What I like to do. If something is as rare as hen's teeth then bodge up a
work-around with standard parts.
This high voltage , but depletion type Mosfet, is an active part of the
tremolo circuit , there is no separate oscillator. So re-biased the mosfet
up a few volts.
Using a TO220 size standard IRF740 enhancement type, fudged pinning, bled
off the 28V line with a 2.7V zener, couple of resistors and a preset pot, to
the gate, now have a fully working work around this tremolo problem. Full
range of .2 to 20 Hz or so and 0 to excessive depth. Its a bit too critical
on preset setting but the owner had not used the tremolo for ages.

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