[R-390] Filter repair

[Norman Ryan]

Hi, All,

I suspect Bob is referring to soldering the bobbin wires to the end cover terminals.  This link might be helpful for better understanding the challenge of soldering hair-fine magnet wire:

http://www.microjoining.com/articles/FlashSoldering_Extending_Applications.pdf

(If clicking the link doesn't work, copy and paste to your browser.)

This mechanical filter repair thread is of great interest to me.  Sometime down the line I want to try my hand at mechanical filter restoration and hope eventually to have results worth sharing with the group.

Carry on and good luck!
Norman


--- On Sat, 3/9/13, Bob Camp <ham at kb8tq.com> wrote:

> From: Bob Camp <ham at kb8tq.com>
> Subject: Re: [R-390] Filter repair
> To: "r-390 at mailman.qth.net Group" <r-390 at mailman.qth.net>
> Date: Saturday, March 9, 2013, 4:07 PM
> Hi
> 
> There is a problem when soldering small diameter copper wire
> known as "neck down". It's an issue once you go past about
> number 36 wire. If you cross section the solder joint you
> can see it fairly well. The diameter of the wire is smaller
> inside the solder than it is normally. The reason is fairly
> simple - some of the copper dissolves in the solder. 
> 
> Here's where the problem comes in - the dissolved area
> extends *above* the surface of the solder. The diameter of
> the wire shrinks before it hits the joint. The why gets into
> how flux and solder do their thing.  The net result is
> that the wire is both weaker and less stiff right at the
> joint. Motion in the wire (think vibration) will break the
> wire at the joint. 
> 
> The solution is fairly simple - dot the joint with something
> that will stiffen things up. Epoxy works, so do various
> other fairly rigid plastics. You don't need very much at
> all, anything that runs up a dozen or so wire diameters is
> more than enough. 
> 
> The other solution is to not use copper wire, go to
> something like steel. Not much of an answer in this case. 
> 
> Bob
> 
> On Mar 9, 2013, at 2:21 PM, quartz55 <quartz55 at hughes.net>
> wrote:
> 
> > I've got the lost magneto wire welded back in the 4K
> filter, but as I say, it's a #34, not 40, and the bobbins
> installed and glued.  But what the heck.  Anyhow,
> I'm pretty tired of it today, I've need to cut some more
> foam PE from the hardline. I find it's easier just to take
> the soldering iron to fasten together the pieces of PE, RTV
> doens't stick, but then I'm using 5 minute epoxy to attach
> the PE to the filter bodies and the bobbins in the copper
> cups.  I've got all the dimensions for all this stuff
> and I'll list it later.  But here's where it stands
> now.  I notice the bobbin wires seem to have lost some
> of their covering, so I'm going to mask them and spray them
> with some enamel so they don't short out when I install
> everything.  It's just a matter if getting it back
> together now and see if it even works.  I'm sure
> soldering those #40 wires on the end caps is going to be
> challenging.
> > 
> > I haven't been looking foreword to winding any of those
> bobbins, but hopefully between all the old filters I have
> and the broken ones, they can be re-used.  They don't
> seem to be different at least between the 455 models.
> > 
> > http://i251.photobucket.com/albums/gg287/DogTi/R390A/filterassembly_zps90ed7d6f.jpg
> >
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