The LND250 had failed, didn't pass the DVM diode, lead swap, test.
Erroneously replaced witha non-depletion type n channel Mosfet but at least
the rumbling had gone. Owner always ran it with minimum tremoplo speed and
depth which gave minimum throbbing.
Still no tremolo function. Now to find a 400V to 500V depletion mode mosfet
from somewhere.
BSS126 and BSP135 are a couple near equivalent device types found

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/