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Fried Resistor in 1985 Marshall Lead 100 Mosfet

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  • #1

    Fried Resistor in 1985 Marshall Lead 100 Mosfet

    Hello,

    I'm hoping that you can help me. I have a 1985 Marshall Lead 100 Mosfet that recently stopped making (any) sound. I opened it up and saw that R46 was burned, and the color codes are melted, so I can tell the impedence.
    Internet searches have been futile, and the few schematics that I've seen are not helpful. Does anyone know the specifics of this resistor?

    Also, C40 is connected serially, but there is no visible damage to it. Should it be replaced as well?

    This amp has been truly faithful for the last 27 years, and I'm performing regularly, so I'm hoping that I can make the repairs myself, quickly!

    Any help is greatly appreciated.

    Thanks,
    -Steve
    Tags: None
  • #2
    Well I am sure there are way better sources than myself here... But I was trying to read that horribly hand written schematic and I see why you found it unhelpful. I see that C48 (positive side filter capacitor) is connected to what I think is R46. R46 best I can read says 210 or 810 ohm???? 2 watt, but not sure as it is like engineer handwriting. Look at the schematic and look at R47 and it almost could read 810 ohm???? 2watt on the negative side of the circuit(either way those resistors are the same values and hand writing is not helping us read the schematic.) I hope I am looking at the right schematic... Anyway my thought is that this resistor failed... Obviously. The next question is why it failed? Did the old filter cap short out and take out the next component inline with it on the circuit board? Was there a power surge? Did it blow a fuse?

    Best of luck and hope you get it working soon, and someone here has a better copy of the schematic.

    marshall-3210-100w-mosfet-amp-schematic-diagram.pdf
    When the going gets weird... The weird turn pro!

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    • #3
      Try googles image search.
      In this forum everyone is entitled to my opinion.

      Comment

      • #4
        This is exactly the same value as three others: R44, R45 and R47. You can just measure any of them. For me it looks like 270 Ohms/2W or 470 Ohms /2W. Check whether the Zener diodes are not shorted. I wouldn't worry about C40.

        EDIT: if you look here: http://drtube.com/schematics/marshall/3210-iss7.gif , you will find that R46 is 270 Ohms/2W and R45 is 470 Ohms/2W (so I was close).

        Mark

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        • #5
          Yeah whoever wrote this schematic wrote 4's funny. I knew when I wrote 210 ohms that seemed funny and 270 makes way more sense. The 7's looked like 1's.
          When the going gets weird... The weird turn pro!

          Comment

          • #6
            Thanks everyone for your comments...

            Dr. Gonz, yes! That was the darn schematic that was driving me nuts!!!

            Markus, great link... I'll check the diodes. Your openeing statement had me a little concerned, but you just made my life (or least my day) a whole lot easier.

            I'll get busy and let you know the outcome.

            Thanks again guys.

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            • #7
              wow seems like im not the only one whos Marshall Lead 100 blew up this week

              my DB1 bridge rectifier just went, so it needs to be replaced, while im at it i decided to replace all susceptible components since it is a 20 something year old amp
              except for the transformer and power mosfets which are expensive i priced everything else (including a spare parts) for 25$
              they had everything but the transistor array ic2, that ill have to get someplace else.
              Here's a listing if your interested
              http://www.mouser.com/ProjectManager...sID=b36058e227

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              • #8
                If you blew the rectifier by using a larger than specified fuse,probably one or more MosFets are blown too.
                Juan Manuel Fahey

                Comment

                • #9
                  I know I'm a little late in answering this query, but thought this might be of benefit to others with the problem of fried resistors in the Lead 100 Mosfet. The resistors R46 and R47, both of which are 270 Ohms 2 Watt, overheat because of a fault in the reverb driver circuit based around TR5 and TR6. Remove both transistors and test the small diodes D9 and D8, both of which should show infinite resistance in one direction. These diodes are prone to failure and can cause the failure of either TR5 or TR6 or both. In addition, failure of these transistors will almost inevitably burn out the input (send) coil in the reverb chamber, which will also need to be replaced. TR5 is an MPSA06 or BC538; TR6 is MPSA56 or BC528. Both are 80V transistors, so don't be tempted to use any lesser device. They should both measure infinite resistance between collector and emitter when removed. The diodes D9 and D8 (originally 1N914) can be replaced with 1N4148.

                  Best wishes, Ray

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