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    #1

    Blown Amp/Mixer

    I have received a GVDaudio Power Mixer BM8800 that has a problem with repeatedly blowing fuses. Upon opening it, I found a voltage regulator that was blown (KA7815). There is also a capacitor that is poorly soldered and is loose (marked in image).

    I looked around for a replacement VR, but as I have little experience with silicon, I would like a second opinion on my selection: http://search.digikey.com/scripts/Dk...296-13999-5-ND

    All of the capacitors are Rubycon/Nichicon/Chemicon, save a few. The two large input capacitors are 4700uF 63V "HD C.E.", which filter the 48V input. There are two 2200uF 35V "NOBBY", which filter the 18V input. Other than that, there are three 1000uF 25V "XINGLONG" capacitors and a 47uF 63V "NOBLE" capacitor. Any of these need to be replaced while I am at it?

    The unit is powered by a massive toroidal transformer, if anyone wants to see how big these things get compared to a SMPS.
    Attached Files
    Tags: None
    #2
    Re: Blown Amp/Mixer

    ouch.. never seen that happen to a good old 78xx linear voltage regulator
    especially since those are supposed to have a thermal protection built in.

    I'd replace the noname craps while i'm at it. i mean.. you're already in there.. so why not..
    looks like it's pretty crappy overall build quality though.. :/

    oh and don't forget to check the diode in front of the toasty 7815
    The 7815 from the digikey link should be fine. it's actually capable of 1.5amp output instead of 1amp of the toasty one, so it should run a bit cooler.. and perhaps not burn up

    Comment

      #3
      Re: Blown Amp/Mixer

      I'm baffled myself as to what could have killed it. Power surge perhaps?

      I assume the primaries on the 40V are fine, replacements for those are expensive anyway. I can replace the primaries on the 18V, however, if need be.

      I'll go ahead and replace the rest. I'm also considering adding a cap before the VR (lack of filtering may have done it in, possibly).

      Also, as I have never worked with audio equipment like this, what aspects would you say would make it a crappy build? Just some knowledge I can use for future reference.

      Comment

        #4
        Re: Blown Amp/Mixer

        Most likely the reg is taken from the main power supply. Equipment used in live events tends to see a lot of dirty power, from generators and such. Probably the AC supply was too high, and the manufacturer's failure to take into account such tolerances, made the rails go over the input voltage capability of the regulator, which is 40 volts IIRC.

        What is its rated power output? It looks like run-of-the-mill Chinese equipment.
        Originally posted by PeteS in CA
        Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
        A working TV? How boring!

        Comment

          #5
          Re: Blown Amp/Mixer

          The unit is rated for 250W per speaker, for a total of four (2 left speaker outputs, 2 right outputs). So an assumed total of 1000W max power.

          I looked at the transistors responsible for output (see attached image) and there are two Toshiba 2SC5200 NPN transistors and two 2SA1943 PNP transistors. Each is rated for 15A, so at 40V that would give a maximum of 600W per channel. So looks good at least on the transistor side. Seems the rectifier bridge (GBJ25J) can handle 25A @ 100C, so that just makes 1000W at 40V. Looks like they're really pushing it on rectification. So probably an overrated unit.

          I could attach an MOV to the input, if you think that would improve unit durability.
          Attached Files

          Comment

            #6
            Re: Blown Amp/Mixer

            Originally posted by Trinite View Post
            that just makes 1000W at 40V. Looks like they're really pushing it on rectification.
            And they're pushing it on regulator input voltage as well. If the reg is indeed fed from the main rail, there is zero headroom. Check that before you replace it.
            Originally posted by PeteS in CA
            Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
            A working TV? How boring!

            Comment

              #7
              Re: Blown Amp/Mixer

              that, plus basically no cooling.. wtf?

              Comment

                #8
                Re: Blown Amp/Mixer

                Originally posted by Th3_uN1Qu3 View Post
                And they're pushing it on regulator input voltage as well. If the reg is indeed fed from the main rail, there is zero headroom. Check that before you replace it.
                The main transformer has two windings. One for the 40V output and another for 18V output. The VR draws from the 18V rail, so it is within spec. It has no heatsink, but there are 3 other VRs that do, so assume the one that blew provided power to a relatively light load (or should at least, anyway).

                Comment

                  #9
                  Re: Blown Amp/Mixer

                  And really poorly designed!

                  All the regulators should be on one big plate or finned heatsink (insulated as necessary) and arranged properly. Not willy nilly. And that 4 transistors does not looks like can do 250W per speaker. Not at all. Regulator tend to die by voltage falling off or shorted, and should had fuse on output or 1 ohm resistor just in case. Exploded regulator means output might had shorted out.

