Koch Amp burnt PCB

[stevesamps]

I have a Koch Multitone with a hole now in the board.

I have isolated the burnt area by drilling a hole in the trace

however the burnt area was across the output transformer primary ugh!

and the tubes test weird also none are shorted but they are way off not matched at all

The thing is my gut says the output transformer with the melted leads
is at the very least a "flakey one

and the tubes for sure
any ideas on how this happened?
trying to get a pic uploaded I guess I have to host it online?

Steve

[Prairie Dawg]

A little bit of flyback voltage from operating without the speaker connected methinks. That or maybe that plus a sorta loose spade connector. If that's what it was, not removing the carbonized section of the board is asking for it to happen again. Carbon is what resistors are made out of. If it was me and it was flyback voltage like that I'd probably relocate those leads to some other part of the real estate.

If the carbonized section is still there, see how much resistance there is across it.

[stevesamps]

carbon is removed well isolated

I had planned to attach the plate wires directly to the points on the board
one directly to pin3 and the other has a flying lead already , so that
burnt area is already isolated and should not effect anything.
there was only one trace there and I cut it, its a burnt Island now...

My concern is the output transformer can I trust it?
I mean if I put new tubes in there and wire it in properly

the amp was still working just lost power
and had that crappy low and distorted sound of a bad OPT.

I normally dont second guess myself just this amp
is way overpriced and custom and all that crap.

plus the owner is looking to resell it so swapping the OPT is drama for him.

myself I would not trust that transformer seems there are 5-6 clues
that say its been very fn hot and is likely shot.

I guess the question is do I even try to fire it up with that transformer
do I try and test the transformer in another amp?

perhaps do the low voltage test to get the turns ratio and Z ratio
that I may do as I have no clue what primary Z they are using.
and it would be a nice "safe way to test the suspect OPT

sorry to ramble on..

[Prairie Dawg]

If you're apprehensive over whether the output transformer is toasted, check it with a megger or borrow an insulation tester from one of your refrigeration mechanic buddies. If it's got a high resistance short that'll find it.

Just as a point of information I dragged out my IRT-2 insulation tester and did a little research on transformer winding insulation resistance values. IIRC correctly Ted Weber (RIP) once told me his output transformers should be >40 megohms. I have an old Bogen output transformer kicking around here since I repurposed the chassis. I tested it at 1kv and the resistance was over 200 megohms.

These are a handy piece of tooling for any amp mechanic. I got mine for $5 at an estate sale along with the Radiotron Designer's Handbook (4th ed) for a buck.

There seem to be a lot of good ones on fleabay with the going price for new Chinese made around sixty bucks.