[Elecraft] KAT100 General Failure

[Jonathan Taylor, K1RFD]

I am testing my KAT100, which had been sitting idle for a number of
years.  It is connected to the K2/100 via the DB9 cable as described
in the KAT100 manual.  The two are interconnected with a coax cable
and Ant1 is connected to a dummy load.

1. Apply power to KAT100.
2. Turn on K2/100.
3. Almost immediately, all LEDs on the SWR bar graph illuminate, the
S-meter bar graph on the K2 goes full scale, and both remain in this
state.  The receiver is silent. (There is no indication of any output
power, as if it were transmitting.)

Pressing the ANT1-2 button on the K2 produces the "not inst" message.
Turning off the K2 power, all LEDs on the KAT100 remain illuminated.
With KAT100 powered off, K2 functions normally.

I have verified the wiring of the DB-9 cable.  What's the next best
thing to check?

Thanks