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Friday, 4 March 2016

Marshall JTM-60 repair.

Steve gave me a lift home from the Indian restaurant with an ulterior motive. There's a duff Marshall amp in the boot, which belongs to a friend of his.

"Makes a noise like a dalek, And. Can you take a look?"


Why not....

Initial tests show it does indeed sound like a dalek. Any audio making it to the speakers is being modulated by huge 100Hz hum.


Removal of the chassis, and I've got a good idea what the fault is. It's no secret I'm not a lover of Mr Marshall's amplifiers. They're OK, but the engineering could be better. This amp dates from 1994, and has a proper valve output stage, 2 EL34's in push-pull, and a smattering of ECC83's doing other things. There's some semi-conductors and op-amps in there too...

What I spotted is the two mains smoothing capacitors. They're located directly above the horizontally mounted EL34's. Not a good idea.

Heat rises, and will have dried out the electrolyte in the caps. Removal of the capacitors shows both to be completely open-circuit! They're 47uF 550V parts. Two new ones are duly ordered....










... and fitted, restoring operation.


Arty valve shots follow... starting to get the hang of the new camera now.





3 comments:

  1. Fascinating, I love the electrolytic capacitor "oven" not a good idea as you quite rightly say. Fascinated as to what camera you are using, massive improvement on previous shots! ;-)

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  2. Thanks Andrew, I finally bought an SLR. It's a Nikon D3100. Seems OK. I could use a different lens though ... and so it begins!

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  3. A most splendiferous choice sir! If Nikon is good enough for NASA...
    've been a Nikon disciple for decades, so the collection slowly grows.
    Splendid resolution in your pictures as is shadow detail, damned impressive indeed for a camera which was released in circa 2010! Keep up the great site of yours, it is a treat indeed when a new posting appears, I look forward to each one!

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