Had a moment to review the PA-77 schematic this evening. There are two
40841 MOSFETs used for timing and T/R control. These are depletion-mode
MOSFETs with dual gates. If required, the quickest means for replacement is
the NTE221 -- although at roughly USD $12 ea., one can study the datasheet
and look for alternative depletion-mode MOSFETs that are available from
Mouser and DigiKey. But, be prepared for SMD/SMT devices that will need a
lead adapter - or you can make your own. Since the NTE221 is already a
leaded component, it may be worth the cost to avoid the hassle of dealing
with surface-mount substitutes.
It might be tempting to substitute with a single-gate FET since its used in
a switching application. However, in depletion mode, voltage impressed on
the gates behave with OR logic and either gate can independently control
source-to-drain current. If you look closely at the PA-77 schematic, the
timing delay MOSFET has its gates tied together. That's an easy substitute.
But the other is used for T/R timing and takes full advantage of the dual
gate structure.
Paul, W9AC
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