JMP100 Transformer and Cap

[Ted]

I have here a Marshall JMP100 with an output transformer with a short from primary to chassis. Not an uncommon fault in big JMPs.
However the one of the main power supply electrolytics has also burst.

Would anybody care to speculate on what the possible relationship between these two faults is? Either of them would render the amp unusable so they must have occurred more or less simultaneously

The best I could come up with is that the large ripple current from trying to supply the shorted transformer took out the cap. Is this credible?

[Enzo]

No, because the short is parallel to the cap. The short grounds off the B+, so the cap now has little or no voltage across it.

[The Dude]

Is it "over-fused"?

[Ted]

The DC resistance to chassis is actually around 30 ohms from the centre tap.

It had taken out the B+ fuse bit not the mains one. I must admit that I didn't check the rating of the old one

[The Dude]

The reason I ask is that 9 times out of 10 when I see this, it's because a tube failed and blew the fuse. Instead of checking or replacing tubes a customer will just put in a bigger fuse. Then, we have what you now have- a shorted output transformer. If you don't check or replace the tubes, don't check the fuse value, and only change the OT, you risk blowing another one. ALWAYS check the fuses for correct value.

Also, did you unhook the CT when testing? You could be reading across something else, especially since you have a toasted cap. Make sure you pull the tubes, unhook the CT and test it that way.

[Ted]

I'd pulled the transformer when I tested it. I just don't want to replace it and the cap without having an idea of why they BOTH failed...

[Enzo]

You may never know. What if your sister got the flu the same day your windshierld wipers broke? related? probably not. Your failures might or might not be related. You are repairing the damage either way. If there is a connection you may or may not ever find it out. Caps usually fail due to overvoltage (unlikely here) or reverse polarity (or exposure to AC) or physical things like too high temperartures. None of that relates to tubes. And tubes fail all the time, but not generally because of anything a cap does.