New and exciting PT80 problem

[Samuel]

Just built my second PT-80 delay, for a friend of mine. It fired up on the first try, everything *seemed* to be working...but when I turn up the repeats past the 12 O'Clock position I get horrible oscillating feedback. I've compared voltages against my working pedal: Same voltages (within reasonable limits) on the PT2399, the SA571, the 78L05, the 78L12, the TL072, and the 2n5088. I didn't bother with the 2n5485 cause that is just part of the Mill Bypass (right?).

I have swapped out every pot except for Delay Time, swapped out the switch, swapped out all three ICs. Same results every time. I double checked the resistor and capacitor values, and doubled checked the electro cap polarity. I've also double checked the pot and offboard component wiring. I'd go through it with an audio probe, but I have no clue what to look for.

Thought I was getting better at the debugging thing, but, uh, HELP!

[mattv]

Mine does that too, but not quite that early in the pot's rotation. I think it's supposed to do that. Have you measured the value of the pot? If you want to avoid this, you might try a resistor in series between the 1uF  (connected to pin 6 of the SA571) and the pot, but *after* the delay level pot. That's just  a suggestion. I can't say if it'll work for sure.

[Scott Swartz]

It supposed to be able to do the infinite repeat feedback thing you are describing.

It will happen at different pot rotations due to parts tolerances on the certain resistors for the SA571.

If this second pedal sounds the same as the first pedal when set for say 3 repeats, then there probably isn't a circuit problem.

For less repeat level simply change the 22K resistor that connects to pin 12 of the SA571 (the sgnal coming from the repeat pot) to 33K or 47K.

[Samuel]

Thanks guys, I'll give those resistor changes a shot.