Well, it's been slow going for my ailing K2. I put the tested U5 - 78L05 back in the board and cut pin 7 of U6. The KPA100 once again responded to the main K2.
I replaced U6 with a machine pin socket, and after waiting another week, I finally got the part from Elecraft.
With U6 replaced, I went through the KPA100 alignment steps. First problem was the SWR Bridge Null Adjustment. While I could get a clear null, the display never showed anything less than 1.1 SWR on 40m.
Next to the Power Calibration. This went fine as well, but when I checked the SWR on all the bands, I received a 1.8 SWR on 80m, and 2.6 on 160m. The highest bands gave a 1.0 indication, but most of the others were 1.1, just like 40m.
I double-checked my dummy load with my external watt meter. It appears they are 1.0 on all bands.
A bit after this, I did some full-power tests:
* 40m - K2 indicates about 70 watts (7 bars), but the external watt meter shows 100 watts
* 80/160m - K2 indicates about 50 watts (5 bars), and sometimes I get a Hi Cur warning. The external watt meter shows 100 watts, or perhaps a little more.
Something is obviously wrong with the power measurement circuits here.
Elecraft suggests that the D16 and D17 diodes are "half bad" -- that is they have been damaged enough to change their characteristics, but not enough to fail open or shorted. Next step is to replace them.
I just got the parts from Mouser this weekend. We'll try more repair work this week. It would be very nice to get the K2/100 up and running again. I'm guessing there might have been some damage from an electro-static discharge of some kind.