[R-390] Filters

[Bob Camp]

Hi

The fancy term for a pinch off used during seal is a "tubeulation". On a big vacuum tube, you cook the thing in an oven for hours / days before you melt the thing with a hydrogen torch. If the nubbin was once a tube, that's the term for it. 

In this case the nubbin may or may not be used as part of the seal. You would need to pull the solder off of it first and see. The radio is rated to run at some *very* cold temperatures. If there is any moisture inside the filter can, you will get frost on the disks as the temperature drops. That would shift the frequency out of spec. 

Unless you plan on running the radio outdoors in Alaska at night in the winter, I would not worry about moisture. If you do want to go through the whole process with what you have, it involves things like vacuum bakes and dry nitrogen filled glove boxes. 

The dividing line is (no big surprise) freezing. The water capacity of air drops quickly as you go below the freezing point. Since we rarely run these radios below freezing, I'd say it's not a big deal. 

If you don't go with the glove box / tube stuff, sealing is still pretty easy. You do one end and let it cool. A very big iron and reasonable speed is the right approach. On the other end, seal as quickly as you can over about 90% of the perimeter. The last 10% gets done in a single swipe (say 10 seconds). If it vents, repeat the swipe. The only real trick is how to hold the giant iron so you get heat into both the header and the can. Oddly enough you can do more damage to the header with heat than to the can.  

Bob

On Feb 17, 2013, at 9:45 AM, "quartz55" <quartz55 at hughes.net> wrote:

> I'm going to attempt to spot weld the wire on the center of the disk with a capacitive discharge.  I need to practice first, so I've got some resistance wire thats lots thicker, but may do for some trials.  If I can manage to get it welded, I'm going to make a form with a piece of teflon wire sheathing inside the form to hold the wire centered in the copper cup and then attempt to weld the wire back on.  I'm not even going to try glue and since the copper cup is soldered or welded into the end resonator disk, it would be impossible to solder it back on.
> 
> I still need to come up with a holder donut.  The hardline I have is Andrew 84147 FSJ4-50B Superflex and the dielectric is foam PE.  I'll put the torch to the stuff and see how it holds up.  Otherwise, I can't think of anything I have laying around except some small teflon sheets and I don't think that would work.  The paper insulating materials in the unit seems to be in good condition, it reminds me of speaker coil form material.   Actually the teflon sheets may be good to replace the stiff paper insulation materials if it's thin enough.  Like the pieces on the end caps.  The small sheets I have aren't big enough for the large piece.  It actually feels sort of like thin gasket material.
> 
> Are there any pictures that someone would especially like to see that I haven't supplied?  G8OAD's web page has good ones.  I'm just filling in with what I didn't understand from his article.
> 
> Here's where I stand right now and I haven't burned the label.  http://s251.beta.photobucket.com/user/DogTi/media/R390A/filterparts_zpsc1292284.jpg.html?sort=3&o=0 The nubbin I was asking about on the bottom end appears to be a hole in the center of the bottom end cap that looks to have been a short tube that is cut off and sealed after soldering I'm guessing
> 
> N3DT
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