Help with a PT80

[Bill Bergman]

Hi guys, Happy New Year !
I haven't built a pedal in a while but I gave my PT80 to a friend for his birthday so I decided to build a replacement for myself. I already had the parts from along time ago.  Anyway the new one works but my old one had a cool effect that the new one doesn't. On the old one when you turned the repeat way up the echo would go into a nasty oscillating feedback loop. I'm guessing I might have used a 47pf cap instead of the 470pf in the op amp part of the circuit. Could that be it or what else might help bring back the oscillating feedback loop?
Also it seems like the delay time is a little less than the old one.
thanks....
Bill

[Dean Hazelwanter]

Hi Bill,

For the (lack of) oscillation, try checking (and if necessary, reducing) the value of the 22K resistor above the 1st half of the 571.

[Bill Bergman]

Wow,
this won't be the first time I've said THANKS Dean ! You've done it again. With your suggestion of lowering the 22k resistor, I replaced it with a 25k pot so I can dial it down. The cool feedback looping is back. It retains the the first attack keeping the original note repeating to infinity and rises in volume. The other pedal seem to go all the way passed the note to more of a ocillating squeal. Not sure which I like best. This may be due to paralleling a 4.7k resistor with the 25k pot. I did that to keep it from zero resistance. Think that might be the difference?
Thanks again Dean,
Bill

[Dean Hazelwanter]

Glad to help!

If you put a 4.7K resistor in parallel with the 25K pot, the resistance will allways be less than 4.7K, regardless of the pot position. Try putting the resistor in series with the pot. Then the resistance can be adjusted between 4.7K and 29.7K.

[Bill Bergman]

Ooops, I meant series, got my terms mixed up.  I am getting 4.7k to 29.7k.

[Dean Hazelwanter]

It retains the the first attack keeping the original note repeating to infinity and rises in volume. The other pedal seem to go all the way passed the note to more of a ocillating squeal. Not sure which I like best.

Not sure what you mean here...

[Bill Bergman]

If I can borrow my original back I'll try and make a sound clip comparison.

[swt]

i know what you mean...i've always wanted to know why all the diy delays i'd made did that. my ibanez champ, the ad9, and other i got, do a regeneration of the note that fades in a fatter way to distortion and oscillation. But the diy repeats the note, and fades away being like a high pitch oscillation riding on top of it, being the note thinner, and almost gone. If someone knows a cure for this...

[disto]

i love the infinate ossilation on delays, didnt boss ad this function to the dd-6 uh i think they called it warp im not sure its been a while since i heard it.

some sound clips would be great though.