unfiltered signal at the output, so now I am suspicious of the off-board Mix pot and associated wiring.

Also, when I touch the pedal during testing it kills hum, should I be concerned about grounding issues or is this pretty normal?
 
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