N6KI said "Be sure the one U buy uses an Opt Coupler typr device and not
descrete transistor(s) - The Optos are far more reliable !!!"
Gee, the 2N398A germanium PNP and the whatever NPN used in the original
implementation of the "K3KU circuit" for keying pos or neg have been in use for
over 30 years w/o failure. They may well pre-date optocouplers AND computer
keying.
I still have a few 2N398A's if anybody wants to build the circuit, though the
MPSA92 used by Logikey in their CMOS4 and by MFJ in their 490 would work fine.
(Note that Logikey and MFJ both use it as a separate neg keying output, not the
K3KU circuit.)
73, Art K3KU
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