[Collins] KWM-2A low output solved

[Bill Abate]

Some months back I noticed I had lower than normal output from the 2A on 
75.  A post to this group suggested I verify the meter circuit 
calibration.  I was also reminded that the meter looks at cathode 
current which includes screen current so my input was less than I 
thought.  So when the 30L-1 started to act up recently, I decided it was 
time for Collins maintenance.  The 30L-1 was misbehaving when I checked 
into the CRA net.  Turned out to be the HV supply.  Yup, the 
electrolytics had died.  Fortunately I bought a set of 120 mfd @ 450 
from a JA at Dayton.  They are the PC board mount type but fit the pc 
board clips nicely.  The L-1 is quite happy now.

Next on the list was the 2A.  Got it out of the cabinet and just 
marveled at it.  The teflon wiring is just so neat and compact.  I wish 
I could operate it upside down, without the cabinet.  Anyway, the first 
task was to check the plate (cathode) meter calibration.  I prefer using 
a constant current source for calibration over measuring the cathode 
resistance and then measuring voltage across the cathode resistors and 
figuring out cathode current.  This way I don't have the 2A on when I'm 
working on it.  The 6146's have to be plugged in though.  I adjust the 
current source for 200 ma. through the cathode circuit and check the meter.

Normally this is easy except that the 2A switches the meter from S-meter 
to plate with a relay.  It's normally connected to the S-meter circuit. 
  I didn't want to unsolder the coil and energize it externally (who 
would change a work of art?).  There had to be an easier way.  So I 
pulled K-4 relay.  The relay pins are flat.  So I merely took a short 
piece of #16 buss wire and flattened the ends with a pair of vice grips 
so that the thickness was the same as the relay pins.  I made two 
jumpers that took the place of the normally open contacts from the 
cathode circuit to the meter.  Just plugged them into the relay socket. 
  Worked like a charm.  The bad news is that the meter calibration was 
right on.

With about 225 watts input I was getting about 65 watts out through my 
Bird watt meter.  Less than 30% efficiency.  One problem was the band 
switch.  Seems I had not exercised it in a while.  Could not get DeOxit 
on all of it because of close quarters and shielding.  So I just rotated 
it about 20 times.  That got the output to about 75 watts.  Still not 
good enough.  Then I found it.  The loading cap calibration was the 
culprit.  The 50 ohm setting was wrong.  I experimented with slight 
changes in loading and got the output to 115 watts.  Now I was happy.

There is a calibration procedure in the manual for the loading cap.  I 
could follow what Collins wanted but I did not understand the why's of 
the procedure.  Since the 75M position has its own calibration padder, 
C-154, I merely found the best value of loading.  I then set the loading 
lever to 50 ohms and adjusted C-154 until I got the same output into the 
dummy load.

The 2A is humming nicely now.  I'm sure there are some more 
knowledgeable people than I that can explain the manual procedure and 
the reasons for it.  I welcome that but this worked for me and I thought 
this might help someone, especially the meter calibration.  Note that 
there are 4 air variables associated with the loading cap and there is 
some interdependence depending on what band you are on.  Since I 
normally use the 2A on 75, I did not check the other bands.  Its 
interesting to note that the manual states that the loading is quite 
broad but that is not what I found into a dummy load.

73,

Bill, K3PGB