[Hallicrafters] Hallicrafters Piston Trimmer Query

[Edward B Richards]

Two suggestions;

1. replace C39 with the correct part.
2. Replace C59 with a new one.

73, Ed Richards K6UUZ


On Thu, 20 Jan 2005 17:56:46 -0600 "Mark Shaum" <k9tr at dtnspeed.net>
writes:
> My SX-111 is on the bench.  After many moons on the shelf it 
> developed
> some instability on 20 meters that was not bandswitch related,
> thankfully.  Component substitution determined the culprit was C39, 
> a 39
> pf N330 dogbone ceramic in parallel with the HFO inductor.  I 
> replaced
> it with a 33 pf NPO temp coefficient disc (which measures 35 pf) as 
> it's
> all I had close to 39 conveniently available.  The 111 drifts a bit
> anyway, so I'm not overly concerned about the N330 temco rating.
> 
> As it turns out that 4 pf makes for a major frequency bump.  
> Correctable
> by adjusting the inductor, but WWV, which shares the coil, is then 
> off
> by an inch or more on the dial. I could easily compensate by 
> adjusting
> trimmer capacitor C59, a 3-30 pf piston style trimmer which is also 
> in
> parallel with the coil.  Except that the lead screw for this 
> trimmer,
> and all other trimmers in the set, are soldered in place.  I wicked 
> away
> all visible solder and still cannot budge the lead screw on this
> trimmer.  Much heat applied, it still won't budge, I fear any 
> greater
> torque will sever the lead screw.
> 
> I know Hallicrafters solder-sealed trimmer lead screws in some other
> sets.  Has anyone actually been able to determine if these are
> re-adjustable at all when the solder is removed or were these some 
> form
> of cheap set, solder, then forever forget adjustment style of 
> capacitor?
> The Halli manual I have for the Mark I calls for adjusting these
> trimmers as part of the calibration routine, so I suspect not all 
> were
> soldered in place at the factory.
> 
> Normal trimmers won't work well here as a delta change of a couple 
> pf
> will put you off by 100 khz or so, making the usual diddle stick 
> trimmer
> twist very touchy.  The piston style trimmer makes sense, as you 
> have
> more fine adjustment room.
> 
> If I can find a small piston trimmer in in the junque box to 
> install, I
> may remove this trimmer and likely perfom some destructive 
> disassembly
> to determine what the innards look like.  But tossing the question 
> out
> first, I'm sure others have been in this situation.
> 
> 73! - Mark
> ------------------------------------------------------------
> Mark Shaum K9TR
> email: k9tr at dtnspeed.net
> http://www.qsl.net/k9tr
> ------------------------------------------------------------
> 
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