[Elecraft] K3s/KPA500 communication issue?

[Mike Dodd]

Occasionally it seems my K3s doesn't know the STBY/OPER state of my 
KPA500. I also have a KAT500 for my Carolina Windom.

Here's the first scenario: K3 in CW mode. I put the KPA in STBY and hit 
TUNE on the KAT and on the K3. When the tune is complete, I send one 
dash and note the 100W LED on the KPA. So far, so good.

Next I put the KPA in OPER mode. The K3 power level is already set to 
about 10W, sufficient for about 250W on the KPA. Now I send a dash.

Occasionally the KPA power indicator shoots up to 700W, and the KPA 
faults. When I clear the fault, the KPA is in STBY, and if I send a 
dash, I see the 100W LED come on. if I then put the KPA in OPER and send 
a dash, USUALLY I get the expected 250W output, but occasionally it 
shoots up to 700W and faults again.

Now for the second scenario. With the KPA in OPER and outputting 250W, I 
switch it to STBY, then send a dash. I expect to see the 100W LED, but 
it does not light. If I look at the K3 RF bar graph, it shows 10W, not 
100W. If I slightly turn the K3 PWR knob, it shows about 10W, not 100W.

Until recently, the K3 ALWAYS switched back to my normal setting of 100W 
when the KPA is in STBY. The fact that it occasionally remains at 10W 
seems to indicate that it doesn't know the KPA is in STBY.

Furthermore, the first scenario, where the KPA output shoots to 700W and 
it faults, seems to indicate that the K3 is sending it 100 watts -- 
another indication that the K3 doesn't know the KPA is in OPER, and thus 
doesn't reduce its output power to 10W.

Is there any way to diagnose this problem, and more importantly, to fix it?

Thanks for any and all guidance.

-- 
73, Mike N4CF
Louisa County, VA USA
Elecraft K3s / P3 / KPA500 / KAT500
Carolina Windom up 45'
http://n4cf.mdodd.com