[Hallicrafters] S-38C dial tracking and AGC

[Glen Zook]

Although the AM broadcast band might be affected by
the "gross" mistuning of the i.f., the other 3 bands
will not be affected that much.  There is no way that
you are going to be able to vary the 455 KHz i.f. more
than 50 KHz (and I doubt very much even that far in
most sets unless someone has actually gotten into the
i.f. cans and done some damage).  On the higher bands,
50 KHz is about the width of a dial marking on the
main tuning dial and it is just not going to make that
much difference.  On band 2, maybe a very slight
difference but on bands 3 and 4 you can't read the
dial of an S-38 that closely!

What I have usually found to be the problem is the
picking of the image instead of the desired frequency
and tuning the circuits to match that.  Usually the
low end is the correct frequency and the high end ends
up being the image.  When this happens, the tracking
is definitely going to be way off.

Also, on virtually every receiver that I have worked
on except those amateur band only ones that invert the
tuning oscillator frequency, the low frequency end is
not set with a trimmer capacitor but is set by a slug
tuned coil.  The high end is set with a trimmer
capacitor.  On those amateur band only sets that use
the inverted oscillator frequencies, then the high end
of the band is set with the slug tuned coil and the
low end with the trimmer.  Although the dial is
reading a lower frequency as the oscillator is tuned
higher in frequency the received signal goes lower in
frequency.  However, on the S-38 this is not the case.
 The slug tuned coil sets the low frequency end and
the trimmer capacitor sets the high frequency end.

Glen, K9STH


--- Don Ehrlich <ehrlich at olypen.com> wrote:

First of all, IF's can be tuned quite a bit farther
off their design frequency than +/- 5 kc.

Consider the typical condition where  the IF is set
dead on 455 Khz and then the dial is positioned
correctly at the bottom of the broadcast band while
the variable cap trimmer is adjusted to receive a
signal at 550 Khz with the dial right on 550 Khz. 
Then, hypothetically, tuning to the top of the band
may then show lack of tracking. 


=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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