[Elecraft] K3 keying an amplifier

[Scott Ellington]

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