Question about Marshall MG100HDFX 100-Watt Amplifier Head

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GM100HDFX head. Fuse blows immediately upon power up when output module on fan-cooled heat sink is plugged in. Increase the fuse and loud buzz comes out through the speaker. No volume control then. Unplug the module with original fuse and headphone output sounds fine for use as line out. Replaced module on fan-cooled heat sink and same thing happens. Symptoms are repeatable. No apparent damage from troubleshooting this way. Used Arctic Silver thermal paste on module instead of white type. Powered off immediately upon buzzing. End of testing. Can you help? My vocabulary of these parts is lacking so please be descriptive... diagrams/pics/arrows? Thank you.

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  • Marshall Master
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NEVER increase or replace the fuse without repairing the unit. Further damage is likely if you do so.

Put a light bulb, say 150 watts in series with the power to the unit to act as a fuse that won't damage things while troubleshooting. The light will light and limit the current.

There are three 47mfd/63V electrolytics on the output module. I suspect one of those is shorted. Observe the correct polarity when replacing. C2 and C3 are most likely. Use my above suggestion with the light bulb and check the voltage across each of these. It should likely be in the 35 to 45 volt range though schematic doesn't show the value..

Posted on Aug 06, 2010

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