Question about Marshall Mg100dfx 100-Watt Combo Guitar Amplifier

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Marshall mg100dfx, internal fuse blowing every time you switch on the unit. What could be the problem

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  • cmangles Sep 30, 2010

    Thank you for the advise it really helped. I have found the problem.
    It is the main power chip IC1. Do you know where i can purchase a replacement For the Marshall Mg100dfx in the uk ?

  • cmangles Oct 01, 2010

    Hi, thank you for that.
    I cannot identify the chip from a number as it has no serial number on it.
    I shall have a look on the digikey site and see if i can match it up.
    Once again many thank's

  • cmangles Oct 01, 2010

    Got the number of the part and it is the TDA7293
    Thank's for all your assistance

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  • Marshall Master
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The way you troubleshoot this is to put a 150 watt lamp in series with the input power cord. The lamp acts as a resetable fuse and will limit the current while you look for the bad component. Start with the capacitors and rectifiers connected to the power transformer.

Posted on Sep 30, 2010

Testimonial: "It was a great help, didn't take long to isolate the problem. Many thank's"

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  • cmangles Sep 30, 2010

    Following your advise i have located the problem.
    The main power chip ''IC1'' has gone short circuit.
    Could you recomend a uk outlet to purchase a replacement ?

  • Fred Yearian Sep 30, 2010

    If the IC is TDA7293 I would procure it from DigiKey.com in the USA.

    DigiKey.com search for TDA7293

    They show a couple, one may have offset leads. I don't know of UK sources, but I suspect they have some there.


    They are about $6 USD.

  • cmangles Oct 01, 2010

    Hi, the chip has no serial number on it. But i shall have a look on the digikey site under that reference number and have a look.
    Many thank's

  • Fred Yearian Oct 01, 2010

    Download a datasheet for it and compare to the electrical connections you find.

  • cmangles Oct 01, 2010

    Thank's for all your assistance, i now have the part number.

  • Fred Yearian Oct 02, 2010

    When poiwering these amps up after a repair DO use the series light bulb trick. Saves fuses should there be another problem!!! I repair a lot of amps and have learned to be cautious.

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There are MANY more likely components than the transformer that will cause the fuse to blow.

Most common are filter capacitors, rectifier diodes, and shorted components in the power amp section.

Transformer failure is very UN-likely. Have a competent electronic tech troubleshoot the unit.

When testing the unit, put a 150 watt light bulb in series with the power line... saves fuses during troubleshooting. This limits the current so one can analyze the real cause of the problem.

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Probably one side of the power amp has shorted and there is likely several semiconductor transistors and other components requiring replacement, but this amp uses a stick IC power amp.

Copy/pasted from the music forum between the arrows, this is likely the problem in your unit:

>>The schematic is posted in the Enzo gallery at www.ampix.org

The 15 leg power IC on the heat sink is a TDA7293V - it is the power amp for the unit.

The chip with small board attached can be ordered from any Marshall dealer or from places like http://www.marshallparts.com/

I usually buy my 7293s from Mouser. They currently have 622 of them in stock and they sell for $6.48 each. Probably won't run out of them soon. << Hint for you: When powering this up after repair, insert a 100 watt light bulb in series with the line cord into the unit. This will act as a automatic reseting fuse which will save fuses if you haven't got all the "cancer" out of your failed amp. It will allow you to do troubleshooting by limiting the current to a safe value. Also, when replacing the TDA part, be sure and use heat sink compound under it and if an insulator was used, make sure you replace the insulator. ALSO clean the area of any soldering residue and flux BEFORE powering the amp again.

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