Markbass Little Mark II

[Enzo]

I have been fixing switchers in one industry of another for 30 years, and the most common failure I see is shorted rectifiers on the secondary side. I agree the switching MOSFETS probably died for a reason, so first thing I'd check beyond them would be the rectifiers. Most switchers just shut down then the rectifiers sgort on the secondary, but don;t count on that every time.

[bma1984]

Thanks again. When you say to power the supply with no load, do you mean disconnect everything past the switching MOSFETS? I can certainly try that after I receive the new parts. I have tested every component I can think of, and the MOSFETS were the culprit. The amp was immediately blowing the fuse in the power stage when powered on.

[J M Fahey]

Not that much.
I would try the supply disconnected from the power amplifier board, just in case it is shorted or misbehaves somewhat.
Do what Enzo says: check that the supply rectifiers are OK. He has seen tons of supplies where they were dead and killed the MosFets.
Does the PSU have any commercial controller IC? Dig its datasheet; many switchers follow the factory schematic suggestion, since they are not on the switcher business but in the musical amplifier business; the switcher is often an afterthought.

[Alex R]

Markbass will supply bonafide repair shops with a new board at a pretty reasonable price. On the ones I've seen, that replaces the SMPS and the rest of the amp into the bargain. Not something I spent much time debating with myself ;-)

[MarkusBass]

I think that there are not that many parts that could fail in this board. Check Zener diodes and diodes on the secondary side. Unfortunatelly, the diodes are shorted by the transformer so you cannot test them in circuit. Another part that could fail is the driver IC (it drives directly gates of the MOSFETs). What driver IC is used in the amp?
Make sure that the 117/230V jumper is set up correctly. It's the main reason (I mean curious user - what happens if ...) why such a SMPS can fail. I think that the driver can be tested without MOSFETs but this is very dangerous (lethal mains voltage on this part of the board - be vey careful). From which part of the world are you (or which mains power voltage you use)?

PS: I did not repair Markbass but I was repairing similar amps. I've seen about 20 bass amps with SMPS from different manufacturers - they are very popular.

EDIT: what about this transistor:http://www.vishay.com/product?docid=91219&query= ? You can even get free samples.

Mark

[jp3zone]

Does anybody know whan the part nr. is for the driver ic in the power supply, The only info on the schematic is its part designation, ie. IC5, nothing else, and Markbass , in their infinite wisdom, have sanded off the actual markings on the ic...

Any help would be appreciated.
Thanks

[oc disorder]

IR 21531

I'm fairly sure its a IR 21531.
I was wondering what the value of the 3 fuses are.
They appear to be half watt ? fusable resistors.
There's no mention of them that I can see
Will edit in the colour bands shortly.
For those interested there is a "service manual" here
Mark Bass Parsek LittleMarkII combo.pdf
Mark Bass Littel Mk2 combo Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

F1 and F2 Black-Red-Red-Silver-Gold not sure about the black first band
although apparently it denotes a fuseable resistor.
so 22 x .01 (silver multiplier) = .22 ohm 5%
and
F3 Orange-Orange-Black-Silver-Gold so Black= 0 in this case no significant 3rd digit
thus 33 x .01 = .33

I'm puzzled by the two different types of colour bands or should I read the F1 & F2 as
0200 x .01 = 2 ohms 5% ?

There are some internal pictures here of a Mark Bass Model.
Markbass LMII: an inside view - TalkBass Forums
There are also variations or different model types and the service manual is the 2005 edition.

[MarkusBass]

Have you noticed that pins 1 and 4 (Vcc and COM) of IC5 are reversed on the schematic (if this is IR21531)?
Also several other parts are not marked, e.g. Zener diodes. Is it on purpose? And if they fail, should I call Markbass Italy?

Mark

[MarkusBass]

F1 and F2 = 0.22 Ohm fusible.

Mark

[J M Fahey]

I *loved* that power supply.
That driver IC sure simplifies things.
I'll try to get some, I've already ordered 100 EI42 cores (the largest reasonably available here), not only will it lower cost and weight but make me easier to pass HIPOT tests and such, to be able to export.
Thanks for the tip.
PS: that PSU is really simple.

[MarkusBass]

It really couldn't be simpler. However, please note that there is no output voltage stabilization (you can only switch primary windings to get desired output voltage), there is no short circiut protection. I also think that some parts are missing on the schematic. E.g. there should be diodes connected to MOSFET gates (typical in half-bridge configuration). Do you want to build it?

Mark

[Alex R]

This is a useful thread. It's good to have an alternative to replacing the board. Thank you for working it out Mark.

[J M Fahey]

Yesw, I want to get those ICs and build one.
I really *need* them for my 300W+ bassamps; not that necessary on the 100W ones.
Anyway I'm sure in , say, 5 years, I'll not be using iron anymore.

[MarkusBass]

You're welcome. But I think it was Oc Disorder who found out what the chip was. It seems that the supply can be easily fixed but there are few pitfalls. The PC board is double-sided so you need some special tools to desolder components (e.g. MOSFETs or the driver IC). Also there are some important security issues when fixing such amps. E.g. if the heatsink of the switching MOSFETs is grounded (connected to the enclousure), the MOSFETs are usually attached to the heatsink with fat ceramic insulators. And this cannot be changed and has to be thoroughly examined after the amp is fixed. It's to best to understand how the SMPS works .

Mark

[MarkusBass]

Yes, I want to get those ICs and build one.
I really *need* them for my 300W+ bassamps; not that necessary on the 100W ones.
Anyway I'm sure in , say, 5 years, I'll not be using iron anymore.

The chip can be purchased e.g. from farnell. I don't know why you are buying EI42 cores. On the photos from Talkbass you can see that the core used in the amp is ETD34. With 125kHz switching frequency it can handle up to 500W.

Mark