[ARC5] Fwd: RAX Project: Busted Trimmer Cap Nuts

Fri May 10 11:18:34 EDT 2024

In the Navy RBS, I had the same problem.  The trimmers in that set had a
pin in the shaft which limited the rotation to a half turn.  The pin was
then touching the ceramic body when the trimmer failed.  Looking for a
spacer between the two, I found that a number 22 solid copper wire between
the pin and the body was a perfect fit.  I simply twisted the wire into a
circle for a simple solution.

Rich KB8TAD

On Fri, May 10, 2024 at 1:41 AM hwhall--- via ARC5 <arc5 at mailman.qth.net>
wrote:

> The RBM sets have a similar problem & this will work for them, too.  I, as
> usual, went the harder road with my RBMs. LOL
>
> Wayne
> WB4OGM
>
> On Thursday, May 9, 2024 at 04:31:25 PM MDT, David Stinson <
> arc5 at ix.netcom.com> wrote:
>
>
> There's been some recent discussion on this and I've been
> asked for input, so I respectfully re-submit this
> from 2017, with an update.
> Dave AB5S
>
>
> --------
> Subject:
> Busted Trimmer Cap Nuts
> Date: Fri, 1 Dec 2017 13:34:53 -0600
> From: David Stinson <arc5 at ix.netcom.com>
>
> 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
>
> (Update: have found one doesn't need to put a spacer
> between every plate.  Three evenly-spaced works
> for me, spacing all the plates, unless there is
> physical damage.)
>
> 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
>
>
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