[GreenKeys] M19 Selector Magnet - magic smoke?

Mon Aug 2 12:59:43 EDT 2021

My favorite 100 mA fuse from two different vendors (Digikey has $5 shipping
for very small lightweight orders):

https://www.mouser.com/ProductDetail/Bel-Fuse/5ST-100-R?qs=MvPYbBWWJysvyCTUIgtvFw%3D%3D

https://www.digikey.com/en/products/detail/bel-fuse-inc/5ST-100-R/1009010

I don't bother to put them in a fuse holder.  I carefully (very short
application of heat) solder wires on each end of the fuse and then enclosed
it in heat shrink, then connect it into the circuit (and the closer to the
magnets the better from a "protect the magnets" perspective).  I expect at
some point we will reach a point where replacement selector magnet coils
will become very hard to find.  On the other hand, fuses should be easy to
obtain in the years ahead.

I have a bunch of these 100 mA fuses.   In an effort to payback/payforward
others on this group who have been so generous to me with their guidance
and gifts of parts, I'd be happy to send anyone in who would like a 100mA
fuse free of charge (for the first 20 people who ask).

Paul, ad7i

On Mon, Aug 2, 2021 at 10:48 AM John Thurmond via GreenKeys <
greenkeys at mailman.qth.net> wrote:

> 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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> >>> 2002-to-present greenkeys archive:
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> >>> 1998-to-2001 greenkeys archive:
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