[GreenKeys] M19 Selector Magnet - magic smoke?

[John Thurmond]

 Thanks for all the feedback on this. I'll check both magnets and see what the results look like.Fuses are a good suggestion - very much a learning process here :)
Is there any particularly good source for new magnets?
    On Sunday, August 1, 2021, 10:26:03 PM CDT, John Nagle <nagle at animats.com> wrote:  
 
 
> This is my first post to greenkeys, but it's obviously *the* place to
> ask these kinds of questions... I have a Model 19 that I saved from
> being scrapped from an SK's estate - I have never worked with
> teletypes, so this is all new to me. He was an excellent radio
> restorer, and it appears to have been working when I acquired it
> (W1AW copy from 2005 still on the
> paper:?https://photos.app.goo.gl/Lh5exkuvKiWi37yK8), but no
> guarantees there. It sat in my garage for 2-3 years, until I
> finally figured out what kind of project I wanted to do with it. It
> turns on and "runs open" (no surprises), but after reading up quite a
> bit, I realized I needed to set up a 60mA current loop to start
> testing it out. I acquired a 120V DC power supply, and I was able to
> get the selector magnets to engage. With initial testing, the
> selector magnet did engage from about ~20V, and this got it to stop
> running open as expected. However, due to my own idiocy and not
> reading the manual for the PSU closely enough, it turned out I was
> running 600mA, rather than 60mA through the selector magnet (I was
> foolishly relying on a current-limiting power supply, rather than
> using a resistor to limit current.) After a few minutes, this
> generated a curl of smoke from the magnet, and I immediately cut the
> power.(https://photos.app.goo.gl/q6bYHMatyfvm1ZzT6). A brief test
> after it had stopped smoking had the magnet still engaging, so I
> decided to continue on with working on an interface. Now that I have
> the interface built, I came back to testing the M19 today to see if I
> could get the interface working. However, I can't seem to get the
> selector magnet to engage at all. I am now running 120V through a 2K
> ohm 10W resistor, and the current is 60mA (for real this time). I
> checked the resistance, which shows ~1 ohm - my understanding from
> reading up on M15s is that this should be ~55 ohms? I am (somewhat
> dangerously) relying on the previous owner's wiring - he connected to
> the selector magnet through the terminal strip on the back of the M19
> (https://photos.app.goo.gl/S3ictP9W6iXpoFkS9), which again, worked
> before, but I thought it was worth noting *where* I had connected the
> over-current previously. Have I cooked the selector magnet? Are there
> any other tests I can run to see whether it's working? Any other
> points of failure I should be paying attention to? I appreciate the
> collective wisdom of this group, and hope you can help. -JohnT K5JBT

If you're reading 1 ohm across the selector magnet, when it's 
disconnected from everything else, it's shorted. You burned through
the winding insulation.

Each magnet coil should have a resistance of about 110 ohms.
55 ohms when in parallel. 220 ohms when in series.

If they are in parallel, disconnect a wire (it's a screw terminal,
or it should be) and check the resistance of each coil separately.
Maybe only one coil is burned out.

There are people on here who can sell you a new coil. Then
you get to do the alignment procedure in the Adjustments manual. It's
not that hard.

Fuses, people, fuses.

                John Nagle
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