[sdiy] OB8 power transformer

[Tony Allgood]

 > An intermittent shorted turn ?

That would be my conclusion too. I had a five year old Peavey guitar amp 
in for repair. It was initially fine for about half an hour and then 
would take out the mains inlet fuse. Changing the fuse, it would work 
only for five minutes or so before blowing again.

I concluded that as the transformer naturally warmed up the physical 
size would change and maybe some loose windings would slightly shift 
causing the wires to rub against each other. The wire's insulation 
coating may have been weak at some point and eventually, kaboom.

I also had a transformer in an old Logan String Melody that would not 
fail but, after three or four hours of being powered up, would get 
stupidly hot (with accompanying 'warm electronics' smell). Testing it in 
isolation on the bench all seemed fine but I found it was very slowly 
heating up. If one wasn't specifically looking for excessive heat rise 
you wouldn't notice anything untoward until the unit had been powered 
for at least two hours - even with no load applied to the secondaries. 
In this case I assumed it was due to losses within the laminated core 
rather than any shorted turns.

Tony

New album out: https://takla-makan.bandcamp.com/album/solway-firth