Ampeg SVT 350 insisting in not working

[camarada78]

I have an Ampeg SVT-350H bass head on my bench that after the repairs appears to be working as it should.

It had two IRFP240 and one IRFP9240 shorted.

So i changed them expecting to be a regular fault with them. It worked for a few days and they burned again. I thought it could be the big caps (since they were at 40% less capacitance. Changed them to new ones. Worked fine for a couple of days, and burned two more 240 and a 9240 again.

So I will not put new mosfets until i find why they are burning but i ran out of ideas and i'm here looking for people with similar problem. everything seems normal (on board). no weird resistors. The only thing different is the relay that is not factory but - according to owner - was like that for years.

Some measurements a little off (compared to the schematic)

I'm getting +/-68v dc instead of the +/-60v dc (i think its because i'm on a 127v AC region instead of a 115AC).
I'm getting +/-17v dc instead of the +/-16v dc (same reason i think)

other than that, i'm not finding anything suspicious.

here is the schem:


Any clues?

 

[CJ]

those amps are tough because you really need matched parts, but in order to match them you need to buy 100 of each!

plus, i have had problems with counterfeit parts so i quit working on them, because even if i fix it right, it is probably going to pop again and put me into a warranty position,

so i feel your pain but have no solution,

 

[camarada78]

i'm searching for circuits to test them. My component tester only goes to one decimal precision and apparently i need at least 2 decima precisions to a better match.