[Hallicrafters] SX-117 VFO Alignment

[Koehl_House]

Well, Darrel, you did not mention if you had access to a manual. I'm
assuming you do. There is one available on BAMA. One thing to keep in mind
is that this vfo tunes backwards...the low end of the band is 4.85 MHz, the
high end is 4.35 MHz. I appears that the C15F is set too high and L16 is too
low. You might consider backing that compression trimmer out and tacking a
small ceramic/air-variable across it. Either is easier to adjust than the
original (I have a R-100 that uses compression trimmers...bear city!).

Here are a couple thoughts. First, I have a note about the VFO in the
HT-44...same circuit. The VFO, a Colpitts type, quits oscillating on the low
end of the band when the tube gets weak. If you graph your offsets vs.
frequency, you will notice that the bottom end of the band falls off
drastically, while the middle and upper end appears fairly linear and flat.
This may be an indication of this type of problem. Second, and this may be
reaching a bit, the oscillator depends upon the ratio of the grid-cathode
and cathode-ground capacitor. If that ratio is wrong the oscillator doesn't
work on the low end of the band (high frequency).

To Summarize:
1. readjust L16 and C15F.
2. change the tube (or try a different tube).
3. look for weird changes in circuit components.

Something to think about...

73, Dick K9FFK

 -----Original Message-----
 From: hallicrafters-admin at mailman.qth.net
 [mailto:hallicrafters-admin at mailman.qth.net]On Behalf Of darrel nichols
 Sent: Monday, October 21, 2002 7:42 PM
 To: Koehl_House; hallicrafters at mailman.qth.net
 Subject: Re: [Hallicrafters] SX-117 VFO Alignment


 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


 _______________________________________________
 List Administrator: Duane Fischer, W8DBF **for assistance**
 dfischer at usol.com
 ----
 Hallicrafters Collectors International: http://www.w9wze.org
 ----
 http://mailman.qth.net/mailman/listinfo/hallicrafters