http://elecraft.85.s1.nabble.com/K3-keying-an-amplifier-tp6033183p6033784.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.
>
> Scott K9MA
>
>
>
> On Feb 16, 2011, at 3:48 PM, Paul Christensen wrote:
>
>>> 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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> Scott Ellington
> Madison, Wisconsin
> USA
>
>
>
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