JCM800 2203 late 80s - background crackle noise

[Greg_L]

That's good, because without power tubes the OT is essentially out of circuit.
To further exclude OT primary to secondary arcing, disconnect the primary CT.

[Greg_L]

that being the case definately replace the diodes after Helmhotz's request on the OT. Diodes are cheap.
nosaj

They should be here tomorrow.

[Greg_L]

Alright, well new rectifier diodes didn't do anything.

[nosaj]

You completely took ot out? If so only thing left is pt. How about scoping pt primaries an secondaries for those spikes?
nosaj

[Greg_L]

You completely took ot out? If so only thing left is pt. How about scoping pt primaries an secondaries for those spikes?
nosaj

I have not disabled OT yet. It's primary center tap is spliced in with a choke wire under shrink wrap. I need to talk to the owner and see what he wants to do before I go tearing all that stuff apart.

I'll have a look at the PT primaries next.

[nosaj]

It's just lifting 3 wires 2 secondaries and a CT.
reason I say scope primaries is to see if seer power source is clean . Also does scope show clean signal if you scope your sig Gen directly?
Nosaj

[Helmholtz]

Could still be many things, like choke (short it to test), ecaps, standby switch (short it to test), board, a tube socket, bad connection...

[Greg_L]

It's just lifting 3 wires 2 secondaries and a CT.
reason I say scope primaries is to see if seer power source is clean . Also does scope show clean signal if you scope your sig Gen directly?
Nosaj

My signal generator is clean. But aside that, I'm getting the spikes even with nothing going into the amp. Just sitting at idle gets crackle noise and spikes in the waveform.

I'll test that other stuff and report back.

[Greg_L]

Could still be many things, like choke (short it to test), ecaps, standby switch (short it to test), board, a tube socket, bad connection...

I was thinking about bypassing the choke just to get it out of circuit and see what happens.
All the caps have been tested and/or replaced.
Standby switch was faulty and was one of the very first things replaced.
The board is not in great condition. It's one of those that hangs off the chassis mounted pots with no support and it is quite warped.
I haven't noticed any problems with the tube sockets but that is certainly a possibility. They've been deep-cleaned and retensioned. They don't seem to be arcing or passing any signal/vontage between terminals.
I've gone over every single tube socket and PCB solder joint and connection in this amp.
All that's left now are transformer/choke things.

[Helmholtz]

All the caps have been tested and/or replaced.

No cap tester will show a HV arcing problem inside a cap and a new cap isn't necessarily good.

[Greg_L]

No cap tester will show a HV arcing problem inside a cap and a new cap isn't necessarily good.

I totally get that. They seem to be behaving as they should though. They're not leaking DC that I can tell.

It's also very possible that I'm missing something totally dumb and obvious because I am just a rookie hobbyist. I'm doing everything I can though so far.

[Helmholtz]

They're not leaking DC that I can tell.

We're not talking leakage here. Completely different story.

[Greg_L]

We're not talking leakage here. Completely different story.

I understand. But what else does one do about caps? I've replaced them all one at a time. With every cap swap the noise was still there. And now I'm finding that the noise is present throughout the amp. It's just that sometimes you can hear it better than others.

I'm really thinking it's a transformer now. Even if I probe ground points I see those little spikes. If I ground the scope probe my signal is clean. If I probe the amp chassis the spikes appear the same as they do scoping the output. So weird.

[Helmholtz]

Ok, if you replaced every single HV cap (wasn't sure about that) without any change, we can exclude them.

[g1]

If I ground the scope probe my signal is clean. If I probe the amp chassis the spikes appear the same as they do scoping the output. So weird.

Are you saying you only see the spikes if your scope is not grounded via it's ground clip?
Is the amp plugged into the same outlet as the scope?