[ARC5] RAX Project: Busted Trimmer Cap Nuts

[David Stinson]

Both early TCS and RAX receivers suffer from
trimmer capacitors with "busted nuts."  These
(typically 50 pFd) trimmer caps are assembled by
compressing a 1/4" nut over the rotor shaft, thus
compressing a spring, setting the proper spacing
between stator and rotor plates.
Over 70 years, the stressed, friction-hold nuts
can crack, releasing the tension on the spring,
forcing the rotor down and contacting the stator,
shorting the cap.
https://photos.app.goo.gl/jTfihh6CKf8Oysdp1

The nut can often be removed with tweezers.
https://photos.app.goo.gl/sG7iXQrj83Aq7ddy1

When you pull the module with the broken cap, you
find:
https://photos.app.goo.gl/yOXVEOPqZsvJXgeA3

It can be hard to find these original trimmers.
Count yourself lucky if you have a direct
replacement.  In early TCS receivers, these can be
"double-mounted"
(two caps on a single piece of ceramic), which
makes them even harder to find.

If one doesn't have a direct replacement (used the
only one I had on another project), what can be
done?
I've tried cleaning the shaft and nut thoroughly
with acetone followed by naptha, then using JB
Weld to glue the nut in place.  Put a small dab of
JB Weld on the rotor shaft, careful to keep it
from being spread all the way to the bottom of the
nut and thus, sticking the shaft forever.
Compressed the nut and shaft with long-nose
Vice-Grips adjusted for the right spacing and left
it sit overnight.

In three attempts, this worked once (the first
time).
The second one got stuck with a small amount of JB
too far down the shaft.  The third popped right
back off.

Thought about it awhile- the idea is to get the
plates separated and stable so the cap can be
used.
Cut some straight strips about 1/4" wide of that
clear, stiff plastic used in "blister" packaging.
https://photos.app.goo.gl/0kouX4ufjRg6kuDo1

Compressed the rotor shaft and inserted these
strips between the shorting plates, fitting
between the rotor shaft and the two posts holding
the stator.
https://photos.app.goo.gl/mjGvhpyOkYY1LSgn1
https://photos.app.goo.gl/jFh9FCdaVuFlvePf1

Once all the plates are insulated, set the rotor
at about 1/4 mesh so that we need move it slowly
in only one direction to find the alignment point
and avoid pushing the plastic inserts out.  Gentle
"tweaking" back and forth at the alignment point
was not a problem.  

Does it change the dielectric constant of the cap?
Yes.  Does it make a difference?  Not a bit.
Several original caps are in circuit this way and
working fine.  Once aligned, we're not moving them
much if at all, so this should save us some radios
being "shelf queens" for lack of trimmers.

GL OM ES 73 DE Dave AB5S