[Elecraft] Requirements for keying a KXPA100

[John Marvin]

So, I got my KXPA100 kit and have assembled it. But I'm having trouble 
keying it with my transceiver (HPSDR Hermes with alterations to support 
driving the KXPA100). My circuit didn't have any trouble keying my 
previous amp (a HFPacker V4). If I manually short the KXPA100 key input 
things appear to work in the limited testing I've done, so I don't think 
there is anything wrong with the KXPA100.

I've measured 5V as expected at the key input, and I've measured the 
current while connecting the KXPA100 key input to my transceiver. I'm 
measuring 0.58 mA when the transceiver attempts to key the amp. The 
KXPA100 manual says that the key current will be 1 mA max (if I am 
reading that correctly) so this seems to be about right. Is there a 
minimum current requirement? There is a 560 ohm resistor in series 
internally, along with  diodes (one in line with the key output to the 
KXPA 100 and one inline with the key output to the internal tx preamp) 
to prevent interaction between the two.

Any ideas? My next experiment will probably involve changing or 
bypassing the 560 ohm resistor, but I have doubts that will fix the problem.

John
AC0ZG