I have found an open detector coil winding in the 'catacomb' coil box for band A of my National NC-200.
There is a visible green (CuSO4) speck where I believe the winding is corroded open. I can get to it with a brush, flux and soldering iron.
Is this 'Litz' wire (many strands of separate cotton-insulated wires) able to be re-joined with a blob of solder for an effective repair? The windings are held together with wax and have a special weave pattern (honeycomb?) that I don't think that I could reproduce by hand.
I don't think that I can unwind the coil safely to the break-point in the delicate winding and hope that I don't have to. Any suggestions?

You can solder Litz wire together at a break. Be sure that every strand gets bonded to the solder. However, a Litz wire coil is most effective when all of the strands remain separate throughout the entire length of the coil.

T.

:You can solder Litz wire together at a break. Be sure that every strand gets bonded to the solder. However, a Litz wire coil is most effective when all of the strands remain separate throughout the entire length of the coil.
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:T.
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I've done this with wire on a winding of an AK model 10 coil. Here's how I did it. I used a sewing needle to lift each end, just like removing a splinter. Carefully slide something beneath the break to protect the other wires. I used a small piece of copper. Be VERY careful not to heat that. Use the lowest temp setting that works. Maybe practice on a similar piece of wire.
Good luck and keep us posted.
Terry

If you could possibly solder each strand together separately, and then keep all of the solder connections insulated from eachother, that would be ideal. However, that would be very tedious.

T.

:If you could possibly solder each strand together separately, and then keep all of the solder connections insulated from eachother, that would be ideal. However, that would be very tedious.
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:T.
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Well things seemed to get worse before they got better. I took a 50x view at the break to see what I was up against. This must have been an old injury that finally corroded open. It looked like the winding had been bumped by something that had left a sharp dent at one time.
I was warming the coil with a hairdryer on low in order to melt off some of the assembly wax. It got too hot and shrunk the end of the polystyrene coil form. This wasn't intentional and my heart sank.
OK now I was able to slip the winding off the coil form, remembering and making a note about its orientation. This may also have been a blessing in disguise because it made it easier to work on.
The winding is an 'over/under' basket-type weave and the other end of the break was under another wire. I used a nylon brush gently to expose the Litz strands as much as possible and gently dug the wire up with a pin. I sharpened a soldering tip and got myself plenty of light and magnification. The first few attempts at bridging the break showed no continuity on the meter. Excess wax would pool up from inside the winding and cover the area that I was trying to solder. I also found a second break in the winding under the cotton wrap. This may have been caused by me over-handling the winding wire ends. Certainly the less handling the better.
Anyways, I now have ~ 6 ohms across the winding which seems about right. I RTV'd it and am letting it sit for awhile to cure before re-assembly.
I realize that this wasn't an 'ideal' repair, but as long as I still have good sensitivity on band A, I'll be happy. Thank You for all the help.

Make sure that there are no shorted turns. Any shorts will be somewhat (if not very) critical for RF coils. Good luck.

T.

It's back together and the reception results are poor.
I could unravel the winding to the point of the break, but I don't think that I could reproduce the 'basket-weave' going back together let alone attach each 'litz' strand separately.
This is extremely fine work and I don't have these kinds of tools. The winding 'ring' measures about 1" diameter, 1/8" deep and 1/8" thick. I also don't want to make more of a mess of the winding than I already have.
I think my only recourse is find a used one or have someone with the right knowledge and tools restore this one or make a replacement from it.
I've seen the knitting and weaving fixtures for how these coils are made. Litz wire is availible. This is just too tiny for me to reproduce or repair. Does anybody offer this kind of service? Thank You.

http://www.nostalgiaair.org/PagesByModel/418/M0012418.pdf

I assume that you did a full alignment, as radios with a tuned RF amplifier stage are very critical to alignment.

:http://www.nostalgiaair.org/PagesByModel/418/M0012418.pdf
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:I assume that you did a full alignment, as radios with a tuned RF amplifier stage are very critical to alignment.
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Yes, the trimmers are peaked and the 'A' band sensitivity is still poor.
I'm looking around for a spare or repair service for this coil.