JCM800 2203 late 80s - background crackle noise

[Helmholtz]

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.

I just don't see what could cause a positive grid voltage except a leaky coupling cap (only exception I could think of would be a gassy tube).

OTOH, many DMMs are not very reliable at low mV readings...

[Greg_L]

The tubes have been swapped and rolled a million times. I'm pretty confident it's not the tubes.

I'll zero in more on that V2a section of the circuit and report back.

[g1]

I just don't see what could cause a positive grid voltage except a leaky coupling cap (only exception I could think of would be a gassy tube).

OTOH, many DMMs are not very reliable at low mV readings...

Grid voltages can be so difficult to read, I'm especially suspicious when the grid leak resistor may not be doing it's job.

[Helmholtz]

Grid voltages can be so difficult to read, I'm especially suspicious when the grid leak resistor may not be doing it's job.

A reading around 1mVDC on a DMM is probably not very reliable and the "fluctuation" could just be caused by the last digit jumping.

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

Now that's relevant and could mean an open grid leak resistor.
As soon as the meter is connected, the resistance of the meter acts as grid leak path and cures the problem.

[Greg_L]

I went through tons more stuff last night....slight improvement.

I've replaced everything in the V1b to V2b path of the circuit. I've removed wires, cleaned board solder pads, re-installed wires, re-flowed solder connections everywhere in the preamp, rewired tube sockets. The crackle is actually a little bit better, but it's still there. I've removed the input jacks and vol pots and replaced them with jumper wires so the circuit still has continuity with no pots...still crackly.

A few sort of new findings....the PCB is very sensitive to tapping and poking with a chopstick in the V2 part of the board.
If I bypass the V1b 10k cold clipper resistor the crackle noise gets a little better.

[Helmholtz]

.
If I bypass the V1b 10k cold clipper resistor the crackle noise gets a little better.

No good idea. That will make V1b draw full current and its ability to amplify gets lost.

Did you measure/resolder/replace R11?

Does the noise stop when you short R11 or short the V2a grid to ground?

What are cathode and plate voltages of V2a?

[Greg_L]

No good idea. That will make V1b draw full current and its ability to amplify gets lost.

Sorry, I meant bypass it with a cap.

Did you measure/resolder/replace R11?

Yes. Replaced with new metal film 470k.

Does the noise stop when you short R11 or short the V2a grid to ground?

Yes and yes.

What are cathode and plate voltages of V2a?

V2a plate - 238 vdc
V2a cathode - 1.39 vdc

[Helmholtz]

Looks as if you have a V2 supply voltage of over 400V? That would be huge.

What happens if you connect a 0.1µ cap from V1b plate to ground?

BTW, for scope measurements the ground clip of the probe should always be connected to a good amp ground. Otherwise the scope uses its own ground as reference, which is likely to introduce noise.

[Greg_L]

Looks as if you have a V2 supply voltage of over 400V? That would be huge.

It's right at it...396vdc. The B+ dropping resistors check out though. There are only two of them, and they both measure right at 10k, as they should. 475vdc at the output plates. I thought the preamp voltages seem a little high but I'm not sure. It's a late 80s 2203 with much less filtering than earlier versions.

What happens if you connect a 0.1µ cap from V1b plate to ground?

Still crackles, but more muffled.

BTW, for scope measurements the ground clip of the probe should always be connected to a good amp ground. Otherwise the scope uses its own ground as reference, which is likely to introduce noise.

I've not connected the scope for any of this more recent testing. When I did use the scope I grounded the scope to the amp chassis.

[Helmholtz]

Can we exclude an external EMI source?

[Greg_L]

Can we exclude an external EMI source?

I think so. We first noticed this crackle when recording in a studio. I took the amp home and started messing with it in my man room. Crackles in there too. Moved the amp to my dining room table where it's been ever since. Crackles in there too. So three very different rooms, all the same crackles.

I'm also wondering if maybe the board has gone bad. It is a little warped as they tend to do in this era of 2203s where the board hangs out into thin air off the pots and input jacks. It's also very sensitive to tapping.

[Greg_L]

At this point the only things I haven't extensively messed with are tube sockets and the transformers/choke.

I've deep cleaned the tube sockets, but have not replaced them.

[Helmholtz]

I've deep cleaned the tube sockets, but have not replaced them.

Did you try to retension socket contacts?

It might not seem logical but I would try a new chinese ECC83 in the V2 socket given your high supply voltage.

[Greg_L]

Did you try to retension socket contacts?

It might not seem logical but I would try a new chinese ECC83 in the V2 socket given your high supply voltage.

I did touch up the tension in the tube sockets but they were good as they were.

I do have a little pile of Chinese 12AX7s. I'll give them a shot, thanks.

[g1]

Also can you check if meter probe at grid still kills the noise?
Maybe some oscillation being manifested as crackle, that the meter capacitance is correcting?