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Thread: Markbass Little Mark II

  1. #1
    Supporting Member Alex R's Avatar
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    Markbass Little Mark II

    Guy came through the door today with one of these Italian amps. First one I've seen. I said, oh, right, it's a preamp, thought you said 500 watts. He said, no it's specced at 500 watts into 4 ohms. And it is. Tiny and light (and silent - the fan runs but the protection relay is in).

    I took a look inside - no big mains transformer. The mains seems to go to two switching mosfets (which have both shorted), and the supply goes after filtering to the output stage from there. Then there is a little transformer for the smaller devices.

    Well I will change the mosfets and cross my fingers, as most of the rest of it is surface mount. The output devices are not shorted.

    Any comments on this design? Any experience with repairs? If it works it is likely to catch on, at least in Europe, as this is an astonishingly light amp! But can it be safe to do away with the isolating effect of a mains transformer?

  2. #2
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    Hi Alex,
    I currently have one under repair, and, having inspected it pretty thoroughly, I guess one of the reasons they're so light is the neodymium speaker (er...magnet), the other reason is the absence of a mains transformer.

    The one I have here is just fine, only the speaker's coil has "gone south", seems like the output devices are rated to withstand the voltage rise due to an open load....

    Though they seem well built, and they have an extremely favorable power vs. weight ratio, I don't like the absence of a transformer and the subsequent lack of insulation from mains. Looks like they revived some of the concepts used by Gallien/Krueger back in the 80s....those amps were extremely compact, but their reliability was rather poor.

    This said, I don't want to judge them prematurely, I think they've been on the market for too short a time to have a dependable reliability record.....I guess time will tell ( My 44 years old AC30 is still going strong, but that's really from another era ), let's wait and see....
    Regards
    Bob

  3. #3
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    Welcome to the wonderful world of switching power supplies!

    I have not seen one of those models, but have seen my share of switching power supplies. They are being used everywhere now-a-days. I've got a Peavey CS-600 on the bench right now that has one. The first time I ever saw one was nearly twenty years ago, in a Walter Woods head. Now every company is using them, especially in high power bass and PA heads.

  4. #4
    Noodle of Reality Steve Conner's Avatar
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    All of the audio stuff, input jack, speaker jack, power amp, whatever, is isolated from the mains. It's only part of the power supply (the two big MOSFETs) that is live.

    Basically how it works is, the mains gets rectified to about 300 volts DC, and smoothed by a few decent-sized electrolytics, this then gets chopped up at high frequency by the two big FETs, and the resulting AC is stepped down though a small ferrite-cored transformer. This is then rectified and smoothed again, to make the DC rails for the power amp. Just like a PC power supply, the only difference is the choice of output voltages.

    If you can't see any decent-sized heatsinks, the power amp is more than likely a Class-D, which works by chopping the DC rails up again (using yet more MOSFETs) into a high-frequency pulse train that approximates the audio signal. This gets reconstituted by a low-pass filter before feeding out the speaker jack.

    The ferrite-cored transformer provides isolation from the line in just the same way as the iron-cored transformer in an old-fashioned power amp. It might only be about 1.5" cubed, but it can still pass those 500+ watts no problem.

    I hate to sound cynical beyond my years, but the MOSFETs don't often blow by themselves. More commonly, something else goes wrong first and takes them out, or they take a bunch of other stuff with them when they go. I think out of all the SMPS I've mended, there was maybe one that had a blown MOSFET and putting a new one in was all it took to make it work again.

    steve
    pontiacpete likes this.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  5. #5
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    Well,
    As I told already I've seen switching power supplies in (bass) amps before, back in the 80s I remember a friend of mine having more than a headache with his GK ( seems to me it was an ML250 but I'm not sure ). Those amps surely had some design flaw, i.e. they were designed with size reduction in mind, so they were prone to thermal breakdowns due to insufficient heat removal.

    I don't want to look romantic, or old-fashioned ( and well, maybe I am ) but, working as an automation engineer, I often deal with Switching PS and PWM inverters and drives, and yes, switching PS are more efficient, but the idea of seeing a switching PS inside an amp makes me sad nonetheless.....Oh, well, I guess I can get used to the idea as long as we're talking 'bout bass amps.....and I know we can't stop progress, but I fear a day will come we'll see a switching PS inside an AC30, a JCM3000 or a Bassman
    Regards
    (sad) Bob

  6. #6
    Supporting Member Alex R's Avatar
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    No, there are big heatsinks in there, I think the power amp isn't too strange.

    The power supply mosfets are on a slab of aluminium.

    Thanks for your explanation about that ferrite core transfomer, I just didn't think all that power could be supplied through that little thing. I've never know how SMPSs work and i'm now a little nearer... a couple of questions to try your patience:

    Is the mains usually rectified by a normal-type bridge?

    The switching chops up the resulting 300vDC so this makes the transformer work - what's the aim of this? How come so small a transformer can be used?

