Another DeArmond adventure

[Mark Hammer]

I've been entrusted to rewind and bring back to life a DeArmond pickup, that was installed by the neck on what is a tenor dobro/resonator, of all things. It's one of these;


What should I know before I attempt to disassemble it?

[big_teee]

If it the Rythm Chief (monkey on a stick)?
There is some great rewind info by SonnyW, on the below thread starting at post 110.
http://music-electronics-forum.com/s...t=monkey+stick
You are also welcome to post you project and pictures in the rewind thread.
http://music-electronics-forum.com/s...ad.php?t=39746
GL,
T

[Mark Hammer]

Separated the top and bottom halves of the cover/base. Going by some other pictures I've seen, this one appears to have been repaired in past, since the top part of the bobbin seems non-original. The challenge would seem to be using gentle heat to soften whatever is used to stick the bottom of the bobbin to the baseplate. I could use some serious guidance on that part. The top image is someone else's unit, and the bottom one is the one I'm working on.

The baseplate would appear to be something non-ferromagnetic; perhaps chrome-plate brass. The part that is perpendicular to the coil is also non-ferromagnetic, with the oblong part and the T-shaped part magnetic. As has been discussed in many other threads, the top and bottom portions of the coilform are thin.

[Mark Hammer]

Used a heat gun to soften the glue and pry the coil off. It's toast. I think I will follow the lead of some other folks and make the "flatwork" from heavy paper, coated-with/soaked-in shellac.

But what I don't know is what the target number of turns or DC resistance should be on this thing. Any clues?

[Mark Hammer]

Got hold of the original DeArmond patent document (thanks for the number, Dave Hunter!), which explains much about the magnet arrangement, and especially the unusual polepiece overlays, atop the coil. Unfortunately, the only information it provides about the coil is that it contains "many turns" of wire. Thanks, Harry. I was hoping for a little more than that.

[Mark Hammer]

I made some flatwork using some very thin copper-clad board I had, making sure to etch off all the copper, leaving only fibreglass. You can see the top and bottom of the finished coil below. I used #42, because I was having some issues with my roll of #43. Not sure if I made the right call. Stuffed as much as I possibly could on the coilform, did some rudimentary potting (the original was not potted), and ended up with somewhere under 5000 turns, and a DCR of 4.7k. I have no idea if this is in the ballpark for this era/type of pickup. I'll remagnetize and wire it up tomorrow and see if sounds any good. If you know anything about the "right" DCR, let me know.