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Re: Mark Bass ,CMD 102P, bass amp of 2011
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N_Cook
Jun 11, 2016, 11:12:54 AM6/11/16
to
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.
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?
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.
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?
N_Cook
Jun 15, 2016, 10:40:37 AM6/15/16
to
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.
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.
Gareth Magennis
Jun 15, 2016, 7:07:50 PM6/15/16
to
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.
headroom?
Gareth.
N_Cook
Jun 16, 2016, 4:57:17 AM6/16/16
to
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
Gareth Magennis
Jun 16, 2016, 4:55:19 PM6/16/16
to
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...
Gareth.
"N_Cook" wrote in message news:njtpl8$9cm$1...@dont-email.me...
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