[R-390] R390A alignment question

[flowertime01 at wmconnect.com]

To help me understand, if each of the filters in a R-390A has a slightly
different bandpass + or - 455 and the IF is tuned precisely to 455 won't you
get some loss of gain anytime you select a filter slightly off of 455.  I
now understand why aligning a radio with one filter you tune for max using
the center of the single filter.  With the R-390A isn't the stagger tuning
to account for the multiple filters giving ample gain for each filter?  I
ask this question because I don't have the equipment to stagger tune the IF
and wondered exactly why it was stagger tuned in the first place.

Ben,

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I think Tisha posted a good response to most of your question. 

The stager tuned decks and straight tuned decks had different IF cans.
While the schematic and part count are exactly the same in the stagger 
tuned and straight tuned decks, the actual circuit response and performance are 
different.
You do not straight tune or stager tune a R390/A deck on whim.
Your specific deck is one or the other. It is not both.
The question is how do you know which deck do you have.

Each mechanical filter will give you a different gain, but not because it 
is off frequency. If your filter is off frequency far enough to cause loss 
you would replace it. The trimmer caps were added to help "balance" the 
differences in gain between filters. Practice is not to balance gain but to "peak" 
each filter for what ever max signal can be achieved.

Once upon a time it was known which contracts were which.

It was never cleanly printed in a TM, thus it is lost.
We will not mention depot deck swapping to add to the mix-up.

There was once a nice article in a monthly Army magazine that provided all 
the ugly details. I am sure some Fellows remember Connie. We could wish 
someone collected that publication and would cull the R390/A articles for us. 
Maybe in the next life time. (:, 

One clue is newer decks have trimmer caps on the mechanical filter. If your 
deck is new enough to have trimmer caps on one end or both ends, your deck 
is new enough to be straight tuned.

The square can with the indent for the nut was to get enough height under 
the square can to clear the trimmer caps. The indent was to keep the bolt and 
nut height under the top cover plate.

And I add some more thoughts.

In the beginning long long ago:
The mechanical filters were centered on 455KHz.
Decks that were stager tuned had different parts than later decks that were 
all centered on 455KHz.
Only one mechanical filter is used at a time.
The whole IF strip and its cans are used all the time.
Les just pointed out to us that the 8KHz filter is in fact 11KHz wide. 
Always has been.
The IF cans without filters have a band pass wider than 16KHz. This in fact 
makes it hard to use a sweep generator to tune the cans when the 16KHz 
mechanical filter is slicing off the corners of the band pass before we can see 
the real band pass of the IF deck cans alone.
The best we can do with the sweep generator is make sure the cans do not 
"crimp" the 16KHz band pass.

Because of the fact of circuit, it was always questioned why any one would 
even try to use the sweep generator to align the IF deck any way. We were 
hard put to find a deck that was stager tuned to start with. Any straight 
tuned deck was just easier to peak with a AN/URM25 set to 455 as determined with 
the frequency counter setting under it on the bench shelf.

While any mechanical filter may not be exact on 455 we do not expect the 
8KHz filter to be off by 4KHz or more. If it was you would replace it. The 4 
and 2 filters can be even further off center and still not be outside the 
16KHz skirts we expect from the cans.

The cans in the straight tuned IF deck do not really come close to having 
16KHz skirts. The cans perform more as impedance transforms than filter 
functions. The can peaks are way up above the flat filter tops. 

As you tweak a can slug around the metered receiver output goes up and 
down, not because you are just moving the band pass of the can around, but more 
because you are getting a better or poorer impedance transform between two 
stages.

Understand the tweak operation is dynamic, and multi faceted. It's not 
simple.

Roger AI4NI
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