[Hallicrafters] SX-28 Dial Tracking Issue

[Richard Knoppow]

     I suspect that the rotor is not centered in the stators.  This can 
come from the tension of the end bearing being wrong. The curve of 
capacitance vs: rotation changes if the rotor is not centered.  If you 
have a means of measuring the capacitor its simple to adjust it to the 
_minimum_ capacitance with the plates fully meshed. If not try adjusting 
to the low end of the dial. Zero beat the generator and try adjusting 
the rotor end bearing. If its off it should move to a higher frequency. 
Find the exact center point.  That may cure the bump in the dial 
calibration.

On 8/8/2016 6:38 PM, Jim DiMauro via Hallicrafters wrote:
> 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
>
>
> ______________________________________________________________
> Hallicrafters mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Hallicrafters at mailman.qth.net
>
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: dfischer at usol.com **
>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL