[R-390] Staggertuned IF Question

[Roy Morgan]

At 10:03 AM 9/14/2006, n4buq at knology.net wrote:
>Are all IF modules capable of stagger-tuning?

Barry,

Certainly, it just depends on how staggered you try to make it.

With the development of 30 mc IF amplifiers for radar systems in WW-II, 
there came a lot of study of the stagger tuning of rf/if amplifiers. Text 
books of the era contain the theory and practical methods for arriving at 
flat passband response.  This depends on setting the Q of the tuned 
circuits, the mutual coupling values,  and separation of tuning points, as 
I remember.

Imagine a strip tuned to one frequency. It will have the well understood 
bell curve pass band shape. It the Q of the coils and their coupling 
coefficients will determine the shape of the transfer characteristic.  If 
you now tune some of the circuits slightly above and below the center, the 
top of the curve will broaden. If you tune them farther and farther away 
from center, you can imagine a double or triple hump characteristic. 
Getting the whole mess to be very flat depends on the Q of each circuit, 
the coupling amounts, and the degree of departure in tuning frequency from 
the center.

>... The IF won't even think about hearing
>a 467kc signal.  I was able to do this on my Motorola, but not this one.

We can assume this is an R-390/URR (the "Non-A"). Notice that T-501 and 
T-506 have resistances across the primary and secondary of the transformer, 
respectively. All the IF transformers also use multiple windings in a 
"feedback" arrangement to arrive at various selectivities. The IF 
transformers are pretty complicated, and  were made with carefully 
engineered coupling coefficients to make this all work out right.

>I seem to recall reading where not all IF modules were designed to
>accomodate stagger-tuning.  Is this true?

It well may be, but this is the first I've heard of it.

>  If not, then I suppose I have
>more work to do.  If so, then do I assume I peak all the slugs for 455kc?

If you can arrange a sweep generator/detector system to do your tuning, you 
will be able to see the effect of your tuning experiments.  An Army 
publication ST 32-152.pdf, entitled "Visual Alignment of Radio Receivers 
R-390/URR and R-390A/URR" is a good place to start.  Go to: 
http://www.r-390a.net/ and look under References, in the Army section. (It 
describes sweepers that do not have retrace blanking, so adaptation of the 
methods is needed for more modern equipment.) There have been articles 
published on how to build sweep generators.  I don't know if FAR Circuits 
has such a board/article available, but someone should check their web site 
and report.  But plenty good enough sweep generators are available on the 
surplus market.

These topics of stagger tuned IF strips and sweep alignment methods are 
both very interesting to me, though I have not yet done very much 
experimenting.  When my Radio Emporium is set up at my new QTH, I certainly 
will.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
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