Marshall Valvestate 80V (8080) problem

[jp3zone]

Hi fellas, I hope you can help me out here.
I got this Marshall valvestate 8080 which has a few problems.

Firstly, it works, but the audio signal has an overtone riding on it, kinda like soft distortion in the background. the normal signal is the as loud as it should be, but the distorted signal is audible when the clean signal starts to decay. It doesnt eminate from the pre amp, as it is audible when plugging the guitar into the fx return. Im i right in thinking it could be the poweramp bias thas of?

I've replaced the following already:

power supply caps, zeners, both output darlingtons + driver, all power amp transistors and diodes, and all opamps. None of the above has helped.

Second problem is that the 1000uf supply caps heat up quite a bit, not enough to scald you, but hotter than I feel comfortable with. You can feel it touching the exposed metal on top of the cap. All the rail voltages also check out, -15.34/ +15.10v and -39.4/+39.4v.

Any help would be very much appreciated.

[olddawg]

Have you swapped in a different 12AX7? As far as the 1000uf caps, Did you use 16v caps? I think I would have used a little higher voltage rating. The old rule of thumb was 20% over the operating voltage and components used to be under rated by at least 20% generally. Depending on where your caps came from nowadays it may be cutting it close. Have you checked for ripple on the rails? You are probably going to need a scope for this one.

[jp3zone]

Olddawg

I've tried different tubes, no result - fx loop is after the tube as well - could it be the tube supply cap i wonder? Pardon my ignorance, but aside from using my scope to track ac signal within the circuit, I'm not sure how to check the ripple on the rails. Could you please guide me through it?

Thanks

[g1]

Sorry to be the one to ask, but you've verified it isn't the speaker?

[Jazz P Bass]

Olddawg

I'm not sure how to check the ripple on the rails. Could you please guide me through it?

Thanks

You check power rail ripple by setting your Volt Meter on volts AC.
Any "ripple" will show up as a voltage.
You will be looking for gross abnormalties.
You can also use the scope.
Make sure the input is set to AC on the scope.

[jp3zone]

Yeah, speaker was my first suspect, as the sound is exactly the same type you get from a damaged speaker coil. But thats not the case here, hooked up 2 different speaker cabs with the same result.

Thanks Jazz, I'll give it a go. What would be considered as acceptable ripple voltage?