[Elecraft] K2/100, KAT100 strange tuning problem

[Solosko, Robert B (Bob)]

Hello All,

	I hope that someone can suggest where to start tracing down a
rather strange tuning problem that cropped up in the middle of working
the IARU on Saturday. The short description is that the K2/100-KAT100
stopped tuning in AUTO mode when in high power mode on 40 meters. Here's
some of the details:

1 - Tuning with the K2/100 and KAT100 on 40 meters has worked fine since
I completed the KPA100 (rev. D) 6 months ago.

2 - I had no problems tuning for about the first 10 QSOs on 40m, and
then the rig suddenly wouldn't tune at all when in high power mode
(there were no lightening strikes during that time) - then tuning became
erratic.

3 - Tuning worked fine when the power was reduced to low power mode: all
I had to do was reduce the power just to the point where the power LEDs
on the KAT100 switched from high power to low power and everything
worked fine - then increasing the power just to the point where the
power LEDs on the KAT1200 switched from low power to high power and the
KAT100 wouldn't tune at all.

4 - If I tuned in low power mode, then cranked up the power to 100w, the
rig transmitted just fine (and I made a bunch more 40m QSOs working like
that)

5 - The rig tuned just fine in high power mode on all other bands.

6 - The problem doesn't seem to an antenna problem as the same thing
happens when the antenna is replaced with a dummy load.

The KAT100 manual has an obscure comment under troubleshooting that
says: "Erratic tuning in AUTO mode: This is much more likely to occur at
high power, and could be due to high RF voltages on the K2 chassis, mic,
key, power supply leads, or KAT100 control cable. First, try reducing
power to 10 watts or less. This will force auto-tune to be done at 2
watts. If this cures the problem, you may need to add RF bypassing to
your power supply, key, or mic; improve your ground system; move the
antenna farther away; or use a "choke balun" between the ATU and the
transceiver." -- 

That actually doesn't tell me much about where to start looking, since
the everything has been working for 6 months (OK, I did get the new MH2
mic, but that was several weeks ago and it's been working fine), and
everything worked for at least the first 1/2 hour on 40m in the IARU, so
nothing's that I changed that could account for the sudden lack in
ability to tune in high power mode on 40m.

Help! Where do I start looking for the cause?

Thanks in advance.

Bob W1SRB