Marshall Valvestate VS65R Static

[rudi246]

Well, replaced the zeners and no difference. I'm about ready to throw this thing.

[Enzo]

OK, so the noise is before CON1, so in the preamp. The noise stays when the tube is gone, so it is after the tube. We have it isolated to that section of the amp.


Run a cord from the FX send to some other amp for a listen. is the static on the FX send?


\From the tube, which you can leave out for now, the signal goes through two sets of tone controls, selected them by IC5. That is the preamp out on that page. On the next page the preamp out signal branches: one through R55 to the FX loop, and the other the loop dry bypass to VR10. The FX return comes back through IC7b then to the other end of VR10. VR10 then is the dry/effects mix control. The output of that IC then goes through IC7c and on to the power amp. Sounds convoluted, but it is a pretty easy path to follow.


So isolate the problem. Is it on the FX send? If so, the source is before that. FX send clean? OK get out a clip wire and ground it. Use the other end to ground the signal path at various points. Pin 5 looks like is the output of IC5. Ground that a moment, does it kill the noise? If so the noise is before that, if no effect, the noise is after. How about the wiper of VR10 - or either end of R59, ground that a moment, noise or not? After IC7, find the junction of C10 and R16, and ground that a moment, noise or not?

[rudi246]

Noise goes through the fx loop. Tested with a Peavey Vypyr amp and a pod with some headphones. The noise also goes throught he headphone jack. Could it still be those ceramic capacitors?

[Enzo]

It comes out the headphones because they are powered from the speaker wires. So we would be surprised if the phones were not affected.

Could it still be those ceramic capacitors?

Don't know, but you are skipping the process. If the noise comes out the FX send, then the source is before that send. Since the FX send is essentially the preamp out signal from IC5, and we know that it is after the tune, that pretty much screams IC5 at us, at least in my view. And you earlier reported that channel didn;t matter, so I tend to rule out the two sets of tome controls. I had mentioned grounding the signal path with a clip wire. Did we try that? Pin 5 appears to be the output of that IC, does grounding that stop the noise? Pins 2 and 7 appear to be the inputs to that IC, depending upon channel selected, does grounding either of those stop the noise?

[J M Fahey]

^^^^^^^^^^ that´s the way to go ^^^^
Do not even imagine other possible noise sources until you have checked where the noise is, relative to IC5 .

[rudi246]

Ok. Will try that. By grounding it do you mean just clipping one end of a wire on the chassis and the other end on the IC pins while it's on?

[J M Fahey]

yes, it is safe to ground signal pins while on for testing purposes

[Mick Bailey]

Just take care not to accidentally short one of the supply rails to ground. Sometimes it's 'safer' to securely locate the lead on the relevant pin and then touch the other end to ground.

[rudi246]

Grounding pins 5, 2, and 7 didn't stop the noise. The only one that made a difference was pin 2 which made it buzz.

If I'm not mistaken, pin 1 is on the bottom left and pin 8 is the top left, correct?

[Mick Bailey]

That's right.

Then move on to IC7b and IC7c. Try grounding pins 1,2,3,6,7 and report back what kills the noise.

There's one problem with grounding pins - if you don't know the power supply is clean, when you ground the output of the last IC in the signal path the noise will stop. But you won't know whether this is a bad IC or bad power rail. That's why you need to ensure your supply is clean at the outset.

[rudi246]

Grounding pin 7 on IC7 seems to stop the static. It buzzes, but no static. I also get a loud buzz from pin 6.

[Enzo]

OK.

When using a ground wire like this, it may ADD some new hum or noise, but the only thing we care about is whether it kills the original noise. I expect it to make hum and stuff in the process.

[rudi246]

I see on the schematic just after pin 7 is C10, R16, and R15. Are those the components I should be looking at?

[Enzo]

ANything is possible, but I tend to suspect the semiconductors first, I'd suspect that IC.

[Mick Bailey]

If grounding IC7 pin7 kills the noise, this tells you that the noise is on or before that pin, so the components that follow it are most likely OK.

Typically op-amps can go noisy. But again, an op-amp only has a limited capacity for supply noise rejection and in most cases need a ripple-free supply. Whilst I still suspect IC7, The fault occurred after you changed components in the PSU, so be prepared for that to be the root of your problem. If there's noise on your supply - or the IC is failing - grounding pin 7 will kill the noise either way.

Ideally find someone with a scope to check the supply rails, or beg/borrow/buy a multimeter set to AC to see if there's anything obvious.