I resumed contest operation after modifying my SteppIR driven element length
for 20 m band. With KPA500 indicating 1.3:1 (LP-100A 1.38:1) I saw no more PA
DISS faults.
If I had known that the KPA500 was this sensitive to load impedance I would,
perhaps, have purchased the KAT500 tuner. However, I found nothing in the
documentation that suggested the KPA500 would fault frequently with a reported
SWR of 1.4:1 and with power and SWR never peaking in to the amber regions.
73,
Andy k3wyc
________________________________
From: ANDY DURBIN <a.dur...@msn.com>
Sent: Saturday, May 26, 2018 11:28 AM
To: elecraft@mailman.qth.net
Subject: PA DISS fault and relationship to SWR
Up until this weekend I had experienced 3 PA DISS faults with my new KPA500.
I provided data to Elecraft and, after a fairly long and informative email
exchange, I was told the fault is very rare and most likely caused by some
problem at my station.
I developed a 10 frames per second data logger which polls and records data
from the KPA500 serial interface as well as the ALARM signal available on the
AUX port. The ALARM signal was also used as a trigger so that 'scope data
could be time correlated with the logger data. CQ WPX CW contest seemed to be
a good opportunity to gather data if the fault could be reproduced. After
experiencing 5 PA DISS faults on 20 meters I abandoned the contest, looked at
the recorded data, and ran some additional tests.
The data show that PA dissipation (DC input power - RF output power) is
strongly dependent of load SWR. That in itself is probably not surprising.
What was surprising was that my KPA500, when operating into an antenna load
that the KPA500 reports as 1.4:1 SWR, has a PA DISS of 500 W when producing 400
W output. What was more surprising is that, although the PA DISS fault is
designed to trip at 400 W ("The PA Diss uses non-averaged data to do a
comparison between the calculated power from PA voltage and PA current and the
measured output power. If it is more than about 400 watts on four consecutive
checks (each 8 mSec apart) then we declare the fault.") it did not trip for a
key down test lasting several seconds.
Sample data for different load condx shown below. None of these key down tests
resulted in PA DISS fault.
SteppIR: LP-100A SWR 1.58:1 PA DC Input / RF Out
/ PA DISS
^OS1; ^WS390 014; ^VI592 151; ^BN05; ^TM047; 893.92 390
503.92
^OS1; ^WS390 014; ^VI590 151; ^BN05; ^FS00; 890.9 390
500.9
SteppIR: LP-100A SWR 1.38:1
^OS1; ^WS428 013; ^VI585 143; ^BN05; ^TM039; 836.55 428
408.55
^OS1; ^WS430 013; ^VI589 144; ^BN05; ^FS00; 848.16 430
418.16
Dummy load LP-100A SWR 1.08:1
^OS1; ^WS440 011; ^VI598 132; ^BN05; ^FS00; 789.36 440
349.36
^OS1; ^WS440 011; ^VI595 132; ^BN05; ^TM043; 785.4 440
345.4
Can someone at Elecraft please explain why PA DISS fault does not trip when key
down PA DISS (calculated from WS and VI serial data) exceeds the 400 W
threshold defined for this fault condition. When the PA DISS fault was seen in
the contest it always happened during the first element of the CW transmission.
Raw logger data, Excel data, and scope traces have been saved. KPA500 firmware
is 1.54.
73,
Andy k3wyc
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