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Marshall Valvestate 2000 avt150 fuse help plus.

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  • Marshall Valvestate 2000 avt150 fuse help plus.

    Hi, I have a valvestate 200 avt 150 by marshall. Blew my first fuse took it apart and replaced it, blew it again. So I eliminated the transformer by plugging one connector in at a time. All was fine until the last two (Blue and Black chord) From what I can tell the black looks like a ground and the blue seems to go to the Positive of some black dohawker connected to the board, that thing is labeled KBU1003G. The spots that the wires go to on the chip have white writing "W102" is the black wire and "W103" is from the blue wire. I can't read schematics any help would be appreciated, thanks.

  • #2
    The "black dohawker" is a 10 amp bridge rectifier.
    W102 goes to the center tap on the secondary of the power transformer.
    It is also connected to ground.
    W103 & W104 is the ac volts from the transformer. Relative to ground.
    The "black dohawker" converts the ac volts into a useable dc volts.
    If the fuse blows when these wires are connected to the board, then either the "black dohawker" is shorted or something downstream of it is.
    In that case it is most probably the ouput ic's. TDA7293.
    You are going to have to take this to a tech.

    Comment


    • #3
      Hm, It was my belief that the transformer converted ac to dc and this is after the transformer.... So does ac power pass through the transformer and those wires don't change to dc until they get to the recitifier?
      Also I'm only having troubles when I hook up the ground or w102 and w103, w104 doesn't blow. Doesn't that mean my problem is with those two wires, If it were past this point why wouldn't it blow w104?
      I don't have the money for the tech so it's forums or the library...

      Comment


      • #4
        Transformers can only operate with AC currents. They transform one AC voltage into another. Then the result is rectified and filtered.

        When you connect only one wire, no circuit is completed, so current cannot flow, so fuses do not blow.

        Furthermore, if one side of your bridge rectifier is shorted, then connecting a hot wire to it will blow fuses, but connecting to the other side may not.

        Check the four sides of your rectifier for shorts, but mainly disconnect all the cables from the two small circuit boards screwed to the heat sink. Each board is as long as a business card but half as wide. And each has a 15-leg IC on it. Those are your power amps, and are the most common rason for blown fuses. See if the amp will fire up and hold a fuse with them disconnected. If so, they are blown. They are indeed the TDA7293V that Jazz mentions.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Ok, I unplugged both boards and was able to plug all wires to the transformer so it's those boards that are bad, if one is bad is the other bad? I ask before I try it because I'm down to my last fuse before I have to buy more. Also where would I be able to find a replacement for those pieces, do I need to replace the whole board or just a part on it? I have a tester (Multometer?) to test electrical current, can I use that to find the bad part (if I can replace just one piece that is).
          Oh as well those fuses are just about impossible to find except for on the web and they cost 3 bucks and 5 to ship.... Could I put a different fuse holder on the chip so that I could buy one inch fuses with the same ratings? I don't know if those fuses are an english thing or what but I've been to every hardware locally and a few scattered about michigan looking for them.

          Comment


          • #6
            They're probably 5 x 20mm fuses, a European size, but you should be able to get them in the USA. I have seen US-made equipment with them in.

            Use the forum search function and take some time to read about a device that we call a "light bulb limiter", it will soon pay for itself at 3 bucks per fuse.

            I believe you can buy the replacement power amp boards from Korg, or whoever handles Marshall in the USA nowadays. Or you can buy the large black chip (the "TDA7293V") separately from an electronics distributor such as Mouser, and replace them yourself, if you fancy your soldering skills.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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            • #7
              They are a 5x20 4 amp 125 volt fuse, which I did find some but, none of the "slo-Blow" type. A piece of what looks like white fiberglass in the middle with a wire coiled around it. I found that style but never in 5X20, only one inch versions... So I was thinking that I could put a one inch fuse holder in the amp and than When I blow another fuse it will be easy to find them here. But like I said those fuses cost about 10 dollars after shipping...
              Also I'm not sure if I'm on the right page now. The boards that were described by Enzo are not black, they are on the Fan housing and like he said, a business long but half as wide.

