JC 120 Issue

[schmidlin]

I'm a tube guy, as you probably are. I have said amp (combo) coming into the shop with a weird issue: It goes "kkkkkkkoew" for half a second when turned on and also when turned off. Any clue where to start?

Other than that, it plays fine.

[teemuk]

Any clue where to start?

Well, it might help us if you explained what this "going kkkkkkkoew" actually even means...

Is it just a particularly weird way to descripe the moderately loud "thump" -type of sound the amp makes when switched on and off? If yes, then the amp is perfectly fine. What you are hearing is a transient signal that is created by voltages in various parts of the circuit rising from zero volts to their nominal value (or in case of powering off, dropping from that to zero volts). The transient leaks from power supply to the signal path and usually gets amplified along the way from preamp to power amp.

Also, which one of the several different JC-120 revisions is it? One from late 1970's can be entirely different than one from early 1980's and onward.

[schmidlin]

Pretty sure it's early 80s.

As for the transient thump, I've owned JCs before, and this ain't it. Not a mild "whump", but an electrical frying sound, kinda like those rice-krispie fireworks. Owner (and I) are concerned it's an issue that might eventually take it down in the field. I'm guessing PS issue.

Thought I'd throw it out there in case anyone has run across this before.

[phsyconoodler]

Filter capacitors come to mind here.Also the AC switch itself.Try another switch.If it still does it look at the filter caps.
Yes,even solid state amps need filter caps replacement on a periodic basis.
80's? Something to think about.

[schmidlin]

I'm agreeing with you. I'll start with all the PS caps. Can't imagine a switch doing that, but I'll definitely look into it.

[schmidlin]

Just a follow up: I swapped out the power caps and all is well. Thank all!