[R-390] Filter repair

[Bob Camp]

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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