[Hallicrafters] SX-28 Dial Tracking Issue

Tue Aug 9 00:15:00 EDT 2016

    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>
> 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
WB6KBL