http://elecraft.85.s1.nabble.com/K3-keying-an-amplifier-tp6033183p6033748.html
Sorry, I still don't think it will work reliably with the ICOM unpowered, if it has a pull-up resistor, etc. However, the ICOM may in fact be open collector/drain, in which case there's no problem. Might be worth checking.
>> I don't think the diode isolation will work in this case.
>>
>> Scott K9MA
>
> If the Icom used an amp key line from an NPN transistor with an open
> collector and a high value (e.g., 10K) pull-up resistor to a positive supply
> rail, then all one needs is a steering diode added to the Icom key line
> configured with the cathode to the junction of the Icom's collector and
> pull-up resistor. The diode's anode points to the Alpha 9500. The K3 with
> its MOSFET output simply parallels the 9500 key line. It doesn't care.
>
> If the Icom is unpowered or powered, a Schottky steering diode will isolate
> the pull-up resistor while still allowing the driving transistor's junction
> to pull low enough to ground. An unbiased or unpowered NPN open-collector
> switching transistor should not pull to ground potential if the rig is
> powered down.
>
> The only remaining obstacle is that when the Icom is powered down, the Alpha
> 9500 key line would see 10K resistance through the pull-up resistor to the
> power supply positive rail. Powered down, that rail may be close to ground
> potential. So, whether a steering diode alone will work is really a matter
> of: (1) the value of the pull-up resistor; and (2) whether the Alpha 9500's
> key line circuitry is sensitive to high-impedance switching. No problem if
> the Alpha uses a photo-transistor on the input key line, for example. But
> if say a pair of both pull-up and pull-down resistors are used ahead of a
> CMOS gate, then the value of the resistors need to be known.
>
> If a steering diode still doesn't work, then a separate switching transistor
> can be used, using an open-collector without a pull-up resistor -- or a
> MOSFET without a pull up on the drain.
>
> Paul, W9AC
>
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