    The weight difference from a standard power supply is quite dramatic (this is the head, not the combo).

    All the diodes and transistors around the mosfets gave me a beep ok so I'm crossing my fingers really hard... but that's a reason why I kept the customer here till I got the mosfets off the board, confirmed they were shorted, and sniffed around for anything else gone, so he understood that he might have to pay out to get this work done and still have no amp, if anything surface-mount has gone too. Because I don't have the gear for SMDs - it was good to be able to show him some so he could understand the difficulty. Is anyone taking SMD work on yet? I guess we will eventually have to...

    I suppose the transformer has to be a suspect?

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    Quote Originally Posted by Alex R View Post
    The switching chops up the resulting 300vDC so this makes the transformer work - what's the aim of this? How come so small a transformer can be used?
    In a nutshell ( and I'm sure Steve can explain this way better and more thoroughly than me):
    Because of the very high switching (carrier) frequency, the ferrite core and the higher input voltage (less current at the primary for a given power); all these factors together improve transfer efficiency, the losses in the "iron" ( er....ferrite) and in the windings are close to zero, all these factors combined together allow for a smaller transformer if compared to conventional (linear) PSs.
    Regards
    Bob

  8. #8
    Noodle of Reality Steve Conner's Avatar
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    It's just a fact that transformers can be made smaller without loss of efficiency, as the frequency of the AC they're transforming gets higher. That's why they use 400Hz on board aircraft.

    We use 50Hz over here, and all our transformers are slightly bigger for the same VA rating than American 60Hz ones.

    Some parts of Canada used to use 25Hz, and the tube radios sold there had a PT twice the size. I think the radio maker kept the same laminations but used a stack twice as tall, the result was a freakish looking PT.

    These switchmode power supplies run at something like 50 or 100kHz. The ferrite core is necessary because regular iron laminations would get red hot from losses at these frequencies.

    The transformer has far fewer turns of thicker wire than a line-frequency type, so it's unlikely to have burnt out.

    The mains is rectified by a normal 4-diode bridge in 220V countries, and sometimes a 2-diode doubler in 120V land. Some newer designs have an active power factor corrector in there too.

    As we used to say in the industry, "Behind every successful SMPS design there's a lot of chicken blood"
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  9. #9
    Supporting Member Alex R's Avatar
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    Quote Originally Posted by Steve Conner View Post
    As we used to say in the industry, "Behind every successful SMPS design there's a lot of chicken blood"
    - don't have a chicken to hand as I used them all up making bad medicine for Scottish Power the last time they overcharged us... way more effective than ringing the call centre. Do you think it might have the same effect if I waved a cooked turkey wing over the amp before I try to fix it?

    So a switch mode power supply is about creating higher-frequency AC with rapid-switching devices so that you can use a smaller transformer? Can it be that simple and I failed to understand it for so long? (story of my life...)

  10. #10
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    Quote Originally Posted by Alex R View Post
    So a switch mode power supply is about creating higher-frequency AC with rapid-switching devices so that you can use a smaller transformer? Can it be that simple and I failed to understand it for so long? (story of my life...)
    More or less, yes, the other part is about how voltage gets regulated, and involves a technology called PWM ( Pulse Width Modulation ) : in ( another ) nutshell, the duty cycle of the waveform is made variable, e.g. you can get minimum voltage with a 10% - 90% duty cycle ( 10 % of the period ON - 90% of the period OFF ) and maximum voltage with a 90% - 10% duty cycle ( 90% of the period ON and 10% of the period OFF ). The filtered voltage is an average, then a voltage error signal is fed back to control the duty cycle to have the output voltage match the desired one.
    Regards
    Bob

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    Sorry for digging up an old post, but this seems like an appropriate thread to ask this question. I'm attempting to repair the two switching mosfets in a Markbass Little Mark Tube 800. The mosfet part # is IRFP27N60KPBF-ND. I tried to order two from digikey but the supplier has recently discontinued the chip. Just wondering if anyone could suggest a direct replacement? Here is the digikey page for the chip if that helps: MOSFET N-CH 600V 27A TO-247AC - IRFP27N60KPBF

    Thanks

  12. #12
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    That part is still certainly being made, but MosFet naming conventions changed -.
    In the old times "27"N"60" meant "27 (Ampere)" N "60(0V)" and so on, while that part name will now be IRFP "somenumber" as in IRFP450 or whatever.
    Search International Rectifier site to find a TO247 case, 27 ampere 600V part, whatever they call it now, and that's it.

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    Thank you so much for the advice. When I search for that part, I can't seem to find TO247 with Vdss of 600V. I'll keep looking since you've got me on the right track. If you find something, please let me know. I'm so excited to get this amp going.

    Thanks,

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    This is pretty close: STW28NK60Z STMicroelectronics MOSFET Power
    Would you recommend this, even though max power dissipation is less and Rds is less? Sorry for my lack of knowledge in this area. Thanks again!

  15. #15
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    It will surely do.
    350W dissipation is monster, and, besides, nominal. It will certainly dissipate around 30W at most, and that because of switching losses.
    Voltage and current match. Lower Rds is a "better spec".
    Looks good.
    Buy 4 , cost is nil.
    For what it's worth, *I* would use those parts.
    EDIT: I forgot, check everything, *something* killed the earlier MosFets.
    See if you can power that supply without load (the amplifier disconnected) ; another problem is that switchers many times don't like variacs or bulb limiters, so you will have to take your chances by connecting it straight to the power line.
    Last edited by J M Fahey; 06-21-2010 at 09:50 PM. Reason: Edit.

  16. #16
    Senior Member Enzo's Avatar
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    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.
    Education is what you're left with after you have forgotten what you have learned.

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    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.

  18. #18
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    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.

  19. #19
    Supporting Member Alex R's Avatar
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    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 ;-)

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    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
    Last edited by MarkusBass; 07-04-2010 at 10:27 AM.

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    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

  22. #22
    Senior Member oc disorder's Avatar
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    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.
    Attached Thumbnails Attached Thumbnails mark-bass-parsek-smps.jpg   fuses.jpg  
    Attached Files Attached Files
    Last edited by oc disorder; 07-09-2010 at 02:49 AM. Reason: Fuse Info

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    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

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    F1 and F2 = 0.22 Ohm fusible.

    Mark

  25. #25
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    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.

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    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

  27. #27
    Supporting Member Alex R's Avatar
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    This is a useful thread. It's good to have an alternative to replacing the board. Thank you for working it out Mark.

  28. #28
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    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.

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    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

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    Quote Originally Posted by J M Fahey View Post
    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

  31. #31
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    Hi Mark.
    I live in Argentina and I use EI42 because that's what's abundant here.
    To be more precise, I have access to a factory which makes them, for me, on a custom order basis.
    They just make the powder, and press and sinter it in whatever die they have available, my choice.
    Their largest regular one is the one I mentioned.
    The cost is nil : I get 200 E42-21-15 for U$100.
    When I go to pick them up personally, I will have a long talk with them about reverb tank magnets.
    They can make them for me, although the 5000 unit minimum order is still a little steep for me.
    Maybe I'll have to provide the die.

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    Please note that there are different core materials possible and not all of them can work with 125kHz switching frequency. I suggest checking ferroxcube catalogues.
    Another problem is that E42 has square bobbin whereas ETD34 has round bobbin. It makes winding the transformer (with ETD34) much easier. And with rounded bobbins you get better parameters of the transformer (lower leaking inductance because windings can be better coupled). I would test first whether E42 can be used in such a SMPS .

    Mark

  33. #33
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    Yea, I know.
    These guys provide datasheets, but I always *measure* everyhing.
    Their material is good, though.
    I know people who uses it; I even built many 12V to +/-42 or +/- 52 converters for car amplifiers and Street use PA systems with those EI42 cores; now I'm going the other way, rectifying 220 VAC to 308VDC and turning that into +/-70V.
    I don't *need* regulation , in fact, remember I'm replacing a heavy 50Hz iron transformer with which I am very happy, except for its weight.
    Winding those square bobbins (I do not have a supplier for them yet, the tests will be with homemade *cardboard* ones) will be much easier than winding the much larger, many many more turns square conventional ones; besides I will use no more than 30 or 40 a Month, at best.
    The problem that IC solves is that I needed a half bridge driver, not an easy thing to do because both halves of the circuit rest 150V apart.
    In fact I do not know how *they* do it.
    My 12V to +/- rails converters were push-pull , driven by oscillating CD4027's, believe it or not.
    The same solution (not yet discarded) would force me to use at least 800V or better yet 1200V Mosfets, not an easy thing to get.
    Link to these fascinating guys:
    Ncleos E
    Pagina nueva 3
    Even more fascinating is that they custom build for *me*, incredible !!.
    They were former Engineers at the local Philips plant, which was dismantled in the late 70's, they pooled their savings and bought most of the machines, at scrap iron prices.
    They already had the know how.

  34. #34
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    OK, so they most probably are able to wind the transformer for you. I have also such a company - they would be happy with an order of 100 pieces but I usually order 1-2 pieces .

    Mark

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    Have you noticed that pins 1 and 4 (Vcc and COM) of IC5 are reversed on the schematic (if this is IR21531)?
    I noticed on the one i'm working on. I was hoping it was an error in the schematic but it is not.
    I verifyed continuity to ground at pin 1.
    So the IR21531 is not a drop in replacement as far as I can tell. I have looked at just about every self oscilating half bridge drive they sell and nothing that has pins 1 and 4 swapped.
    I'm stuck trying to find one of these. Any one have any luck?

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