[Boatanchors] [BoatAnchors] Scott SLRM - Low BFO Injection

[David Stinson]

On 9/8/2019 12:27 PM, Ben Hall wrote:

> On 9/8/2019 9:43 AM, David Stinson via BoatAnchors wrote:
> This one had the "busted trimmer nut" problem in the LO of Band 3.
>
> Hi Dave,
>
> I went looking thru the archives (both my local and the subscription 
> archive) and couldn't find anything about the plastic insert solution. 
> Got a copy you could forward me?  In the past I've fixed the split 
> nuts by soldering the busted collar nuts back on, but it's a 
> super-pain in the rear and not always successful. So I'd love to hear 
> a better method!

Hello, Ben!
This is what I do.  Works well for me.  From a post in late 2017, and a 
short article in Jan 2018 ER:

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 youself 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 replacment (used 
the only one I had on another project), what can be done? I've tried 
cleaning the shaft and nut  throughly 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 seperated 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

Carefully ompressed the rotor shaft with channel locks 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



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