[Collins] (no subject)

[Dr. Gerald N. Johnson]

On Wed, 2007-03-14 at 12:03 +0000, w2cqm at juno.com wrote:
> 
> 
> Thanks to all of those who offered suggestions to remedy a Collins 
> 30L1 tuning problem. My main concern with the initial query was 
> determining whether or not a bad tube could cause serious impedance 
> mismatches that would  ultimately  effect the loading capacitor 
> sequence. Some suggested changing several of the obvious parasitic 
> components in the plate circuit;and only then consider the tubes for 
> replacement. Cleaning up the bandswitch controlling the additional 
> banks of loading capacitance  made equally good sense.  I'll do that 
> and save the tube replacement for last. Again, what I found odd was 
> that resonance on 80m was with minimum (zero)antenna tune capacitance; 
> and on 40m there there was full loading (meshed) capacitance. I found 
> that somewhat unusual for an antenna matching design. I'd expect some 
> partial value of capacitance on that control. Some individuals took 
> issue with the fact that I expected some detuning (or reduction of 
> power) when rotating the antenna tune control. Perhaps my choice of 
> words was no clear.  I mentioned the "dip" with the antenna tune 
> control to indicate I was at the extreme ranges of capacitance on each 
> of the two bands tested. Thanks again to all who took the time to 
> share their thoughts. It was very much appreciated.  Best regards Ron 
> W2CQM/3  
> 
The wording of your post indicated a general lack of knowing how the Pi
network tunes and works and now you admit it left out details.

There will be a position of the load control (not at all an antenna
tuning control) that gives maximum output, but the plate tuning
capacitor has to be resonated. The load control only sets the impedance
transformation ratio while the plate tuning resonates the whole network.

There are four sections to the loading capacitor. Two are used on all
bands, a third is added for 20 meters and all four are used for 80 and
40 meters. 

We focus on tubes (parasitic suppressors should have the most effect on
spurious outputs on 10 meters), bandswitch contacts, and loading
capacitor because tubes are the most delicate, but are not necessarily
frequency sensitive. Bandswitch contacts and the loading capacitor are
frequency sensitive.

In a basket case linear, damage to all components is possible. Look for
dirt or bent plates in the loading capacitor that cause it to short when
turned towards maximum capacitance. 

The situation where you get maximum power output at minimum C (maximum
loading) on 80 but normal C on 40 looks at the capacitor being bent. You
could check that with power off and using an antenna analyzer on the
output connector while pushing the antenna relay manually. The Z should
change smoothly for all loading capacitor positions. If it shows an RF
short, there's the problem. There is an RF choke across the output (and
if its been changed to one with too little L, output and loading can be
odd) that prevents making such a test simply with an ohmmeter. Unhook
the RF choke and you can test with an ohmmeter.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer