JCM 800 2205

[Jazz P Bass]

A JCM 2205 showed up for repair. Previous repairer replaced all the electro & coupling caps. Including the power caps.
New MM output tranny & choke.
New Svetlana EL34 tubes.
Came to me quiet as a mouse, no input.
Plug in a guitar, turn it up 1/2 way, it sounds like an angry hornet nest.
Digital scope reads the buzz aa 60Hz.
If I go from send, to another amp, it is fine.
Insert anything into return, the hornets are back.
Here is the screwy part.
I have a digital scope. If the leads are hooked up to the output, scope turned on or turned off, no hornets.
I thought it was haunted. had it on the bench, all hooked up for surgery, no buzz.
Put it back in the cabinet, buzz. (Took me a while to figure out the scope part)
Any idea where the hornets are hiding?
(PS: this goes way beyond single coil PU hum)

[Alex R]

Is your scope screen grounding it?

[Jazz P Bass]

JCM 800 hornet buzz

Grounded to what?
I guess it is.
By the white return wire.
The scope mains plug is two prong.
Line in to the amp is a two wire variac.

[booj]

Does it buzz when plugged into a grounded outlet?

[Jazz P Bass]

JCM 800 2205

Yes it buzzes on a 3 wire outlet.
I may have found the problem
CA3046 was leaking.

[Jazz P Bass]

CA3046

Replaced the switching IC, CA3046.
That was not the answer.
The Hornets are back.

[Jazz P Bass]

Test setup

It appears that my test setup was (is) deficient.
I run my variac (2 wire) through a Kill-a-watt meter.
With the meter connected, Hornets.
Removed the Kill-a-watt, silence.
I have had well over 100 tube amps (among others) on this setup.
Never experienced this awful buzzing.
That means something is different about this one amp.
Yeah, well.
The amp is going back to the customer.
I will probably lose sleep wondering on this one.

[gomez63]

I'm sure you covered this, however, did you try swapping the output tube pin 3 wires (to plates)?

[gomez63]

FWIW, I have a 2210 that came in that is got me going from component to component after having the bees come on in the boost channel and then the channel switching fall out. I replaced the CA3046 and switching is back, but dull boost channel.

[Jazz P Bass]

Jcm 2205

I'm sure you covered this, however, did you try swapping the output tube pin 3 wires (to plates)?

Thought about it. Now I will do it.
The amp is back.

[gomez63]

FWIW, I have a 2210 that came in that is got me going from component to component after having the bees come on in the boost channel and then the channel switching fall out. I replaced the CA3046 and switching is back, but dull boost channel.

Funny thing...dull boost channel traced to poor connection of grounded shield wire on the bottom of the component board..go figure!

[Jazz P Bass]

JCM 800 2205

Well I finally have this amp quieted down.
Previous repair tech had the ground wires all tied together.
I separated the power transformer leads from all others.
Cap grounds are at one spot.
What really nailed it was running a ground wire from the input jack ground lug right to the chassis.