[Elecraft] KPA100 SWR Bridge Alignment:

[Martin AC6RM]

Howdy;

   I am in the final stages of testing the KPA100.  I am getting some
strangeness on SWR readings.  I have the K2/100 connected to an
MFJ-260C dummy load through an MFJ Versa Tuner II (941-E) in bypass. 
The K2 is being powered by an Alinco DM330MV 32A switching power
supply.

   In summary, I am seeing -- I think -- exaggerated SWR readings on the
K2.  Here's the story:

   When I first did the SWR Bridge Null Adjustment (per p44) with C1's
adjustment slot perpendicular to the rear panel of the PA, I made no
actual adjustment because the LCD reading read 1.0, and a voltage check
of pin1 of U5 showed 0.019v.  "Cool!", I thought.

   Then during the power tests, at 20 watts I was getting HI REFL all over
the place, so I knew something was wrong.  I went back and re-did the
null adjustment.  I found that adjusting C1 gives me either 9.9 or 1.0;
nothing in between.  There's quite a range on C1 that will give me 1.0
so I found the middle using the voltage on pin1 of U5.  I was only able
to get this voltage down to about 0.06.  As I swung the capacitor back
and forth, it jumps from the 60millivolt range to about 10volts at
either side of that 1.0 range.

   I then redid the power calibration.

   After these adjustments, I re-did the power tests at 20watts and
everything looks great.  The 941's meter is matching the display and
the power supply's current readings are consistent.

   Then I tried 100watt test per the last step of p47.  Again, I am
getting HI REFL.  I'm on 40m.  I drop the power to 75watts.  No
problem, 1.0-1 per the display; the ammeter reading on the power supply
is consistent, and I show 75watts on the 941's meter.

   I checked each band.  Each band has a different power rating that turns
on the HI REFL:

80m = 42 watts
40m = 100 watts
30m = okay -- no HI REFL at any power setting
20m = 68 watts
17m = 60 watts
15m = okay -- no HI REFL at any power setting
12m = 48 watts
10m = 28 watts

   I decide to connect to my antenna (a High Sierra HS1500 with a bunch of
radials on my ground-floor apartment patio) and try it there.

   I set power for 20 watts.  For any given band I am seeing about a 5:0-1
SWR on the 941's cross-needle display when the antenna is not adjusted
correct, but the K2 is showing 9.9-1.  As I move the antenna's coil in
to range, the 941's cross-needle meter gracefully drops the reflected
needle to zero, but the K2's display goes from 9.9-1 IMMEDIATELY to
1.0-1; no in between readings.

   I've redone the bridge null adjustment and the power adjustments
several times, but this situation doesn't go away.

   Any ideas?

   Many thanks and 73,

   Martin AC6RM (k2#3021)