[Elecraft] KPA500 AUX input

[Jack Brindle]

Phil is essentially correct. It should work just fine.

The KPA500 has protective diodes on its band inputs to eliminate  
potential problems with external devices supplying power into the  
KPA's microcontroller when it is powered off. These diodes will also  
block the KPA's +5V from getting out.

I haven't tried it, but I would expect that an unmodified original K3  
would work just fine with the KPA in this situation. The required  
pullups are on the the KPA side, and you indicate that the W9XT device  
has its own pullups, so the requisite logic states for all devices  
should be in great shape.

One word of caution: if the pullups in the decoder go to any voltage  
higher than +5V, be sure to add protective schottky diodes in the  
decoder. This might be a good idea anyway in case you add additional  
devices. 1N5711 diodes will work just fine. Cathode towards the K3,  
Anode towards the internal decoder logic.

Jack Brindle, W6FB


On Apr 25, 2011, at 8:58 AM, Phil & Debbie Salas wrote:

> The KPA500 Band Data inputs do require the pull-up resistor mod on  
> the K3.
> However, if your Top Ten decoder has pull-ups to 5VDC, then you  
> should be
> able to use the Top Ten as your pull-up source as long as the Top  
> Ten is
> turned on (if the Top Ten is off, then there will be no pull-up and  
> the
> KPA500 won't respond to the K3 band data input without the K3 pull- 
> up mod).
> You'll need a true DB15HD splitter cable so as to connect both in  
> parallel.
> The only reason to diode-isolate the Top Ten is if the Top Ten loads  
> down
> the band data output from the K3 when the Top Ten is turned off (I  
> don't
> know if this is the case).
>
> Phil - AD5X
>
> "... I currently use the AUX output from the K3 to drive my 'Top  
> Ten' type
> band decoder (actually, it's a W9XT) to drive the antenna selection  
> relays.
> Having an early K3, the pull-up resistors are in the decoder, not  
> the K3.
>
> The question, should anyone venture an educated guess: Can I drive the
> KPA500's AUX input in parallel with my existing decoder using a Y  
> cord, or
> should I isolate the K3 from both the KPA500 and W9XT with diodes?   
> Ralph,
> VE7XF"
>
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