[Hallicrafters] SX-28 Dial Tracking Issue

[Richard Knoppow]

On 8/8/2016 8:04 PM, Richard Knoppow wrote:
>     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.
>

     A little more since this is not very intuitive.  Since the 
capacitor will have too high a value you will compensate it at the low 
end by reducing the amount of inductance of the coil.  At mid range the 
coil will be too small but the added capacitance from lack of centering 
will not compensate so the frequency reads lower than the actual 
frequency. At the high end you compensate with the trimmer but the value 
of the trimmer is small compared to the mid frequency value of the main 
capacitor, so it won't correct the error.
     The lack of centering may be visible but even a small amount will 
cause problems so check it electrically.
      Please do not resort to bending capacitor plates until you have 
exhausted everything else. If they are not bent they were never bent at 
the factory and something else is the problem.

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