[Hallicrafters] SX-28 Dial Tracking Issue

Tue Aug 9 11:13:21 EDT 2016

Rich, this article might help. It goes through the various production changes.
Joe Connor
SX-28 NOTES

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SX-28 NOTES
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    On Tuesday, August 9, 2016 10:26 AM, Rich Oliver <kc9gq7 at gmail.com> wrote:

 My SX-28 is a bit peculiar in that it has a fused power plug and there is
no provision for internal fuses.  Another oddity is that the tuning cap
shield cover plate is smaller than most.  I take these differences to mean
that this is a very early unit.  Since there was a change to the capacitor
shield I wonder if there might be a change in the tuning cap, itself.
Unfortunately I cannot easily access it right now (bad back).  Has anybody
else encountered an SX-28 like this?

73, Rich

On Tue, Aug 9, 2016 at 6:38 AM, Jacques Fortin <jacques.f at videotron.ca>
wrote:

> Hi all,
>
> I have found at least two sources that gives the same tracking adjustment
> sequence.
> The procedure is called three point tracking alignment and involves padder
> capacitors in the local oscillator tank circuits.
> This can be resumed as:
> 1.      Adjust the COIL INDUCTANCE to obtain the proper alignment at the
> CENTER of the dial.
> 2.      Adjust the TRIMMER CAPACITOR to obtain the proper alignment at the
> HIGH end of the dial.
> 3.      Adjust the PADDER CAPACITOR to obtain the proper alignment at the
> LOW end of the dial.
> 4.      Repeat the sequence until no improvement is possible.
>
> This should be the original procedure used at Hallicrafter's….
> Explained under Oscillator tracking at : http://users.tpg.com.au/users/
> ldbutler/Superhet.htm
> Once properly done, re-alignment can be performed using only LOW and HIGH
> adjustment points (two point tracking), as described in the SX-28(A) manual.
>
> 73, Jacques, VE2JFE
>
>
> -----Message d'origine-----
> De : Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] De la
> part de Richard Knoppow
> Envoyé : 9 août 2016 00:15
> À : hallicrafters at mailman.qth.net
> Objet : Re: [Hallicrafters] SX-28 Dial Tracking Issue
>
>    I forgot about the padders. I would certainly check those before doing
> anything mechanical to the tuning capacitor.  Sometimes you can get things
> on by setting the low end with the coil and the mid band with the padder.
> Do this a few times until both are on. Then do the high frequency trimmer
> and check the low and mid again.
>
> On 8/8/2016 8:29 PM, Jacques Fortin wrote:
> > Hi Jim,
> >
> > My best guess is that somebody have played with the local oscillator
> > padders
> > (FP1 to FP6 in the manual, or C7 to C12 in the schematic).
> > If someone, in the life of this set, have tempered with those, it can
> > well explain why you succeed to align the bands ends and be off at the
> centers.
> > The effect of the padders is like changing the shape of the variable
> > capacitor rotor plates (like spreading the adjustment sections in
> > other radios).
> > Both the SX-28 and the SX-28A manuals are specific about the FP
> adjustments:
> > DON'T TOUCH THOSE.
> > But you know, some never reads the manuals....
> > Just try the following to see if this can be related:
> > On any given band (say the highest one) just carefully note the actual
> > position of the related padder (C7) then purposely turn it a quarter
> > turn in either direction, then realign the ends using C103 and S14.
> > How is the center tracking, now ?
> > It can be better or worse, but at least you will have a starting point !
> > The TM11-874 schematic gives the padder capacitor values for the
> > SX-28A, but I do not find any alignment procedure for those...
> > Maybe other people will know better (I hope).
> > I am not "in" my own SX-28A right now, but I will be during the next
> > year for sure.
> > Please make us aware of your findings !
> >
> > 73, Jacques, VE2JFE
> >
> > -----Message d'origine-----
> > De : Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] De
> > la part de Jim DiMauro via Hallicrafters Envoyé : 8 août 2016 21:39 À
> > : Hallicrafters <hallicrafters at mailman.qth.net <mailto:
> hallicrafters at mailman.qth.net> > Objet :
> > [Hallicrafters] SX-28 Dial Tracking Issue
> >
> > Hi All:
> >
> > I recently finished restoring a 1941 vintage SX-28. The rebuild
> > included all paper caps, a few micas that were out of spec and all out
> of spec resistors.
> > I proceeded with the IF alignment, which seemed to respond well to the
> > procedure in the manual, and then I did the RF alignment. That¹s where
> > things didn¹t seem quite right.  I set the low frequency and high
> > frequency alignment points on each band, and the main tuning dial
> > lines up perfectly at each of the low and high end alignment points.
> > The problem is that the frequency accuracy of the dial degrades as I
> > move from the end points to the center of the band, with the error
> > being greatest at the center. I don¹t expect a radio of this type to
> > track like a Collins PTO would, but the tracking error is excessive
> > when compared to other radios that I restored, like my NC-200 and
> > HQ-129-X, both of which are nearly spot on from one end of each band to
> the other.
> >
> >
> > I took detailed measurements that show the magnitude of the error on
> > the SX-28. Here are sample measurements from each band: Band 1 aligns
> > perfectly at the alignment end points of 600 kc and 1400 kc, but the
> > approximate band center shows a 20 kc error (i.e. an 800 kc signal
> > reads 780 kc on the dial); Band 2 shows a 50 kc error at 2000 kc; Band
> > 3 has an 80 kc error at 4000 kc; Band 4 is off by 200 kc at 7700 kc;
> > Band 5 is off by 300 kc at 14000 kc; and Band 6 is off by 500 kc at
> > 28000 kc. Again, in all these cases the dial reading agrees with the
> > signal frequency at the high and low end alignment points, and in all
> > cases the dial reading is lower than the actual frequency.
> >
> >
> > Interestingly, the error in terms of the physical movement of the main
> > tuning capacitor seems to be consistent among all of the bands at the
> > band centers. To clarify, the difference in the main tuning dial
> > logging scale (the metal disk on the main tuning knob) between the
> > actual and dial frequency at the center is about 30 points on all
> > bands. If it was due to a problem with the gear train I would not
> > expect the end points to remain accurate after repeated tuning up and
> > down the band. I took care to ensure that I didn¹t align on image
> > frequencies. The band spread dial is installed correctly and I did the
> > alignment with the band spread capacitor set for minimum capacitance
> > (100 on the logging scale). There are no obvious physical issues with
> > the main of bandspread tuning caps, and there is no evidence that the
> > capacitor plates had ever been ³knifed.² Any ideas would be
> > appreciated. The radio looks and sounds great, but the magnitude of the
> dial error is driving an OCD person like me nuts.
> >
> > 73,
> > Jim
> > WA2MER
> >
> >
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>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>
> WB6KBL
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