Marshall MG15 CDR Strange Issue

[misterc57]

Had very weak fizzy distorted output. I traced the issue to 4558 IC5A (clean signal at pin 3, bad signal at pin 1).
Replaced IC5 with a new 4558, amp sounded great but after three minutes of playing audio drops and is replaced by a hum.
Pattern... turn amp off, wait, turn amp on. Sounds fine for a few minutes and issue returns.

I have replaced all filter caps, C33, C34, C31, C32, C27, C28. I have a stable V with -14.9 and +14.7 on the rails. However the issue persists, after three minutes of playing the audio drops and is replaced by a hum, while V remains constant.

Simpler scenario... Amp ON, nothing plugged into the input, both volume pots at zero. No sound then after three minutes a hum starts.

Would love suggestions on this one. Schematic I am using is attached. Thank you


Marshall+MG15CDR.pdf

[dmeek]

After sound dies, check for DC voltage at all op-amp output pins. Maybe a missing ground reference somewhere.

[Jon Snell]

The two filter capacitors don't look right ... are they the correct way around?

[misterc57]

The two filter capacitors don't look right ... are they the correct way around?

Now that I look at the schematic it looks like one is backwards,