[Hallicrafters] S-38C dial tracking and AGC

[Don Ehrlich]

Craig, Glenn, et al,

The suggestion about selecting the correct image is a very good one.
I had forgotten about that.  That is important on the higher frequency
bands when peaking the rf coils, though, not the IF.

Glenn is presenting flawless logic regarding the possible effect of a
few kc change in IF frequency on tracking.  Unfortunately, my
experience is that it can make a big difference.   How can I resolve
this contradiction?

First of all, IF's can be tuned quite a bit farther off their design
frequency than +/- 5 kc.  You or I would not do that (ha!) but a
series of previous owners turned loose with a screwdriver while tuning
for maximum volume sure could.

The effect of IF frequency on tracking is more complicated than just
shifting the apparent position of the dial a few kilohertz.  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.  Now adjust the IF
freq.  and do it all over again.

... is everybody still awake? ....

OK, remember that the trimmer that sets up the bottom of the band has
relatively little effect at the BOTTOM of the band and so it had to be
changed quite a bit after changing the IF frequency to keep the dial
at 550 Khz with a 550 Khz signal.  But at the TOP of the band that
trimmer has a lot of effect so the main tuning capacitor position at
the TOP of the band changes quite a bit ... and thus the dial pointer
position ... and thus tracking.   This effect is far greater at the
top of the band than the few Khz you might have moved the IF
frequency.

It's getting late and I'm not sure if I will still believe this in the
morning.  I can't wait to find out.

Good night all,

Don  K7FJ


----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: "Craig Roberts" <crgrbrts at verizon.net>;
<hallicrafters at mailman.qth.net>
Sent: Thursday, December 11, 2003 12:59 PM
Subject: Re: [Hallicrafters] S-38C dial tracking and AGC


> Considering that the i.f. frequency in the S-38 series
> equipment is 455 KHz, and the fact that you can't vary
> it more than +/- 5 KHz (and probably less), the i.f.
> frequency alignment isn't going to make any difference
> in the calibration!  It only means that the injection
> oscillator is going to be shifted up to 5 KHz in
> either direction and, frankly, you can't tell that
> difference in the calibration of the S-38 series dial!
>
> Now, you can run into problems with images and align
> the dial at one end or the other on the image which is
> 910 KHz away from the desired frequency.  That will
> cause all sorts of problems with the dial accuracy
> since you think that it is on one frequency at the low
> end and another at the high end.
>
> Single conversion receivers are VERY prone to images
> especially on the higher frequency bands especially
> with lower i.f. frequencies.
>
> Glen, K9STH
>
>
> --- Craig Roberts <crgrbrts at verizon.net> wrote:
>
> Regarding the dial tracking, he makes a very good
> point about IF alignment.
>