What causes this noise?

[drummer4gc]

More details and voltages coming soon...

Can anyone identify a cause for a pedal making noise like this? It's a Polychorus. Noise does respond in various ways to knob twiddling. I've replaced all caps in the circuit to no avail, and the voltage regulator looks like it's putting out a proper +15. Any initial thoughts before I go pin by pin?

Schematic found here on p. 8: http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_id=28500

[GiovannyS10]

Fireworks!!!!!

Hey man, i some times ago i did a common pt2399 echo circuit, and i had a 'similar' problem, but more continuous. See it:

The problem was only a little short between the in and a V+...  I think i measures 5V more or less. I can't remember... I could simulate the problem in the video. I think this rule not apply to your project. But i am sharing my experience, because maybe it can direct you in the right way!
Your problem seems a little bit like mine [the sound] the difference is the mine seems have a continuous shorting, and yours seems intermittent... Maybe its a completely different problem hahah, i never worked with your circuit before, and i have few experiences with chorus.

Best regards!

[drummer4gc]

In going through the circuit checking voltages, I noticed that the "A" voltage supplying many of the chips was unstable and bouncing. I removed the LM311n chip, and there is no more noise (or effected signal, obviously. I'm guessing the issues were being caused by this chip loading down the power supply somehow? im about to order a replacement, unless anyone thinks it could be caused by a different issue?

[ElectricDruid]

If taking a part out makes a circuit squeak, it is not justified to conclude that the part you removed must be a squeak suppressor.

In your case, you might just have disconnected whatever it is that's causing the problem by removing the LM311. It *might* be the problem, but it's by no means guaranteed. Since you've removed the effected part of the audio, it's not a surprise that the noise is gone.

I'd try tracing the audio through with an audio probe and find out how far you can get before the noise appears. If you're right about "power supply", the noise will be all over.

I'd test the following, in this order:

Pin 1 of U1, the TL072 inverting amp at the input
Pin 7 of U1, the differential mixer
TP#4, the output from the NE571
Pin 1 of U3, the pre-delay filter
pin 7 of U3, the post-delay mixer (or TP#5 seems to be the same thing)

At one of these stages you'll have the noise. My guess would be that it'll be the first stage after the delay, but it's only a guess.

HTH,
Tom