[Hallicrafters] SX-28 Dial Tracking Issue

[Charlie T, K3ICH]

I'm coming in a little late on this topic, but are you absolutely sure you
have adjusted the LO for the proper injection above or below the dial
frequency?

If the LO is supposed to run above the dial, or high side injection, and you
have it running below the dial frequency, it may explain your situation.

Sorry if this insults your intelligence, or, if you have previously cleared
this point up.

Then again, this may be a moot point since you would probably run out of
adjustment range on either the coil or the cap, making it impossible to even
get the end points set properly if you have the LO on the wrong side.

It's an easy trap to fall into, especially on a radio that's had the golden
screwdriver treatment.  I always have a counter monitoring the LO and check
the manual to be sure where it should be running.

Forget the signal generator for the dial tracking.  Just use your counter
and adjust the LO to where it "should" be, then tweak the rest of the front
end with the signal generator.

73, Charlie k3ICH



-----Original Message-----
From: Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] On Behalf
Of Richard Knoppow
Sent: Monday, August 08, 2016 11:32 PM
To: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] SX-28 Dial Tracking Issue



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
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