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,

[misterc57]

So one of the large filter caps was installed backward. Looks like that ruined the cap as it has lost over 50% capacitance so I reinstalled one of the old ones that test good.

However the problem has not gone away.

So I monitored IC5 and was seeing odd V readings. Testing with nothing plugged in I saw V slowly rising at pin 1 and then the hum kicks in after about 3 minutes and then the V at pin 1 was jumping around for a while and eventually V became stable with the following...

rails +/- 17 V

IC5
pin 1. 12.2V
pin 4. -14V
pin 7. 12V
pin 8. 13.3V

[dmeek]

An open wiper on the treble pot could cause this

[Jon Snell]

IC5.
If pin 1 is 12.2v pin 2 will also be as will pin 3. 5a is purely a buffer and is tied to ground through VR6, 5 & 4. Does R13 rise to 12.2v?
Is that what you have?
If pin 1 has 12.2v pin 5 will be close making pin 7 similar as will pin 6

There will be enough information above to find out why the DC level is rising.