[Elecraft] RE: Problem with KPA100 power output; high current

[Courtney Krehbiel]

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