Debugging

Frequent questions about dead or faulty pedals. Whether you repair, mod or build pedals, debugging is important. Threads about general debugging problems irrespective of the circuit involved...

Debugging

Postby MuzzledAxe » 23 Jun 2012, 11:21

Does anyone have a list for the best way to debug

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Re: Debugging

Postby Nocentelli » 24 Jun 2012, 17:30

There is a good procedure outlined at diystompboxes.com, but it really covers all the information you should gather if you expect people to be able to help you debug over the internet.

I would suggest checking the following if you're attempting to debug yourself:

1) The basics - is your guitar definitely connected to the input? is the volume turned up? is the guitar cable to the input good? is the cable to the amp good? is the amp on and volume turned up? All of these things can be checked in one go if the bypass works (I.e. If you've boxed it up with a footswitch, or you've got the circuitboard in a looper)
2) The circuit - are you 100% convinced the schematic/layout you're using is verified?
3) The component connections and orientation - are you 100% certain you have connected all the parts as they are supposed to be according to the schem/layout? I'm talking electrolytic cap orientation, transistor pinout, pot connections, everything grounded that need to be, every connected to +9v (or whatever) that needs to be, vref arrangements (if applicable) are sound, nothing is touching/shorting that shouldn't be, and all component values are what are specified (I.e. you've not put a 1M resistor where a 1k should go)
4) The essential connections - is power definitely getting to all the +9v (or whatever) rail? Are all ground connections grounded?

If you're certain the above are all present and correct, I'd suggest audio-probing to be the next step: the idea is to follow the signal path and find where it goes wrong. You can do this a couple of ways, but what works for me is a simple flying lead from the output of the effect circuit - On the breadboard, I have a simple plastic looper box with a DPDT bypass toggle, with send and return wires leading to the circuit input and output respectively. To audio-probe a faulty circuit, I connect the send to the circuit input and use the return lead as a flying lead. I start by touching the return lead to the send lead to check that all the points addressed in 1) above are sound; I then move the return lead along the circuit from input to output (with as paper copy of the schem/layout in front of me) until the audio drops out. Where this occurs will hint strongly at what the problem is, probably something from 2) - 4) that has not been noticed.

This method will also work if the circuit is boxed and the bypass is good: just croc-clip or solder a flying lead to the fx return lug of the footswitch and use that: it might be advisable to have a capacitor on the end of the flying lead if you're worried about sending dc to the input of an expensive amp: a quick google of "audio probe" brings a lot of useful links.

If you find where the audio drops out but cannot find the fault at that point, I'd advise measuring voltages at the CBE of transistor and all pins of any IC ready to post at a forum to see if anyone more experienced can spot an obvious (to them) flaw.
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Re: Debugging

Postby MuzzledAxe » 25 Jun 2012, 23:17

Cheers

I just thought it might be a good idea to have a plan when needed.
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