PT-80 Delay - Feedback issues?

[mp3j3rk]

Hi,

I've found a few threads talking about PT2399 based delay pedals having some issues with feedback due to self oscillation and mods to reduce that to levels that are audibly tolerable.  I built the General Guitar Gadgets PT-80 (all build documents located: http://www.generalguitargadgets.com/effects-projects/modulationecho/pt-80-delay/) with a MAX1044 charge pump about a year ago , and it appears that there is something similar to the mods that I have read about already included in the circuit.  My thought is that I may be having a different problem.  The other threads stated that the feedback occurred when adjusting the "Repeats" pot, but I have issues with self-oscillation when the "Delay Level" pot is just past the noon setting regardless of the position of the "Delay Time" or "Repeats" pots.  I am pretty sure that I should have a little more play on the Delay Level pot before I hit uncontrollable feedback, or is that completely off-base?  Any one else with the PT-80 experience this? 

I am still learning much about audio circuits, but I am thinking that the SA571 compander and the components surrounding it are probably relevant, and likely pins 9-16 of the 571.  Possibly a bad 571/2399?  I have one more of both of those ICs that I could swap out (thankful that I used IC sockets here).  JD also provided a document with measured voltages on all ICs so I think I'll double check those again.  I checked the 2399 right after I built it when I was having problems with the MAX1044 (bad diode), and after fixing that issue I had good voltages there, but maybe something has changed.

Thoughts?  Suggestions?  Am I making mountains out of molehills?

[Mark Hammer]

Well if your pedal screams at you, that's a mountain, not a molehill.

I can't think of anything normative that would cause that sort of reaction from that control in that position.  Something is definitely wrong.

I'll just quickly add that my own PT-80 is very well-behaved, so it's not a problem with the board or design.

[ElectricDruid]

The Delay Level control shouldn't be able to cause feedback in *any* position, so there's definitely something wrong.

I checked the schematic:

http://www.generalguitargadgets.com/pdf/ggg_pt80_sc.pdf


Are you sure the Repeat pot and Delay Level pot wiring is all correct?

Tom