JCM800 2203 late 80s - background crackle noise

[nosaj]

Ha I've actually been through all that already. I was watching it in the dark last night. No arcs.

Can't see arcing in the OT in the dark can you?
nosaj

[Greg_L]

Can't see arcing in the OT in the dark can you?
nosaj

No, I was looking through the tube sockets and dropping resistors and stuff.

At this point I might have to assume it's something in the choke or OT or PT.

I have not messed with the rectifier diodes. Could they cause something like a constant crackle? My B+ rail seems pretty steady and healthy.

[The Dude]

Can you see any noise on the B+ rail with a scope?

[Greg_L]

Can you see any noise on the B+ rail with a scope?

I haven't really seriously scoped the B+ rail. I'm a little leery of putting my scope to that high of voltage.

Is there a safe way to do it?

[Greg_L]

Another thing....I've played the amp now. It sounds pretty awesome.

With an actual guitar plugged in and the gain turned up a little, the general noise of a guitar being plugged in overpowers the crackle. You can't hear it. But as soon as you roll the guitar vol off the crackle shows up. If you have the master volume up and the gain way low, the crackle is there. If you turn the gain back up, the crackle fades away.

[The Dude]

What's your scope input voltage max rating? Do you have a 10X probe?

[Greg_L]

What's your scope input voltage max rating? Do you have a 10X probe?

I do have a 10x probe, and a 100x probe.
10x probe is good for 600vdc.
The scope itself says max 400v.

[The Dude]

I think the typical 2203 has less than 500V B+ (measure first to be sure), so you should be fine with the 10X probe.

[g1]

I think I was caught out on this before and with the scope on AC coupling, there will be more than 400V there no matter which probe. (500V B+).
You are fine with the 10x probe if scope set to DC coupling.
The scope probe can't divide down the DC when the scope coupling cap is blocking DC.

[Greg_L]

Ok thanks gents.

so just to clarify....

10x probe...DC coupling and my scope hopefully won't explode?

I'm getting about 475vdc on the output tube plates.

Where should I probe the B+ rail? At a filter cap? Right off the diodes?

[The Dude]

If g1 says so, listen to him, but I don't see how that could be. A 10X probe just contains a voltage divider from probe tip to ground, so I don't see how AC or DC coupling would matter. Maybe I'm missing something and I certainly don't want to be responsible for blowing up your scope.

[Greg_L]

Ok so I gave it a try at the lowest DC voltage spot...V1a plate. It's a lower voltage than my scope's rating and probe rating.

With DC coupling I couldn't even get the DC trace onto the screen at a low enough resolution to see noise. At 100v per div the trace was there, smooth and straight as an arrow. When I'd fine tune to actually see it the trace it shot off the screen and no amount of vertical adjustment could get it back. That could be operator error on my part. I'm hardly a scope whiz kid.

But I decided to go all in and set it to AC coupling and I could zoom in on the DC trace. AC coupled, 10x probe, down to 200mv/div and there it is....lots of hairs and crap squirting off the DC voltage.
I also tried the back end of the last B+ dropping resistor - same result. DC signal with lots of scratchy hairs coming off of it.

It's the same hairy crap that shows up at the output.

And when I scope the output at the speaker jack and listen at the same time, the spikes in the waveform correspond with the sound it makes.

[g1]

If g1 says so, listen to him, but I don't see how that could be. A 10X probe just contains a voltage divider from probe tip to ground, so I don't see how AC or DC coupling would matter. Maybe I'm missing something and I certainly don't want to be responsible for blowing up your scope.

I thought same, but was told the divider's path to ground is in the scope, not the probe. And on AC coupling, that path is after the cap. Without the schematic it's harder to describe. And I may not be describing it right.
Now what is at the other end of that cap, and whether the full DC ends up across it, is another question.

[g1]

lots of hairs and crap squirting off the DC voltage.
I also tried the back end of the last B+ dropping resistor - same result. DC signal with lots of scratchy hairs coming off of it.

It's the same hairy crap that shows up at the output.

And when I scope the output at the speaker jack and listen at the same time, the spikes in the waveform correspond with the sound it makes.

Have the rectifier diodes been replaced?

[Greg_L]

Have the rectifier diodes been replaced?

No. They're one of the very few things left in this amp that have not been messed with.

I don't have any on hand at the moment. I'm gonna go try to find some locally tomorrow.