[Collins] Low Grid Drive - KWM-2

[Robert & Linda McGraw K4TAX]

I fully agree with Jerry in that general tweaking without confirmed reason 
to tweak is most likely going to introduce more unknown problems.  Also, 
another bit of advice, check the 2 watt carbon resistors and to some extent 
the 1 watt values too.  With age these devices tend to migrate up in value 
even if they show no outward evidence of discoloration.  They can be checked 
in the circuit with no power applied.  Measurements with a DVM should 
indicate the value of the resistor or lower.  If you find one measures 
higher, remove one end connection, re-measure it to confirm its value.  If 
out of tolerance, replace it.

If you find it necessary to replace the frozen or broken trimmers, I found 
that Surplus Sales of Nebraska has "rebuild" kits for these.  For a few $$ 
more, exact replacements are available.

73
Bob, K4TAX


----- Original Message ----- 
From: "DrGerald Johnson" <geraldj at ispwest.com>
To: <ai2q at adelphia.net>
Cc: <collins at mailman.qth.net>
Sent: Wednesday, February 16, 2005 7:45 PM
Subject: Re: [Collins] Low Grid Drive - KWM-2


>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.
>
>
>
>
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