Marshall DSL 100HR crackling static V4 phase inverter.

[Indyryder02]

I can’t be the only one to have come across this..
amp came in with blown HT fuses. Replaced em. Biased up fine. No bias irregularities or drifting etc.

what it does have is annoying static / crackling sound i generally associate with bad solder joints, tubes, plate resistors, transistors, etc.

I can only detect the noise with audio probe and scope between the plate resistors, and the power tube grids.

After trying different tubes, I’ve reflowed all of the solder connections in this area, tried replacing plate resistors, C84 and even C41 as it got damaged while I was trying to heat cycle the area with a heat gun, which was making the noise go away….
however freeze spraying certain areas after the heat has not narrowed it down any further… it’s pretty well narrowed down as is.. which is driving me nuts that I still can’t figure it out…

pulled board again, Reflowed more solder joints..
still makes the noise… chop sticked everything…

anyone else come across this?

Had to step away as I don’t even know what else to try or look for. Ugh!

Kicking my ass!

[Helmholtz]

what it does have is annoying static / crackling sound i generally associate with bad solder joints, tubes, plate resistors, transistors, etc.

I can only detect the noise with audio probe and scope between the plate resistors, and the power tube grids.

Don't understand.
Don't you hear the noise with a speaker?
What is between plate resistors?

Did you clean/retension tube socket contacts?

[Indyryder02]

Don't understand.
Don't you hear the noise with a speaker?
What is between plate resistors?

Did you clean/retension tube socket contacts?

In other words. I can’t detect the noise outside of that part of the circuit.. all tubes are removed except v4..
audio probe on plate side of plate resistors for v4…
noise is seen and heard between the plate resistors and the grids of the output tubes.. not on the phase grids or anywhere outside of that part of the circuit.

Yes. Cleaned tube socket as well. I’ll double check tension.

[Helmholtz]

I can’t detect the noise outside of that part of the circuit..

No wonder with all other tubes out.

Ok, so the noise is likely generated by the PI stage.

[Indyryder02]

No wonder with all other tubes out.

Ok, so the noise is likely generated by the PI stage.

Yes..
but more importantly no where else in the v4 section.. ie grids.. just on the plates only. Unless it’s too small for my scope to see or audio probe to hear.. think I’d see something..
it’s definitely affected by heat, which makes the noise disappear when heating c41 and c44… c41 was already replaced.. but it seems to react the most.

[Helmholtz]

Yes..
but more importantly no where else in the v4 section.. ie grids.. just on the plates only.

Well, it's an amplifying stage and noise would be about 30 times lower on grids or cathode than on plates.

[g1]

definitely affected by heat, which makes the noise disappear when heating c41 and c44… c41 was already replaced.. but it seems to react the most.

I'm looking for C44 in the PI circuit but not seeing it, where is it?
Did you try disconnecting the C84 cap between PI plates? Have you replaced coupling caps C37 and C38? I've had PI coupling caps cause crackling before.

[Indyryder02]

I'm looking for C44 in the PI circuit but not seeing it, where is it?
Did you try disconnecting the C84 cap between PI plates? Have you replaced coupling caps C37 and C38? I've had PI coupling caps cause crackling before.

Sorry, C44 was not replaced, just in the same area affected by heat, but I couldn’t find it in the circuit either.. so leaving that out of the equation..
i replaced the c84 but don’t think I tried it removed..
I have not replaced c37 and 38, just reflowed solder to them.. I’ve not ran into them crackling before, but that’s certainly worth a try then! There’s really just so little in this small area.. I can heat the area up and the noise goes away.. unfortunately I can’t pinpoint it any further with freeze spray.

going to try coupling caps next, as there just isn’t much left that hasn’t been replaced or resoldered there. Hate when you’re so close and still so far away! Haha!

[g1]

If you already replace C84, you don't need to lift it. Not likely that 2 of them would both crackle (unless replacement was too low voltage rating).

[Indyryder02]

If you already replace C84, you don't need to lift it. Not likely that 2 of them would both crackle (unless replacement was too low voltage rating).

Replaced the coupling caps. Still have noise but seems to have changed a bit.. or I’m losing my mind… could also be that! Haha

i even lifted r131 and r132 just to remove that as a possibility..

Anywhere I probe between the output tube grids and the phas inverter plates has the noise. I can hear with the audio probe and see it with the scope. I can’t hear or see anything on the PI grids or cathodes.

with R34 lifted from the PI plate.. the noise was gone.. with R35 lifted, the noise was of course much lower but still present… I replaced R34 with a bigger watt resistor.. still no change…

what else could be left?

edit. Lifted c41, noise went away…

rechecking solder joints etc

[Helmholtz]

Is HT 6 clean? In rare cases a bad filter cap can cause strange noises. Check C23 and R10.
Also please post PI cathode and plate voltages.

Do you still have noise at PI plates with both coupling caps lifted?

[Indyryder02]

Is HT 6 clean? In rare cases a bad filter cap can cause strange noises. Check C23 and R10.
Also please post PI cathode and plate voltages.

Do you still have noise at PI plates with both coupling caps lifted?

I lifted c41 from the grid and the noise stopped. I can’t see or hear the noise there with my scope or audio probe.. think I’d see something at least… I know it’s not amplified like the plate side, but usually I can see some small noise on the grid if that’s where it’s coming from… has to be there though if it goes away with the cap lifted though right?

ugh. This of course will no doubt wind up being something really stupid. Anytime I have this much trouble with suck a seemingly simple issue…( it wasn’t even one of the initial concerns with the amp! It’s just driving me nuts)

PI VOLTAGES 1. 117v, 6. 121v, 3 & 8 17v.

[g1]

If the crackle goes away with C41 lifted, it must be coming from before C41. Is the crackle there when both masters are turned all the way down?

[Indyryder02]

If the crackle goes away with C41 lifted, it must be coming from before C41. Is the crackle there when both masters are turned all the way down?

Ok.. I figured so.. starting to question everything! Lol

crackle.. which has sort of changed into More of a glitchy whooshing sound is there regardless of master volumes settings.

[Helmholtz]

If the crackle goes away with C41 lifted, it must be coming from before C41.

Or from a leaky C41. Leakage doesn't need to be steady.

Sorry just saw that C41 was replaced.

Need to see page 1 of schematic.