Question from Ed (etech):

Hi all,

This old style shadow meter has an open coil and I would like to rewind it. Does anyone know exactly how to remove the bobbin from this assembly? I have looked at Chuck’s web site, but don’t see this older style coil and vane assembly.

Alan Douglas:

I did one of these for somebody last year, and if I recall, I taped the whole thing to the face of my coil-winder mandrel and rewound it with the vane in place.

While cleaning off the dining-room table I found my notes. The coil used #41 wire, 5520 turns, and measured 1760 ohms.

#40 wire will probably work but you need to be sure to get enough turns on. The magnetic field is proportional to the number of turns and you need enough to control the vane, with the limited B+ current available.

Ed (etech):

I successfully managed to rewind the coils of both defective shadowmeters I have here. The first thing I did was to disassemble the shadow meters.

Disassembled shadowmeter

Then, I stripped the old wire off the bobbins. On the first one, I actually unwound or spun the wire off of the old bobbin. I decided that it took too long to do that, so on the second bobbin, I just cut the old wire off with an Exacto knife. That was indeed much faster.

Stripped bobbin

After that, I had to find a way to mount the bobbin assembly to the winder without destroying the vane. There was 7/32 hole in the rear of the bobbin, I found that I was able to use a 1/4 – 20 bolt and thread the hole in bobbin. Following that, I cut the head off of a 2 inch long 1/4 – 20 bolt and used it for a mounting shaft, threading it into the rear of the bobbin. I put a drop of super glue on the shaft at the bobbin surface just to hold the bobbin temporarily in place as the shaft was turned in the winding machine. Also, you have to be careful not to thread the 1/4 – 20 shaft too far into the bobbin, as the vane will be damaged.

Winding machine fitted with shadowmeter bobbin – shown partially wound.

I used #40 wire and wound the bobbin until it was full. I measured the resistance of each new coil. The first ended up at 1290 ohms and the second ended up at 1270 ohms. Not bad since I took no measurements until the bobbins were full. Both original working shadow meters I have here measured between 1100 and 1200 ohms, as supplied by Philco. I tested each of the newly wound shadowmeters using a 9 volt battery. Both operated just fine, providing a nice deflection with the 9 volt battery. All I had to do then was finish reassembling the shadowmeters. They went together easily and work fine. 

BTW, Here are a couple pics of my homebrew winding machine, in case you haven’t seen it.

Winding machine

Foot switch