[Elecraft] Troubleshoot power control problems on KPA100

[Gregg Lengling]

I've been working on repairing my lightning damaged KPA100.  I had to
replace the hi-current opamp and 2 FETs that control the VRFDET line.
The VRFDET line seems to be working properly now...as I have the same
voltage at the 5 watt setting (2.72 volts) as the 50 watt setting.
Which should be proper as the circuit is a 10 to 1 to make up for the
gain in the amp.  However everything tracks properly in the QRP
mode....I can set the power output at anywhere from 1 watt to 10 watts
and it tracks as well as can be expected.  However when I start getting
into the QRO mode....the tracking ends.  25 watt setting gives 30 out
but the display reads 25, at the 50 watt setting I've got 120 out but
the display says 50.  At the same time the SWR reading climbs....this
leads me to believe that maybe the 1N5711's are breaking down at high
power.  The check okay but that doesn't mean they are under high output
power.


I am figuring this out right...because at this point to replace those
diodes will be fun...probably won't have to remove the board from the
heat sink, but I'll have to take the back panel off.

Am I right or wrong...gents???


Gregg R. Lengling, W9DHI, Retired
Administrator http://www.milwaukeehdtv.org

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