Debugging original Sans AMp GT2

[Rastus]

hi guys, I have an orginal Sans Amp GT2 that produces an almighty squeel and squal when plugged in.  How should I go about debugging this?  I have read posts on debugging but dont know an order to work through or even how to do it.  Im basically a beginner at this, I have built three pedals from schems in here ( ive been a memebr for a couple of years) but ive never had to debug as everything worked as it should have on the first try.  Im good with a solderiong iron and good at following instructions but my knowledge level is pretty poor.  I have a suspicion there may be a faulty pot, just becasue it has been a bit noisy in the past , but I dont even know how to use my multimeter to check this!  ARrghh.  Any help and guidance is enormously appreciated, cheers, rastus

[therecordingart]

This is a DIY unit or a Tech21 built unit?

I've built a few GT2s and they oscillated like crazy when connected to the bypass switch but were fine when disconnected from the bypass switch.

[Rastus]

hi , its a Tech 21 built unit and has always worked properly in the past.  Im guessing the debugging needs to be different than a diy one in that we know it was all good once.   My guess is a component or two has failed but how do I go about determining this?  cheers

[therecordingart]

I'm not a debugging expert, but my mind links oscillations with components in the feedback circuit. You could build yourself an audio probe and walk from the input of the circuit to the output until you find the faulty stage. Looking at the schematic for this on the Tonepad website (not sure how close it is to the real thing) I keep looking at the 220pf caps used in the feedback circuit of the opamps. Mess with those and you got yourself some instability.