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Marshall tsl122 Bias problem

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  • #31
    Thanks for the reply but how can I order a replacement board when an authorized tech for my country doesnt even know for this problem and he wont be willing to order a new board i already asked him about that problem and he said it's internet rubbish how can i get a new board then?

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    • #32
      You will need to find another Marshall tech. I'd advise doing that anyway as the one you've been talking to is either fobbing you off or very short of experience with Marshall amps. Marshall don't offically admit to this problem but believe me they know about it.

      Marshall themselves have a repair dept in the UK and it is possible to call them up.


      What country are you in?

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      • #33
        I'm from Croatia. I though of calling marshall factory and ordering a new board but then i found out that only authorized techs can do that.

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        • #34
          This is the board & the part number.
          Link: Search | Antique Electronic Supply
          Find out if they will ship to Croatia.

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          • #35
            There you go. One further point ... Marshall cured this problem by using a new material, you can recognise the new boards, which are rather darker green than the old ones, because you can see the glassfibre weave beneath the surface of the material. The older boards have no glassfibre and no weave visible at all. If your board has this woven material you do not have the bias drift problem in its standard form. But I'll bet you a tenner you have the old plain light green board.

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            • #36
              This board leakage measured with a really high Z input meter, like a HP410C or B but probably not with a DMM.
              A common cause of boards to have leakage is during fabrication, the resin and curing agent are in the wrong ratios so the resulting board becomes hydroscopic and absorbs moisture, even can swell a bit. Old phenolic boards did this also but due to a different process.
              I have helped such boards by desiccating them with heat and vacuum after a thorough soaking in alcohol for a few days but a new board sound like it is easier if you can find one.
              None of the factories support their gear in other countries, no parts are available and no service centers. We have that problem here, the distributors refuse to even repair out of warranty units and will not supply parts. That really holds back the market from ever developing, which here would be the second largest to the US. The first company that take customer service seriously here will be rewarded with capturing a major market. It is really really dumb to write off all these other countries and millions of potential customers with a lot of disposable income to spend, and then complain that sales are flat or weak.

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              • #37
                Thank you all guys, you have really been of great help, I'll try to do everything to avoid the purchase of a new board so here's what I plan on doing next :

                1. Isolate whole bias circuit from the main board and recreate it on a new board which I will place somewhere inside the amp, or near the power tubes on the chassis, I plan on recreating the whole bias circuit after those two 220K resistors (R67, R69)

                2. Remove screen voltage traces from the board and put the screen resistors on a new board placed somewhere in the amp and have only 1 wire from each resistor on the new board to pin 4 of every power tube, and 1 wire going to R63 on which the screen voltage circuit is further connected, I think R63 is far enough from the power tubes and bias circuit and therefore will not cause more problems with drifting

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                • #38
                  Hey, Good luck.
                  As you stated, nothing to lose.

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                  • #39
                    Originally posted by Jazz P Bass View Post
                    Hey, Good luck.
                    As you stated, nothing to lose.
                    Thanks , will keep you posted on how thing are going

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