Mark Bass ,CMD 102P, bass amp of 2011

N

N_Cook

Guest
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.
 
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?
 
"N_Cook" wrote in message news:nh21r3$1c3$1@dont-email.me...

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.
 
On 12/05/2016 20:34, Gareth Magennis wrote:
"N_Cook" wrote in message news:nh21r3$1c3$1@dont-email.me...
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.
Click to expand...

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
 
"N_Cook" wrote in message news:nh2nlv$n5q$1@dont-email.me...

On 12/05/2016 20:34, Gareth Magennis wrote:
"N_Cook" wrote in message news:nh21r3$1c3$1@dont-email.me...
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.
Click to expand...

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.
 
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.
 
At least there is vertical clearance on the pins of the replacement
audio TOP-3, to slide on some ferrite beads.
 
"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.





https://www.youtube.com/watch?v=1IT5ogF4C-8



Gareth.
 
On 12/05/2016 14:56, N_Cook wrote:> 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.
Click to expand...

I've now got all the full-rated replacement parts, and will be replacing
in steps.
In place of the ground off marking driver, IR2153 is working in there
with 2x expendable sacrificial cheap IR740 pwerfets in there, running
60% then up to 80% of mains, wiht the 2 main PA power rails isolated. So
again the only? Mark Bass circuit out there has pins 1 and 4 swapped
over, deliberate disinformation to go with grinding off business. Next
step will be to remake those rail connections to the original 2x2 audio
powerfets and run at 60% up to 80% mains.
Then what is the next step, add the 2x 200V rating n and p audio
powerfets plus minor mods again at 60 to 80% mains then replace the
IR740, or replace the proper smps 600V powefet pair and go to 100% mains
and original 4 audio powerfets, then add the 2 replacements and mods?
 
I suspect its use of SMPS type powerfets in the audio side of this ,
IRFP240/9240 pairs rather than audio grade with whatever is inside of
those to reduce chance of oscillation in // use.
Anyway replaced those 2 and running with IRF740 in the smps , 60% mains,
then put in 2x STW33N60 and amp running ok at 100% mains, fingers crossed.
Mods were 20 amp stubs of wire from heatsink to ground pins of the 2
33uF,100V 105deg F caps, I uprated voltage on the replacements as +/-82V
unloaded.
Added 180pF over the 180pF of C2 to reduce f spec of the amp, bass amp
anyway.
Aded back to back 4.7V zeners over the back to back 6.2V zener gate
protectors of the audio amp, as owner only goes to 60% volume.
Added blocks of cured silicone sealant in the easy air paths forcing the
air through the heatsink vanes.
For the replacement 240 and 9240 ground back the flange bits of the gate
pins to take 10mm long ferrite beads, unknown spec <>4K ohm over ends.
 
Aded back to back 4.7V zeners over the back to back 6.2V zener gate
protectors of the audio amp, as owner only goes to 60% volume.




You might want to reconsider that. Isn't that restricting the amps
headroom?



Gareth.
 
On 16/06/2016 00:07, Gareth Magennis wrote:
Aded back to back 4.7V zeners over the back to back 6.2V zener gate
protectors of the audio amp, as owner only goes to 60% volume.




You might want to reconsider that. Isn't that restricting the amps
headroom?



Gareth.
Click to expand...

yes, going the wrong way, so removed.
On the air-movement lack of design, instead of drilling small screws
through the h/s
for the pcb mounting screws, they deep milled off the corners so making
it even less likely the air will pass through the restriction of the h/s
vanes
 
I'm well glad I'm in a position to refuse to repair this kind of bollocks.


Gareth.








"N_Cook" wrote in message news:njtpl8$9cm$1@dont-email.me...

On 16/06/2016 00:07, Gareth Magennis wrote:
Aded back to back 4.7V zeners over the back to back 6.2V zener gate
protectors of the audio amp, as owner only goes to 60% volume.




You might want to reconsider that. Isn't that restricting the amps
headroom?



Gareth.
Click to expand...

yes, going the wrong way, so removed.
On the air-movement lack of design, instead of drilling small screws
through the h/s
for the pcb mounting screws, they deep milled off the corners so making
it even less likely the air will pass through the restriction of the h/s
vanes
 
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