                  FYI: good linear regulators have thermal overload built in, this one is fairchild brand.

                  Good one tend to have 8 transistors and two even four transformers for that claimed wattage much and 1KW from that transformer. Impossible.

                  PS: be really wary if the amp transistors are true copy, there were plenty of fakes out there on that particular part numbers were popular ones to copy using really underrated semiconductors. Make sure you get these from good supplier.

                  Cheers, Wizard

                  Comment

                    #10
                    Re: Blown Amp/Mixer

                    Couldn't find any specs on the transformer, but going by weight (2.2kg), it would seem to be comparable to a 250VA toroid. So, more like 250W combined max output for the entire unit - and even that is probably stretching it.

                    Comment

                      #11
                      Re: Blown Amp/Mixer

                      Originally posted by Wizard View Post
                      Regulator tend to die by voltage falling off or shorted, and should had fuse on output or 1 ohm resistor just in case. Exploded regulator means output might had shorted out.
                      Tested the output path, no obvious shorts. Seems that it provides the selection voltage for switching between 12V and 48V phantom power on the mic connections. So no major load on it, apparently. Odd that it would blow. However, it does seem that it draws directly from both 18V rectifiers, one supplying +18V to the input pin and the other supplying +18V to the common pin. Perhaps too much dirty power for it handle? There is a spot on the PCB to add a filter cap, so I ordered one for that.

                      Comment

                        #12
                        Re: Blown Amp/Mixer

                        Never seen a reg blow due to input ripple. It might just be the occasional dud part - for example, the 78L05 supposedly has short circuit protection. I tried it out and it left a hole.
                        Originally posted by PeteS in CA
                        Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                        A working TV? How boring!

                        Comment

                          #13
                          Re: Blown Amp/Mixer

                          the 78Lxx's don't have any protections against short circuits or too high temperature.

                          all the others do though..
                          - the 78Mxx (500mA)
                          - the "normal" 78xx (1A ; the ST 78xxCV up to 1.5A)
                          - the 78Sxx (2A)
                          - and the pretty rare 78Txx (3A) in TO3

                          Comment

                            #14
                            Re: Blown Amp/Mixer

                            Then why does everyone list it in their datasheets in big bold text? http://www.datasheetcatalog.com/data.../0/78L05.shtml
                            Originally posted by PeteS in CA
                            Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                            A working TV? How boring!

                            Comment

                              #15
                              Re: Blown Amp/Mixer

                              no idea.. i've never seen a TO92 78Lxx where the short circuit protection actually worked.. it always blew them ._.

                              the normal 78xx's just shut down and continue working after a soft restart (if the short is gone obviously..)

                              Comment

                                #16
                                Re: Blown Amp/Mixer

                                From what I can see, all the traces running out of the VR terminate in resistors, the lowest of which are around 460 ohms. Hoping then that it was just a defective part.

                                As for build quality, this is laughable. I dug a bit deeper into the unit and found an improvised connector cut out of a larger part (great way to cut corners...literally). Check the photos below:
                                Attached Files

                                Comment

                                  #17
                                  Re: Blown Amp/Mixer

                                  It looks like it's been cut with a hacksaw.
                                  Originally posted by PeteS in CA
                                  Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                  A working TV? How boring!

                                  Comment

                                    #18
                                    Re: Blown Amp/Mixer

                                    And a few other deficiencies I've noticed. The power switch is located on the neutral side, leaving everything before it live, even when off. Also, there is nothing attached to the ground connector.

                                    Comment

                                      #19
                                      Re: Blown Amp/Mixer

                                      Originally posted by Trinite View Post
                                      The power switch is located on the neutral side, leaving everything before it live, even when off.
                                      More common than you might imagine. Here in Europe the mains plug isn't polarized so it can be inserted either way. It's a 50/50 chance which end goes to the power switch since no one bothers with double pole switches.

                                      Originally posted by Trinite View Post
                                      Also, there is nothing attached to the ground connector.
                                      Built-in ground lift! Woohoo!
                                      Originally posted by PeteS in CA
                                      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                      A working TV? How boring!

                                      Comment

                                        #20
                                        Re: Blown Amp/Mixer

                                        Originally posted by Th3_uN1Qu3 View Post
                                        Built-in ground lift! Woohoo!
                                        There is a 75V potential between the transformer center tap and ground

                                        Although I would assume it is not very high current.

                                        Comment

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