[Collins] Ailing KWM-2

[Jim M.]

Also, look at the CRA or CCA website
(http://www.collinsradio.org/html/servebull.html) and read KWM-2 Sefvice
Information Letter 1-75, which describes various instabilities.  It says
a common cause is capacitor C123, an axial lead ceramic feedthrough
(.001 I believe) which is mounted immediately to the right of the 6CL6
driver in the PA cage.  As they age they become frequency sensitive. 
This also went bad in my KWM-2A.  I was able to purchase a replacement
from SSN but they are a bear to replace.  

"Jim M." wrote:
> 
> This has sometimes happened to mine because of corrosion or poor
> electrical contact between the little cans that cover the exciter coils
> and bandswitches.  They are supposed to provide sheilding that prevents
> feedback.  If this shielding degrades, feedback occurs and the rig may
> oscillate "out of band", hence the strange dip frequency.  You can
> loosen the nuts holding the cans from the top and move them around to
> break down the corrosion, see if that helps.  Be sure to retighten the
> nuts but not too tightly.
> 
> Donna & Robert Littleton wrote:
> >
> > I am continuing to try to diagnose a problem in the transmitter stages
> > of a 1963 vintage WE KWM-2.  The transceiver suddenly developed
> > problems about two weeks ago.  I have gotten the receiver section back
> > in operation, but the problems in the transmitter stages continue to
> > elude my detection.  Here are the symptoms:
> >
> > I get a larger than normal dip in the the plate current when tuning the
> > PA.  The current dips way down to about 70 mils.  Also, the dip occurs
> > off frequency from its normal location.  For example, when the radio is
> > set for 20 meters, the dip occurs when the tuning knob indicator is
> > within the 21 mhz markings.  The same is true on all other bands - the
> > pointer is in a higher frequency range than it should be at the point
> > of dip.  In addition, there is absolutley zero power output;  however,
> > my frequency counter senses RF around the PA cage at the correct
> > frequency (14.2 mhz in the particular case checked).
> >
> > Here is what appears to be working okay:
> >
> > The finals and driver - changed with no effect.
> > TR relay (K3)- Continuity checks (with no power, of course) indicate
> > that it is functioning correctly.
> > I see no obviously smoked or overheating components.
> > The band switch cord to the PA compartment and the switch itself within
> > the compartment are working as they should.
> > I have checked the resistance and voltage values of the pins of each
> > tube in the transmitter chain, and all are either right on the money,
> > or very close to specified values.
> > The plate voltage blocking capacitor.  I do not have a capacitance
> > tester, but the capacitor is not shorted and is leaking DC to its
> > output side.  I assume that if it were open, I would get no dip at all
> > in the plate current.
> >
> > Can anyone help me diagnose this problem?  I'm about out of guesses.
> >
> > 73,
> >
> > Bob, KB4X
> >
> > Sponsored by the Collins Radio Association
> > http://www.collinsra.com
> Sponsored by the Collins Radio Association
> http://www.collinsra.com