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Mark Bass ,CMD 102P, bass amp of 2011
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N_Cook
May 12, 2016, 9:56:12 AM5/12/16
to
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.
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.
N_Cook
May 12, 2016, 11:30:21 AM5/12/16
to
1 of 3 of each n and p type paralleled audio powerFETs gone ohmic all
round, IRFP240 and IRFP9240.
So a month ago 1 failed then the other soon after, both failing safe and
non-functional? then both going shorted at the last power-up?
round, IRFP240 and IRFP9240.
So a month ago 1 failed then the other soon after, both failing safe and
non-functional? then both going shorted at the last power-up?
Gareth Magennis
May 12, 2016, 3:34:19 PM5/12/16
to
"N_Cook" wrote in message news:nh21r3$1c3$1...@dont-email.me...
This is one manufacturer I refuse point blank to repair.
Gareth.
N_Cook
May 12, 2016, 4:08:56 PM5/12/16
to
I started looking at any common factor between the failed powerFETs.
They are not symetrically disposed, but a 100V 33uF main +rail decoupler
is physically near one failed FET and the other rail cap near the other
FET. I'll check the ESR but will replace them anyway. Some sort of very
large dV/dt or high level hf/rf that knocked out the adjacent FETs but
got attenuated along the common lines to the other pairs of FETS perhaps.?
Add some ferrite beads as well?
Needs something adding , instead of just replacing the expensive
powerFETs especially if the caps ESRs are good
Gareth Magennis
May 12, 2016, 6:02:26 PM5/12/16
to
"N_Cook" wrote in message news:nh2nlv$n5q$1...@dont-email.me...
That's why I won't repair this stuff.
Gareth.
N_Cook
May 13, 2016, 4:14:32 AM5/13/16
to
good ESR and uF value of those 2 caps and the main ps rail caps.
Thin ground traces going to those 33uF caps, so looks like some of those
otherwise mysterious retrofit pieces of overlain wires will be going in
there.
Thin ground traces going to those 33uF caps, so looks like some of those
otherwise mysterious retrofit pieces of overlain wires will be going in
there.
N_Cook
May 13, 2016, 10:20:50 AM5/13/16
to
At least there is vertical clearance on the pins of the replacement
audio TOP-3, to slide on some ferrite beads.
audio TOP-3, to slide on some ferrite beads.
Gareth Magennis
May 13, 2016, 3:12:57 PM5/13/16
to
"N_Cook" wrote in message news:nh4nl9$2lp$1...@dont-email.me...
At least there is vertical clearance on the pins of the replacement
audio TOP-3, to slide on some ferrite beads.
Gareth.
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