Hi All,
I found my problem, and thought I repost my findings back to the
reflector to help others who may have similar issues. There are a few
red herrings in my troubleshooting observations, and I am deeply
grateful to Tom Hammond, N0SS and Ron, ZL1TW for their time and advice
to help me zero in on the problem.
First of all, the problem with C224 was most likely caused during
construction test when I errantly reversed the RF leads going to the
KPA100. I'd conveniently forgotten about this momentary assembly error.
The installation of the DSP module was also a red herring. Thanks to
several who suggested I trim the leads on the newer version DSP module
to make it sit closer to the control board and less likely to short to
the KPA100 heatsink/shield when the cover was on. Done. It fits much
better now.
The real cause of my trouble was dead 1N5711 diodes in the internal SWR
bridge circuit. The diodes in both the KPA100 and KAT100 were fried and
seemed to look like 2 to 4 Kohm resistors. I'd been comparing
resistance readings between the two circuits thinking it wasn't likely
that both were dead. Wrong! The problem wasn't obvious to me because
of the effect of the surrounding circuit, but closer testing with the
diode feature on the DVM got me very suspicious. Once out of the
circuit, they were obviously bad. With the RF detector circuit dead and
measuring no output power, the K2 feedback control process just kept
trying to drive the unit harder until it reached full bore and
high-current would ensue.
As a helpful hint, I was able to remove and replace the diodes from the
top side on both the KPA100 and KAT100 with minimal disassembly. I
looped a very fine piece of steel wire under one diode leg, and pulled
up gently while applying heat. With one end now free, it was easy to
heat the other lead and remove the diode. There was just enough room to
get the tip of a solder sucker in to clean out the hole, and I dropped
the replacement with leads pre-trimmed and resoldered from the top. It
was about a 30 minute operation for both the KPA100 and KAT100. I
figured this approach was less traumatic to the hardware than the
disassembly necessary to reach these diodes from the bottom.
Thanks again to all who helped!
-- Courtney Krehbiel KD6X
________________________________
From: Courtney Krehbiel
Sent: Friday, March 02, 2007 8:32 AM
To: elecraft@mailman.qth.net
Cc: Courtney Krehbiel
Subject: RE: Problem with KPA100 power output; high current
Hi All,
I have K2 S/N 5788 with KPA100 which I recently completed. Everything
seemed to be working perfectly, including the KPA100. Over the weekend,
I removed the top cover KPA100 to install the KDSP2 module. I was very
gentle with the KPA100. The receiver worked fine after assembly, so I
moved the unit across the room from the workbench to desk and rerouted
the antenna coax. The first time I tried to light up the KPA100, the
power supply folded back due to high current.
I've spent the last few days dusting off my troubleshooting skills, and
have come up with the following observations. I've also tried to review
and follow troubleshooting hints from this reflector. My test equipment
is limited to a DVM and an old external analog SWR bridge, so I'm
somewhat handicapped. I'm hoping someone has some ideas or similar
experience.
1. K2/KPA100 was working originally as far as I could determine during
construction and alignment steps. Tested ok into dummy load and
KAT100/long wire on the air.
2. Problem is not band specific. I'm using a good 50 ohm dummy load for
all tests.
3. K2 works normally into a dummy load antenna with KPA100 removed.
4. C224 .047 uFat AUX RF output of K2 had an internal short. Replaced
this with new part. Not sure if this could have been cause or effect.
No impact on observed problem after replacing part.
5. Have repeatedly seen HI CUR on K2. Moved current limit to 425 mA
from original setting of 350 since I thought the DSP module might have
pushed things over the top. This makes it better with the top open.
Putting the KPA100 back on the K2 causes the supply to fold back. (I
expect this is due to a better ground connection between the KPA100 and
K2.) All measurements made with KPA100 removed and sitting to the right
side of K2.
6. Power output seems to be at or near max both <10 and >10watt setting.
Relays click at the right time, and power output into a dummy load on
the analog SWR bridge takes a significant jump when high power kicks in.
(I used the K2 solo to "calibrate" the external SWR bridge.) The amp
seems to be working.
7. Power control doesn't seem to affect power setting or power output.
Control is very jumpy at the far CCW end of the range, but that's the
extent of the adjustment ability.
8. I've carefully looked at the built in SWR bridge and components
around U5 and U6. I used a jewelers loupe to check each connection
during assembly. It's possible I missed something, but I was very
careful and methodical.
9. Both R26 and R27 seem are set near middle of range. I read 26.8Kohms
to ground at both U5 pins 3 and 5.
10. VRFDET seems to sit between .8v and 1.4v depending on band and
whether high or low power. There is no movement in VRFDET when I adjust
the Power control. (Just like no change in output power level on
external analog SWR.)
11. DVM measured frequency seems to correlate to tuned frequency. I
doubt the circuit is oscillating.
I would be very appreciative of any ideas, recommendations, or
suggestions.
Thanks very much, and 73,
-- Courtney Krehbiel
KD6X
K2 S/N 5788
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