[R-390] Re: Trim Caps Science Experiment

[Drew Papanek]

Roger wrote:

>I do not have a sweep generator and would like someone to conduct a science 
>experiment for us using said test equipment.

>Selecting an IF deck with filter trim caps. Applying the sweep generator to 
>the IF at 455Khz plus and minus 16 Khz. Monitoring the output of the IF 
>deck with suitable scope to go with sweep generator.

>While sweeping a mechanical filter and observing the output band pass, 
>slowly vary the trim caps. Repeat this procedure for each mechanical filter 
>and adjust both top and bottom cap for each filter.

<snip>

The same could be done without a sweep generator although much more 
laboriously.

The test would reveal something about the relative Q of the mechanical 
filter drive and pickup coil circuits.  If the Q were quite low then the 
peak from adjustment would be quite broad and adjustment would have little 
effect.

>If the filters do not shift frequency or bandpass width, then any other 
>change in filter response from adjusting the trimmer caps can be "faked" by
>making other adjustments else where (IF gain adjust) in the receiver.

That would be true in the low Q case.  If the Q were higher yes, we might 
still be able to compensate using the IF gain adjustment but then there 
would be excessive gain ahead of the filters.  The total gain of the IF 
strip would meet spec but the higher gain before the filters would make the 
receiver more susceptible to intermod from close-in signals.  (Perusing the 
"Cost Reduction" Engineering Report" revealed that most of the intermod came 
from the first IF amp stage, ahead of the filters.)

Would that really matter?  How many of us use our R-390a's in close 
proximity to multi-kilowatt transmitters?

>Thus rendering the trim caps nice things to have and diddle with but not 
>required to make the minimum passing receiver.

Fiddling with the adjustments is part of the joy of having one of these 
radios.  The joy would be compounded if the adjustments were done as part of 
a modification that YOU did.

There are risks involved in adding the caps to a non-cap deck.  The foam 
innards of our aging mechanical filters likely have deteriorated, making the 
filters very susceptible to damage from mechanical shock.  So, during the 
drilling and blasting that cap installation would entail, it might be best 
if the filters were removed.  Would the benefit exceed the effort?

How about selecting fixed caps to resonate the filters just as it was done 
at the factory?  That would be simpler and safer.

How about soldering small ceramic trimmers across the filter terminals?  
Mouser and others carry some very compact units which look like they could 
be used here.  Likely one would have to add some fixed capacitance in 
parallel, but the end result would still be adjustable.   If  for 
convenience, perhaps just the most-used filters could be resonated in that 
way.

One could procure a deck having the caps but no filters (I think Fair Radio 
still has decks so un-equipped) and transplant the filters from the 
functional but capless deck.

>Thanks for taking on the project and reporting your results. Just what do 
>them caps really do for us?

I am an armchair  "engineer" and leave that exercise to someone else less 
"motivationally challenged" than I am.

Drew