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Mark Bass 600 something or another...

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  • Mark Bass 600 something or another...

    My shop is 14 miles from the U.S. and Mexico border. I have a lot of clients who play in bands on both sides of the border or back and forth and a lot of the times running on an overloaded and overworked generator to power their impromptu outdoor festivals (social gatherings). Sometimes they stick the 120 into the 240 or visa versa and a brief light show usually ensues causing their equipment to show up at my door.

    Here, I got a Mark Bass 600 something or another amp with a blown mains fuse. First thing was to replace the fuse and bring it up on the limiter… the limiter glowed brightly… yeah, it's a dead short, so as Australia's Dave Jones would say: "Let's take it apart!". Here's a shot of the amplifier/power supply board or, main board once I got it out of the chassis:

    Click image for larger version

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    On the left is the power amplifier and on the right is the power supply, nicely segregated. My first suspect for the short was the power MOSFET's in the amplifier but a quick diode mode check of their leads didn't show anything out of the ordinary, no shorts, no opens just what you would expect from a good working amplifier. I think that part of this board is O.K. so that leaves the power supply side. I can see two big MOSFET's on a heatsink in the power supply so this leads me to believe this is a split bridge resonator type of supply, not a traditional feedback coupled SMPS. Both MOSFET's were dead shorts. There's not a lot to this supply so I began the task of removing all the big silicon, heatsinks and a couple banks of filter capacitors that were just in the way. Now I could check the little things and even see them, also give the board a good close inspection:

    Click image for larger version

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    One problem here is that I don't have a schematic for this particular Mark Bass (Parsek) amplifier, I do have a closely related Mark Bass amplifier service manual but it's not exactly the same, mine is older for one thing. The manuals schematic for the power supply is nearly identical to the one before me, some of the reference designators are different and a few minor areas of the circuit have changed for the better or worse, I don't know. It's a good guide none the less as far as I can see. Here I have marked with red "X"s the shorted semiconductors I have found. I have not found any passive components that are bad or damaged so far but I do have reservations about IC5, the IR21531D split bridge driver. The one on the board has it's markings filed off (nice!!), anyway, it's timing circuit sets it to 100Khz and all the surrounding circuitry support it being an IR21531D… not an S.

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    As always I get to wondering what happened to cause this? The split bridge driver is rated to handle up to 600 volts so if it was a power surge then that was some power surge, was it that? I don't trust that chip anymore and plan to replace it. All the front end mains circuitry appears to be unaffected by whatever the event was but that stuff is very overbuilt and can probably take a bit of a beating.

    Anyone have prior experience or things I should watch out for, I'm working with an unsettling amount of guesswork here.
    ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

  • #2
    That's funny because at the moment I have the same power amp/supply to be fixed (my version is a little bit different).
    If you have T22 and T23 shorted, I would look at R76, R 77 and R78 (whether they are open).
    There was no need to remove the whole heatsink. You should only remove the part of the heatsink for the MOSFETs. This is much easier. You assemble it back by screwing the MOSFETs to the heatsink (with insulators) and then assembling the heatsink and soldering the transistors. But first check R76 and R77.
    What input voltage was selected when it failed? Often users set it incorrectly and this cause the amp failure.
    I wonder what you will find. I need to order few parts for the amp so I have to wait few days for them to arrive.


    Mark

    Comment


    • #3
      Originally posted by MarkusBass View Post
      You should only remove the part of the heatsink for the MOSFETs.
      That's the first thing I removed and it was nice to see that part of the heatsink separable. The rest of the junk came out because I found a short in the secondary rectifiers and that assembly is a magic trick to remove. I figured let me get a clean playing ground where I have an excellent chance to trace the board and confirm the circuits as well as getting in there to test the SMD's and small parts, fuses etc. The main heatsink part came out once again to get better access to the secondary rectifiers and allowed me to inspect the mains front end chain which is gooped down with a lot of silicon and difficult to see clearly. I have checked all the major little parts like those resistors and all looks good so far... I will continue checking until the rectifiers arrive.
      ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

      Comment


      • #4
        As always I get to wondering what happened to cause this?
        As Enzo often says , (unless there´s a gross mistake or abuse): "who knows why it failed?" (assuming it failed "by itself") .
        Parts fail , only variable is when.
        That´s why Service, Service Manuals and Techs exist.

        Seeing your dead semiconductor list, probably a secondary side rectifier failed, said short stressed and killed Switching Mosfets, we are not sure yet about IR2153

        Remember schematic is wrong on purpose, it mislabels +V and ground pins.
        Juan Manuel Fahey

        Comment


        • #5
          Originally posted by J M Fahey View Post
          Remember schematic is wrong on purpose, it mislabels +V and ground pins.
          Wow! that one flew right over my head... +V and ground pins... really? It looks like a correctly labeled schematic to me... the circuit looks about right to me as well... I'm just going to hammer the square peg into the round hole anyway, it might be fun... but, seriously? +V and ground?
          ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

          Comment


          • #6
            Originally posted by Sowhat View Post
            Wow! that one flew right over my head... +V and ground pins... really? It looks like a correctly labeled schematic to me... the circuit looks about right to me as well... I'm just going to hammer the square peg into the round hole anyway, it might be fun... but, seriously? +V and ground?
            Maybe he gave up cheating in your version, which caused a lot of anger against him, but this is the original circuit, as posted in the official Service manual.
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            There was an urban legend (maybe started by himself) saying that the original erased ID switcher was actuallt an IR2153xx but mounted upside down, with its back towards the PCB and exposing its "belly" ... which if followed would have caused the destruction of the Power Supply.

            Mind you, the error was on the schematic, but the PCB was actually fine.

            In any case, the current wave of resentment comes from the fact that he originally became known because of the "Made in Italy" amp and impressive quality, again Italian made B&C speakers.
            Yellow cone, but I guess they are made out of Kevlar and that is a popular colour.

            Only small problem is that current MarkBass amps are made in Indonesia (not bad by itself) BUT with much cheaper generic Indonesian speakers ... with yellow painted cones of course.

            Problem is he apparently "forgot" to make such news known to users ..... who eventually found out anyway.

            If you search, there´s quite a few threads speaking about this.
            Juan Manuel Fahey

            Comment


            • #7
              Juan,

              You are referring to Markbass Little Mark II while Sowhat is working on Markbass 600. In this case the schematic is slightly different and there is no mistake on the schematic regarding the IR21531D integrated circuit. For me it seems that R79 is missing on the schematic but I can see it on the PC board. So maybe there are other "surprises" on the schematic. I also wonder why the schematic is slightly different from LittleMark while it seems that the PC board is the same .

              Mark

              Comment


              • #8
                Dear Mark, both amps "should be" different, at least model is, but that´s not the point .

                The point is that the IR2153x was shown with the *wrong* pinout in the official MarkBass Service Manual and when people asked, they were told the wrong story about "custom chips".
                There´s some posts about it in TalkBass and elsewhere.

                That, coupled to the IC marking obfuscating (to put it mildly) sent people on the wrong track.

                It was hinted that it was a custom made Switcher while it actually was the plain vanilla one.

                The old Proverb goes "once bitten twice shy" so in this case, since there was already one example of mislabelling, it wouldn´t hurt warning Sowhat
                Juan Manuel Fahey

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                • #9
                  See... ya hang out with the right people and you learn something, that's what my daddy told me. I had no idea of this devious Italian deception, but I do now thanks to you. I've see this type of stuff here and there but I was unaware of the Parsek deception. I do of course enjoy it when I come across a chip with the markings sanded off.
                  ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                  Comment


                  • #10
                    The other thread about Markbass and the IR21531D chip was on this forum. Here: http://music-electronics-forum.com/t10881/
                    It is important that the replacement chip is with the letter "D" (with integrated diode).

                    Comment


                    • #11
                      Originally posted by Sowhat View Post
                      I do of course enjoy it when I come across a chip with the markings sanded off.
                      I'm quoting myself which is wrong but I have an additional thought about this chip. If it were indeed a custom chip why wouldn't you leave the marking on, there's no way anyone is going to find errata on a custom chip except for the people who designed it, your secret would be safe, no, it's ground off to hide the obvious. Just work on modern computer equipment for a while with all that hidden firmware lurking about, a lot of it in self destructing, tamper proof architecture, those are all FPGA's and the like where the part number tells you very little about what the application of the chip really does in the first place. There's plenty of skull-duggery going on in the business without having to grind off part numbers... how juvenile!
                      ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                      Comment


                      • #12
                        If you don´t like sanding, what about board gooping?

                        Juan Manuel Fahey

                        Comment


                        • #13
                          That's funny. A little more goop and you won't even need a chassis anymore. Make it into a ball and bounce/roll it to the gig.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            Originally posted by J M Fahey View Post
                            If you don´t like sanding, what about board gooping?
                            Mmmmm.... goopa goopa! Yeah, I'm not a big fan.
                            ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                            Comment


                            • #15
                              This one is fixed and out the door now. The IR21531D was OK but I replaced it anyway. It appears that D24 dual diode failed in the secondary rectifiers and this took out T22 and T23 the two switching MOSFETS on the primary, a pretty straight forward repair once all the dust settled. Thanks all for your support and comments.
                              ... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!

                              Comment

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