Mark Bass ,CMD 102P, bass amp of 2011

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?

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

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.

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

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.

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.

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.

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