JCM800 2203 late 80s - background crackle noise

[Greg_L]

Is the screen clip of the scope probe connected to anything?
As the scope is showing stuff that’s not audible, it may be a red herring. Perhaps better to try to find somewhere to probe that better represents the issue.

Thanks for responding.

The scope ground is connected to the amp chassis. I don't often use the little pigtail on the probe because it gets in the way and I don't want it to touch anything it shouldn't. Is that wrong?

In the first video you can see how the little noise spikes on the scope trace correspond with the crackle coming out of the speaker. Every crackle has a visible spike. The noise is audible with the master vol turned up. The noise is not audible with the vol turned down, but you can still see the noise at the output on the scope trace. It's like it's always there. Always. If I pull preamp tubes the audible noise goes away, but the noise spikes still show up in the trace.

I've gone through and one-by-one replaced plate resistors and bypass/coupling caps, removed input jacks, bypass the volume pots....the noise is always there.

[g1]

I hadn't realized there were 2 videos in that earlier post. I had only looked at the embedded one and there was no audible crackling. Now I looked at the first link.
I hear a constant crackling sound, but I can't agree it is definitive that it corresponds to the spikes on the scope, the sound is almost constant.
Agree with Jon Snell that it sounds like what you often hear from noisy plate resistors.
Suggest you put aside the ground spikes for now and proceed with the tube pulling Jon mentioned in post #7.

[tschrama]

If MV kills it and Gain makes no difference, it must be comming from between those two points. And prolly from something with large DC over it. You could temporally solder a large cap over the anode resistors one by one, to see if it stops. Idem with the CF resistor.

[Greg_L]

I hadn't realized there were 2 videos in that earlier post. I had only looked at the embedded one and there was no audible crackling. Now I looked at the first link.
I hear a constant crackling sound, but I can't agree it is definitive that it corresponds to the spikes on the scope, the sound is almost constant.
Agree with Jon Snell that it sounds like what you often hear from noisy plate resistors.
Suggest you put aside the ground spikes for now and proceed with the tube pulling Jon mentioned in post #7.

Will do thanks. I'm going back to square F*#%ing one with this thing.

[Greg_L]

If MV kills it and Gain makes no difference, it must be comming from between those two points. And prolly from something with large DC over it. You could temporally solder a large cap over the anode resistors one by one, to see if it stops. Idem with the CF resistor.

Thanks, that's a good idea. I get it, and will try it.

[Greg_L]

Man this thing is killing me. I'm about ready to just give up.

Every time I think I've found something....it's not it. The crackle continues. I've isolated what I think is the noise in the preamp, then it's in the tone stack, then it's in the phase inverter, I've actually gone through and temp replaced pieces one at a time. Nothing fixes it.

Every coupling cap, every plate resistor, cathode resistors, bypass caps, re-flowed solder connections under the board, re-flowed socket pin connections, cleaned and re-cleaned and re-cleaned pots, jacks, and tube sockets, replaced tubes, NOTHING FIXES IT!

So frustrating.

[g1]

Go back to isolating the problem. If turning down the master always kills the noise, then it can't be in the PI circuit and must be ahead of the master.
If the noise is there even with the master down, then it must be after the master, or in the supply.

[Enzo]

And don't confuse symptoms. If it seems to be in the tone stack, pulling the PI will stop it too. That doesn't mean the PI is at fault.

[Greg_L]

Yeah I'm gonna give it one more try from scratch.

Here's what I know:

The crackle can be turned down with the master volume.
The crackle responds to the EQ controls.
Preamp volume has no effect on the crackle.
Swapping tubes does not help.

I didn't notice this before, and I did check, but there is a little DC vacillating on V2a grid. It's definitely not 0v like it should be. I've replaced the coupling cap before that stage so I'm not sure how a little DC is trickling onto that grid. Maybe a bad tube socket?

[Helmholtz]

I didn't notice this before, and I did check, but there is a little DC vacillating on V2a grid. It's definitely not 0v like it should be. I've replaced the coupling cap before that stage so I'm not sure how a little DC is trickling onto that grid.

So the DC is not constant?

How much is it.
Is it positive or negative wrt ground?

Have checked /replaced R7?

[Greg_L]

So the DC is not constant?

How much is it.
Is it positive or negative wrt ground?

Have checked /replaced R7?

I have checked and replaced R7. No change.

The DC bounces around 1mv. Sometimes a little less, sometimes a little more. It's not steady. It seems to range from about .7mv to 1.5mv.

And when I measure that grid, touching the MM probe to that grid pin makes the crackling stop.

[Helmholtz]

The DC bounces around 1mv. Sometimes a little less, sometimes a little more. It's not steady. It seems to range from about .7mv to 1.5mv.

And when I measure that grid, touching the MM probe to that grid pin makes the crackling stop.

Again, positive or negative wrt ground?

A constant negative 1mV would be quite normal.

You might have a bad/noisy R11 or bad tube socket contacts.

[Greg_L]

Again, positive or negative wrt ground?

A constant negative 1mV would be quite normal.

You might have a bad/noisy R11 or bad tube socket contacts.

It's positive DC. About +.7 to +1.5mv dc. It fluctuates.

[Helmholtz]

It's positive DC. About +.7 to +1.5mv dc. It fluctuates.

Positive DC is typically caused by a leaky coupling cap (C7). Not all new components are good components.

[g1]

You might have a bad/noisy R11 or bad tube socket contacts.

This sounds more likely to me than a second bad coupling cap (assuming it was replaced as part of troubleshooting).
A resistance check from the the grid pin of socket to ground might show. Measure from tube side of socket.