[Hallicrafters] Hallicrafters Piston Trimmer Query

[Mark Shaum]

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
------------------------------------------------------------