Mark Bass

[Valvehead]

hey fellas--need a scem. Mark Bass model = mini CMD 121P , Combo Head 2 . Its a small 1x12 combo amp . This one had 1 cap completely melt down and 1 other is burnt + 2 , 5w resistors next to them. Need to know what C87 is , and id like some ideas as to WHY the catastrophic meltdown .

[J M Fahey]

Designers tend to repeat pet ideas over and over, guess this one should be close enough.
FWIW thisVersion 3 is VERY close to earlier Version 2, what I said before.
https://elektrotanya.com/parsek_mark...wnload.html#dl
Think the latest version has a Class D amp, in that case we are in trouble.

Post a gut picture or two to have a better idea of what you have.

[blindboybenton]

Hi you won't get the diagram from markbass , tried for years to get mark bass to provide. The amp in question has been repaired and long gone. Too my memory i replaced the main board with a new one. im in the uk i got from a uk supplier price was good.. well worth doing then you have a new bass amp
Please note, that im retired now and my business has been given to someone else
i can't give any more info
bbb

[Valvehead]

[Valvehead]

parts burnt....1 cap not pictured was totally melted

[Valvehead]

Designers tend to repeat pet ideas over and over, guess this one should be close enough.
FWIW thisVersion 3 is VERY close to earlier Version 2, what I said before.
https://elektrotanya.com/parsek_mark...wnload.html#dl
Think the latest version has a Class D amp, in that case we are in trouble.

Post a gut picture or two to have a better idea of what you have.

im not seeing my parts on there , looking for the 2 , 4R7J 5 watt resistors to start with . The 2 burnt caps ( one is a .1uf/400v ) and 5w resistors are connected to the relay and the 2 big inductors (coils)

[Greg Robinson]

Sorry for the necropost, but I just repaired A CMD 102P (same amplifier, different speakers in the combo) with the same fault. Figured here was as good as anywhere for posterity.

At least in my case, C86 - 680nF 400V failed first, causing R53 and R54 (4R7 5W) to overheat, leading to C87 (330nF 250V) and C88 (100nF 400V) melting, as they are right next to each other.

All these components, along with L2 and L10, form the class D output filter. L2 and C86 form the first pole, L10 and C88 form the second, and C87 is in series with the parallel pair of R53/54 to form a Boucherot cell/Zobel network. Without C86 to work with L2 to do the bulk of the filtering, R53/54 overheat, with escaped magic smoke and extreme EMI emission being the result.

[Jazz P Bass]

Thank you for that information!

[andrewleech]

At least in my case, C86 - 680nF 400V failed first, causing R53 and R54 (4R7 5W) to overheat, leading to C87 (330nF 250V) and C88 (100nF 400V) melting, as they are right next to each other.

I just want to say a huge thanks for posting this!

My partner's a bass player and loves her CMD 102P, perfect size for both practice amp and to take on stage where a PA does the heavy lifting!

At rehearsal a week or so ago, there was a "hot" smell, enough to turn off the amp and abort playing!

I'm the elec guy, opened it up the next day and left it running for a few minutes, yep a couple of _very_ hot 5w resisters. Nothing visibly cooked though, everything still looked fine around the silicon used on those parts.

If it wasn't for this post I probably wouldn't have had a lot to go on. I pulled off the larger 0.68uF cap and while it still looked visibly fine, capacitance measure on my good multimeter read only about 10n so yeah, not much use.
Quickly ordered a few replacement options for this and a couple each for the 100n and 330n just in case.

She had a gig in a week at that point, I was taking a gamble that it was only the same problem. Backup plan would have meant borrowing a friend's amp, so manageable. Cutting it fine, the parts arrived in 6 days!

Got the 0.68uF fitted and turned it on, thankfully, no more hot resisters!

I noticed there's a rather significant hiss from the tweeter, not sure if that was normal or because the other caps need changing.


Pulled the other two caps of and wow, they were not visibly fine behind the silicon!
One had a black round hole burnt right into it where it was sitting against the resisters. The other was bulging on the sides and cracked open underneath.
So yeah both got replaced.
Tweeter still hisses, going to ignore that for now.


I then went about adding some support to the parts. Knowing that most silicon isn't safe for electronics, I thought some acrylic based flexible sealant I had already would do the job nicely. Gooped it on around the caps, left it overnight to dry.

Next day, morning of the gig, test the amp and nope, doesn't work anymore! _any_ input of any level lights the clipping led, output never turns on (no sound from speaker, not even hiss)

Realised the only thing I'd done since previous test was the sealant, pulled it apart again and carefully pulled it all off with knife and tweezers. Some had smeared on the resisters and IC near the 0.68uf. Some, under the caps, hadn't actually dried. In hindsight that should have been obvious.
Oh well, got it all cleaned off again and thankfully it worked!
Though it seems to take a couple of seconds longer at power on before the output relay clicks on, so there might be a little leakage there somewhere still.

Either way it got her through the gig, and in a few days I'll try again with sealant, with electronics safe silicon this time...

Thanks again for sharing!

[drewl]

I have a newer model, Momark 800 with a similiar power supply/power amp in which one of the coils is burning up.
It took out C86 which I replaced with a .22uf (After reading this I'll install the correct value) and replaced the one coil that is getting hot.

Amp works but the coil is still getting very hot.

If this is the Zoebel, I'll try replacing the same associated parts on this amp, although the high frequency signal deosn't look too high, at about 1.5 vac around 220 something KHZ