[Elecraft] KPA100 Fault - Follow-Up

[Steve Wilson, G3VMW]

You may remember that a couple of weeks back I asked the guys on this
Reflector if they could offer any fault advice when my KPA100 PA went up
in smoke after I tried to drive my Alpha 91B amplifier on 40m.  There
appeared to be a huge RF feedback loop and the end result was that the
KPA100 amplifier would not operate at all.  Once the KPA100 amplifier
was removed, the basic K2 worked fine up to the normal 12-15W output.

Kurt Cramer W7QHD suggested that the VRFDET line might be missing on the
KPA100 and that possibly, U6 EL5146C was faulty.  In fact, that was
pretty close and thanks to further advice from Sam, KL7V/W5 it turned
out that the two 1N5711 Schottky SWR detector diodes, D16 and D17 were
open circuit.  A closer inspection also revealed a flashover had
occurred between C1 and the adjacent metal pillar.  C1 was damaged
beyond repair and I had to replace it.  I got some more 1N5711 diodes
and replaced the faulty ones with some difficulty - it is a tight space
for large fingers.

Gary Surrency at Elecraft advised me that some people have reported
problems driving Alpha amps, since they don't all present a 50 ohm
impedance on the input. He advised that it would be a very good idea to
fit the KPA100 shield upgrade kit and also the new KPA100MODKT
modification kit, which involves some tricky component changes to the
KPA100 to allow full output under high SWR conditions and also improves
60M operation.  The KPA100 firmware has to be upgraded to version 1.09
for the KPA100MODKT changes to work.

The two KPA100 mod kits and the PA firmware arrived in the UK pretty
quickly from Elecraft.  I completed the KPA100 mods earlier this week
and re-installed the KPA100 in the K2.  I'm pleased to report that there
seems to be no problem at all in driving my Alpha 91B and I have had no
problems with feedback.

However, tonight, I got a report of some ripple on the CW note of the
K2/100 running 100W (no Alpha 91B attached).  I've had this before and
re-dressing the connecting leads inside the K2 by removing the RH side
panel cured the problem previously, but this didn't seem to work today.
I use a Kenwood PS-40 switched mode PSU with the K2/100 and swapping to
a SEC 1223 PSU made no difference at all.  The guy giving the report
said that when the KPA100 was switched out and I was running 10W from
the QRP PA, then the signal was clean!   The K2 wasn't close to any
other AC PSUs and I am aware of the warnings about keeping the K2/100 at
least 18" away from strong magnetic fields and PSUs.

Has anyone else had problems like this?  

The KPA100 is proving to be a marginal upgrade for me given the problems
I've encountered.  Q9 had to be replaced because it wasn't switching
properly and I have had to replace one of the small RFCs in the RS-232
line after it went open circuit.  Elecraft have been outstanding with
their technical support but I'm hoping not much more can go wrong!  

73 de

Steve Wilson, G3VMW


Steve Wilson, G3VMW
Bramham, Wetherby, West Yorkshire
Email: steve@g3vmw.demon.co.uk