[Collins] Low Grid Drive - KWM-2

[Dr.Gerald Johnson]

I don't agree with using alignment as a test tool. In my experience, alignment never 
found a bad part, and with the parts aged like the trimmer capacitors. Those frozen 
trimmers can break when frozen and an alignment is attempted. Then the "test" has 
introduced NEW problems that didn't need to be introduced.

Yes alignment does drift with time but not nearly as rapidly as tubes age. Tubes are 
in sockets for good reason, they need regular replacement. The rest of the parts 
don't age so fast so they are soldered in place. Alignment is most necessary AFTER 
replacing tubes because the input and output C of the replacement tube has a 
greater variation than the circuit can stand. There's an old rule of thumb for IF 
stages that if the tuning C can be at least 100 pf, the expected variations between 
tubes won't mess up the alignment too much. Unfortunately in the higher bands of 
the S-line/KWM-2, the circuit tuning C gets to be smaller than 100 pf so the few pf of 
tube differences show up. In these radios that difference can be tuned out by 
ignoring the manual alignment for the RF stages where it says to set the highest 
band trimmers to a certain location, then tune the slugs, then tune the trimmers on 
progressively lower bands. Rather go to the highest band and peak the 
preselector/driver tuning and then while rocking that tuning, peak the two trimmers 
at the changed tube. Then ALL the rest of the slugs and trimmers should be close 
enough to not need to be changed because the tube change capacitance has 
been compensated in those high band trimmers that are always in the circuit.

Using alignment as a test tool just makes work that has to be repeated when the 
real fault is found.

Check the tubes FIRST, then look for the more exotic things like bad switch 
connections and weak crystals and bad trimmers, but bad trimmers will only affect a 
single band (except the set for the highest band).

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.