[Elecraft] KPA100 problem

[Steve Wilson, G3VMW]

I wonder if anyone can offer a few words of advice or perhaps point me
in the right direction please?

I have a K2 serial #1508 and a KPA100 PA, which have worked perfectly
for quite some time now.  Today, something very strange has happened
after I connected the K2/100 to my external linear amplifier.  After
driving the amplifier OK, I am now getting no RF output and high current
readings on the K2 when I press tune or key the rig.

I now have the KPA100 out of the K2 case and in the test position on its
back.  These are the symptoms:

1. With the KPA100 completely disconnected, the K2 works perfectly with
12-15W output on all bands.

2. With just the ribbon cable from P1 of the KPA100 connected to the K2
and J1 of the KPA100 connected to AUX-RF on the K2, dummy load on the
SO239 of the KPA100, I get HiCur readings and no output from the K2.
This is effectively the set-up for the SWR bridge null adjustments and
should be basic K2 straight through and KPA100 effectively out of
circuit.

As soon as I disconnect P1 on the KPA100 from the K2 control board
header and return the output lead to the K2 BNC I can get RF output
(from the basic K2).  With the KPA100 connected, the relays click and I
can hear signals on RX OK with an antenna on the SO239.  It suggests
that the T/R switch works OK since everything is OK on RX.

I have no high current supply connected to the KPA100 for any of these
tests above.  All the KPA100 test voltages (Page 43 in my manual) are
correct.

Can anyone suggest why I might be getting the HiCur readings and no
output as soon as P1 on the KPA100 is connected to the K2 control board
header?  Previously, the K2 and KPA100 has worked perfectly - 100W plus
output and no HiCur readings.  The Cal Cur on the K2 is set to 3.5A and
hasn't changed so I assume the problem is on the KPA100 board?

Thanks in advance for any pointers.

Steve Wilson, G3VMW
Bramham, Wetherby, West Yorkshire
Email: steve@g3vmw.demon.co.uk