[Collins] KWM-2A low output solved

[Gerald]

On Fri, 2005-08-26 at 00:06 -0400, Bill Abate wrote:
> To all who responded on or off the list, thank you for your comments. 
> Hope the recounting helped some of you.  As far as my tune up procedure, 
> don't forget I was tuning into a 50 ohm dummy load.  The 50 ohm setting 
> should have been valid.  Why else put that marking there?  I did rock 
> the loading control but it did not make much difference.  The 50 ohm 
> setting was that far off.  I used a wattmeter as well as the plate meter 
> for tune up.  It wasn't until I moved the load control appreciably that 
> the output picked up.
> 
> I went back and reread the tune up instructions.  Collins does not 
> mention the load control setting until the final adjustment.  Then you 
> adjust it for 230 ma.  Again, if 50 ohm setting was close, it should 
> have worked out fine.  Since the instructions do not give a starting 
> point for load it is reasonable to start at the 50 ohm marking.
> 
> Quoting from the Service Instructions for loading trimmer adjustment, 
> "These trimmer capacitors are adjusted to provide the required total 
> output capacity for matching 50-ohm antenna loads on the amateur bands 
> with the INCR LOAD control set at the 50 ohm mark.  Normally, they will 
> not need adjustment since, when the PA is properly loaded, the tuning is 
> relatively broad".  To me the refers to the broadness of the loading 
> control not the broadness of the impedance matching.
> 
> Hope this is of some help.
> 
> 73,
> 
> Bill, K3PGB

The 50 ohm setting is GENERALLY valid, providing the trimmer for the
band hasn't changed value either by fact of age, insect deposits, or by
an unauthorized screwdriver tightening the "loose" screw in the trimmer.
While the compression mica trimmer is pretty good at a wide variation in
capacitance and handling decent RF current its not all that great for
handling high voltage because of the mixed air and mica dielectric. The
low dielectric constant of air next to the high dielectric constant of
mica forces most of the voltage to be across the air when the two
dielectrics are in series between plates. And so when the air gap is
especially small the air often breaks down and arcs, leading to heating
and damage to the solid dielectric. That damage usually makes the solid
dielectric conductive by releasing most everything (on a puff of smoke)
but the carbon. That changes the effective capacitance of the capacitor.

The fact that the variable load capacitor had to be reduced in value
(moving the lever towards a higher R load) hints that the trimmer had
too much C for the load and so some of the dielectric has been damaged,
some air displaced by spider webs and spiders, or the trimmer screw has
been tightened.

On 75 meters where the padder capacitances are a great deal larger than
the variable loading capacitor, moving the loading capacitor lever has
little effect. But on 10 m where there is no padding capacitor needed,
the loading capacitor lever has a lot of effect and so there is no scale
to the loading capacitor arm beyond the 50 ohm points set by the
trimmers IF SET CORRECTLY. That also means that the radio will load to a
wider range of impedances on 10m than on 75 meters. That is a normal
limitation of a transmitter output pi network.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer