Marshall DSL 100HR crackling static V4 phase inverter.

[Helmholtz]

What is the most sensitive input setting (mV/div) of your scope?
Do you use a 10x or a 1x probe?

[Indyryder02]

What is the most sensitive input setting (mV/div) of your scope?
Do you use a 10x or a 1x probe?

Looks like 5mv. Probe has both options. Using 1x.

[Helmholtz]

Looks like 5mv. Probe has both options. Using 1x.

Ok, now what is the noise level at the plates?

And what is the scope's noise floor with 5mV/div setting and open input (no probe connected).?

[Indyryder02]

Ok, now what is the noise level at the plates?

And what is the scope's noise floor with 5mV/div setting and open input (no probe connected).?

About 20mv or so.. it’s jumping up and down. Can’t get it to steady.

noise floor is less than 5mv

[Indyryder02]

Did you check if HT6 and the bias voltage are clean?

You could also try shorting the node between C89, R41, R42 to ground.

Freeze spray can help to locate a noisy/unstable components.

I did. I replaced C23. And lifted R131 and R132 without change.
shorting the node you mentioned above kills the noise.

[Helmholtz]

About 20mv or so.. it’s jumping up and down. Can’t get it to steady.

noise floor is less than 5mv

You probably guessed why I asked.

It only takes less than 1mV at the grid or the cathode to produce a 20mV noise at the plate.
Probably hiding within/below the noise floor of the scope.
No wonder you can't "see" it.

[Helmholtz]

shorting the node you mentioned above kills the noise.

Shows that this is where the noise enters.

[Indyryder02]

Shows that this is where the noise enters.

So I’m not totally losing my mind!! Ugh!

I know what it’s not…
I’ve replaced R144, C36, TR1 and TR2, C139, C138,C23
C41,C29.. prob more and it still makes this rustling noise. And I can’t believe I still haven’t found it yet! Haha
not much left in that area…
grounding R146 kills the noise too, does this mean it’d be behind that in the circuit? Between R146 and HT1 and AGND, PGND?
only a few resistors left in that area.

[Helmholtz]

I would try freeze spray to localize noisy components.

[Indyryder02]

I would try freeze spray to localize noisy components.

This is what I originally tried… I could get the noise to go away after heating the circuit up with a heat gun… noise would go away.. then tried to freeze individual components to make the noise return.. but I couldn’t get it to isolate any one component. Ran out of spray. And most of the components in that area have all been replaced by now.

[Helmholtz]

Did you try to disconnect the NFB (R41 or connector9?

[Indyryder02]

Did you try to disconnect the NFB (R41 or connector9?

Yes.

[Indyryder02]

Ok. Replaced R146, R141, R148, R142, R143…
I’ve replaced every part in this part of the circuit and it’s still doing it… this beats all.
ugh. Truly at wits end.

If I ground the inside of R143 the noise goes away.

there’s not really anything left. Less the board is conductive somewhere etc and it’s bleeding through. ♂️

[The Dude]

FYI: These amps have a known issue with a yellowish glue that becomes conductive causing arcing. There's not some glue in the problem area or the power supply causing your problem is there? Look for a glue area that might have darkened in color.

[Indyryder02]

FYI: These amps have a known issue with a yellowish glue that becomes conductive causing arcing. There's not some glue in the problem area or the power supply causing your problem is there? Look for a glue area that might have darkened in color.

There was some of this glue. And some was very dark on one contact point.. I’d removed it all and cleaned the board with alcohol. But still the noise remains…
Really feeling beat down. Now I’ve somehow made it worse! It has a big hiss noise now that varies with the resonance and presence controls. So something with one of those last resistors I replaced. Ugh. I hate replacing parts like this without any real confirmation etc.. just makes things worse sometimes.
Need to step away from it for a few and regain my thoughts.
What a P.O.S.