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

Fri May 10 12:34:28 EDT 2024

Thanks Dave and Rich for those helpful tips!
The RBMs and TCS rcvrs I did years ago I guess I was very lucky with only
minor issues.
 I have the RAX rcvrs I'm going to be working on and Dave's tips on those
caps will be helpful to say the least.

As an aside...looking for one more RAX shock mount to compete the 3
radios..any leads on one would be greatly appreciated.

Paul
N6FEG

On Fri, May 10, 2024, 8:19 AM Rich Post <kb8tad at gmail.com> wrote:

> 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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