"N_Cook" wrote in message ...

On 12/05/2016 20:34, Gareth Magennis wrote:


"N_Cook" wrote in message ...
Owner switched on, blue led came on as normal , but between that and 20
seconds grabbing guitar lead, had blown the mains fuse, no input, usual
load. No problem in previous use, may be a drop in volume , as for the
last month had to use 6 instead of 4 on the volume, but uses different
cabs so cannot be certain of that.
Both IRFP27N60K ohmic low all round. With trade mark ground off 8pin
SMPS supervisor, from combined effort here a few years ago, probably
IR2153 ,if like the other flavors of these amps.
Any view as to what would cause this sort of failure? and pre-emptive
mod around the supervisor circuit?
I don't like the great thick slabs of ceramic insulator pads, should be
used on bathroom splash-backs IMHO , not heat conductors.
The main amps not checked over yet.






Good luck with that.

This is one manufacturer I refuse point blank to repair.


Gareth.

At least a close enough schematic is on elektrotanya.
I started looking at any common factor between the failed powerFETs.
They are not symetrically disposed, but a 100V 33uF main +rail decoupler
is physically near one failed FET and the other rail cap near the other
FET. I'll check the ESR but will replace them anyway. Some sort of very
large dV/dt or high level hf/rf that knocked out the adjacent FETs but
got attenuated along the common lines to the other pairs of FETS perhaps.?
Add some ferrite beads as well?
Needs something adding , instead of just replacing the expensive
powerFETs especially if the caps ESRs are good






Yeah, yeah yeah.

That's why I won't repair this stuff.

Gareth.