[Hallicrafters] SX-117 VFO Alignment

[darrel nichols]

On Monday 21 October 2002 10:51, you wrote:
> Darrel:
> Would you please expound on your statement that you "...cannot get the dial
> to calibrate...". Exactly what do you mean by calibration? Calibration
> depends upon the adjustment of both L16 and C15F. Getting these right can
> be frustrating.
> This VFO only tunes 500 kHz. I would be looking for an off-value component
> before I started bending plates.
>
> I own two such receivers and calibration on both is well within the reach
> of C38, the front panel CAL RESET. Adjustment is only plus or minus 20% of
> "mid-mesh" for this control when changing bands and can generally be left
> alone when QSYing in-band.
>
> I will note a discrepancy between both of these units and the factory
> manual. C15F is shown as an integral part of the tuning capacitor (closest
> to the front panel) in the manual. On both receivers this capacitor is/was
> a 5-25 pf, NPO, ceramic trimmer. The compression trimmer is totally absent
> and it appears as if it was never installed...ever. Further, on one of
> receivers I found it necessary to change that trimmer to an air-variable
> (very small EFJ-type). For whatever reason, it was quite sensitive to
> temperature changes and located in an area prone to draft. The receiver was
> quite drifty and never actually settled down. Both receivers now are rock
> solid (10s of cycles/hour) after warm-up.
>
> 73,
> Dick K9FFK
>
Mine must be an early model (serial 252009) and does have the integral
trimmer in the cap only.  That setting is quite flakey.  No small wonder they
upgraded it during the run.

I've been back and forth between the trimmer and coil in both directions
attempting to find a "linear portion" of their range with no joy.   At the 
give-up point yesterday, charted on freq at the high end, with the dial
off by +8 KC at 400 and 300.  +.5 at 100 and -17 at the bottom end.
These readings are taken with a counter at the VFO ouput connector.
Clearly the cap needs some warping and will take a special touch
from the readings I am seeing.

Stability is within the limits of a design of this nature so do not suspect
component failure at this point.  Don't see any owner enhancements in
the circuit.   

I'll look it over some more before I take heroic measures.

Darrel, N0DBX