JCM800 2203 late 80s - background crackle noise

[Greg_L]

Hello all. Merry Holidays.

I have a late 80s 2203 on the bench. It came in badly needing filter caps - they were all bulging and leaky - and just an overall freshening up. The amp worked and sounded good besides the bad filter caps and slight background crackle/static noise. I replaced the filter caps and bias caps. The standby switch was falling apart too and not making good connections. Replaced standby switch.

The static/crackly background noise is still there. It goes away when you turn down the master volume or pull preamp tubes. Here's what I've done:
1) Swapped tubes around. Rolled in a known good 12AX7 in each spot - no change.
2) Deep-cleaned tube sockets and pots and jacks - no change
3) Flipped the board and touched up solder joints and cleaned off old flux - no change
4) Touched up wire connections at tube sockets - no change
5) Manually grounded the input and wiper of master vol pot - that killed all noise and signal, as expected.
6) Poked prodded and shimmied wires around with chopstick. Poked for sensitivity around resistors and caps - no findings there.
7) Verified resistor values and looked for DC where it shouldn't be - this all seems okay.

So I'm getting stumped. I'm sure I've overlooked something simple maybe. Please share any thoughts or ideas you have and I'll try it.


This is the schematic that goes with this particular amp.
https://www.thetubestore.com/lib/the...-Schematic.pdf

[mozz]

Freeze spray and a heat gun. Pulling tubes you should be able to narrow it down. You can also monitor voltages.

[Greg_L]

It's definitely something before the MV, but that's a lot of stuff.

Forgot to mention with the other things...

Grounding the first two grids - no change.
Preamp vol all the way down - no change
New output tubes - no change
Turning down the master vol and/or grounding the vol pot lugs kills the noise. So maybe it's something in the tone stack?

[Enzo]

So isolate the problem. The noise comes from somewhere. You found it was before the master control. So keep looking. which controls do or do not affect the noise in ANY way?

Get your scope on the speaker output, now you know what the noise looks like. Now go through the preamp stage by stage looking for it. Pins 1 and 6 of each tube is a good place to start.

[Greg_L]

So isolate the problem. The noise comes from somewhere. You found it was before the master control. So keep looking. which controls do or do not affect the noise in ANY way?

Get your scope on the speaker output, now you know what the noise looks like. Now go through the preamp stage by stage looking for it. Pins 1 and 6 of each tube is a good place to start.

Gotcha. That's coming up next.

[Greg_L]

Ok so this seems weird....I'm seeing the noise spikes on the scope. I'm hearing them through the bench speaker. I've got the time axis sweeping slowly so I can see and hear the noise crackles at the same time.

The weirdness is the noise seems to be in the grounding. If I probe the chassis, or any ground point, I'm getting noise spikes on the screen that coincide with what I'm hearing in the bench speaker. So I guess tomorrow I'll be redoing every single ground? Does this seem plausible?

[Jon Snell]

It probably has nothing to do with the ground points.
If the noise does not change in volume adjusting the pre amp volume control, Remove V1 ... any change if yes, replace the 100k resistors and check the disk ceramic coupling capacitors.
Remove V2 ... has the noise gone?
If it has, the problem is probably one or all of the 100k resistors. That is assuming the ECC83 (12AX7) is quiet.

[Greg_L]

It probably has nothing to do with the ground points.
If the noise does not change in volume adjusting the pre amp volume control, Remove V1 ... any change if yes, replace the 100k resistors and check the disk ceramic coupling capacitors.
Remove V2 ... has the noise gone?
If it has, the problem is probably one or all of the 100k resistors. That is assuming the ECC83 (12AX7) is quiet.

Yeah I was thinking maybe plate resistors or caps but I was hoping to pinpoint the problem without having to just throw parts at it. My skills are not there yet.

[catalin gramada]

Post a track...mp3 or whatever

[Greg_L]

Post a track...mp3 or whatever

Ok will do later today.

[Jon Snell]

Keep logical and note your findings. You will soon find where the noise emanates from.
No need to throw parts at it.

[Greg_L]

Okay still no progress. Whenever I think I've had a breakthrough....nope. Back to square one.

Here's what I'm dealing with...
In this first video I'm scoping the output and I have a bench speaker attacked. You can hear and see the noise spikes. When I turn the volume down, you don't hear the noise but they're clearly still there in the scope trace.

https://www.youtube.com/watch?v=07SKvD2SqR0



In this second video V1 is pulled and the preamp volume is all the way down. I can't audibly hear the crackly noise but it still shows as the familiar spikes at the output. V1B plate wire also seems to be very sensitive. Flicking at it with a chopstick makes a lot of noise even with no tube installed. And then flipping standby to turn off the HT still yields spiky noise at the output. And then turning the whole amp off, still that spiky noise at the output. Removing the probe all together finally shows a smooth no noise waveform. I'm perplexed.

[Greg_L]

Anyone wanna peek at those videos and chime in please?

[Greg_L]

Damn ok then.

[pdf64]

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.