              Zoinks... ok just found the black boxes... I was thinking it was the entire chip. To make it right the black boxed are literally attached to the fan housing with white goo on the back (correct me if I'm wrong) they are labeled TDA...
              These are the pieces I need to replace?

              Comment


              • #8
                Yes. They are the TDA7293V chips that Enzo was talking about.

                The business card shaped thing is a "board" - printed circuit board.
                The black box with many legs is a "chip" - integrated circuit, informally known as a silicon chip.

                If you can't find slow-blow, regular should do. It's actually better if they blow quickly at this stage in the game. No reason not to put a 1" holder in, if you can make one fit.
                "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                Comment


                • #9
                  The chip that I'm seeing doesn't have a V at the end, I don't know if that makes any difference. Would I be able to cut the prongs leaving them attached to the board and just solder the new chip to the existing prongs? I found the chips here
                  Output Modules
                  Is this a good price or is there somewhere better.
                  Do I need more "Fire"putty to put on the back of it or should the stuff left over suffice?

                  Also why would these go bad? The amp is about 12 years old so maybe that's it. or is it a sign of worse problems?

                  Comment


                  • #10
                    You can do whatever you want as long as the prongs end up connected solidly to the right place. If any come loose the chip may blow again.

                    Price looks OK, I guess. Mouser might be cheaper but they might have a minimum order or shipping fee. It is important to get the right one: if you don't order the "V" type then there has to be a sheet of insulating material between the chip and the heatsink (or "fan housing" as you called it.) If this is missing, it will blow again. If you didn't find such a sheet in there when dismantling it, you should get the "V" part, as it has the insulator built in.

                    That is not "fire putty", it's Thermal grease - Wikipedia, the free encyclopedia - sometimes you can wipe the old stuff onto the new chip, but you should be able to get it cheaply from your local computer store anyway.

                    Valvestates seem to blow up now and again. I guess maybe because it was abused by cranking it up into a load of too low impedance, running it really hot, shorting the output, whatever.
                    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                    Comment


                    • #11
                      TDA7293

                      I thought the "V" meant vertical.
                      As oppossed to "H", horizontal.
                      Both IC's have the B- dc voltage rail tied to the metal back plane.
                      You do not need an insulator because the fan housing has isolators to keep the heat sink away from chassis ground. Although a small amount of grease is called for.
                      One way to tell if the ic is bad is to measure the resistance from pin #13 (B-) to #14 (output) & pin #15 (B+) 7 pin #14 (output).
                      (Consult the attached data sheet for pin assignments).
                      That is usually how I see them fail. Shorted.
                      Once you get the board out & on your bench, it is o/k to clip off the old ic.
                      The old stub of a pin can be removed by using a solder sucker.
                      Or by wetting (melting) the solder joint with your iron & GENTLY tapping the board on a hard surface.
                      Please keep in mind when reinstalling the power connectors, that the power rails must be drained of any residual voltage.
                      Or you just may "POP" your brand new TDA7293 chips.
                      Attached Files

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                      • #12
                        Ah, sorry, the V is for Vertical. I'm getting confused with the LM3886TF.
                        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                        • #13
                          Thanks for the attachment. I'm finding that only one is bad, should I still replace both of them. How much can I screw up with soldering, I'm not terrible just never anything this small. On one chip I got a reading of "0" and on the other it was 3-4

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                          • #14
                            Ok, the one that reads "0" has a crack and what looks like a spot that burnt/exploded.

                            Comment


                            • #15
                              "The other one reads 3-4."
                              If you are indicating 3-4 ohms, that one is toast also.
                              What you are reading when you go from either power rail to the output pin is the output transistor(s) inside the package, Collector to Emitter.
                              It cannot be a low resistance reading.

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