Minor correction – late in the production life of the Model 14/15/19, Teletype Corp came out with a second type of selector unit that usually has a DPDT toggle switch mounted on it that could be switched between 60 mA and the later standard of 20 mA by connecting the two coils in either series for 20 mA or parallel for 60 mA (note that no one that I know of has ever come up with an explanation of why it is 20/60 instead of 30/60 – it just is).  The new selector is called the “Holding Magnet” type and the original type came to be called the “Pulling Magnet” type.  The Pulling Magnet selector cannot be rewired to operate on 20 mA because  the two coils are already in series.

If someone removed the switch and hard wired the magnets or if you have just a loose magnet, it is easy to ID which type of coil you have.  The original Pulling Magnet coils are round (and larger).  The Holding Magnets coils are square (and smaller).  You can also ID the two types by opening the loop or turning off the DC supply and letting the machine run open.  The armature of the Pulling Magnet selector will not move or jump around as the motor runs.  The armature of the Holding Magnet type will jump as the drive shaft rotates.  The armature of the holding magnet type is held in the Mark position if the pulse is a Mark and allowed to fall away if a Space.  Hence the name.

Robert Downs

From: [email protected] [mailto:[email protected]] On Behalf Of Tom Hunter
Sent: Friday, December 31, 2021 23:14
To: [email protected]
Subject: [GreenKeys] Model 15 remedial question Inbox

Jason,

You need to measure the current flowing through your model 15, even with a multimeter in series with your loop.

It should be 60 Milliamps. I have voltages from 80 to 180 in my various loops, but in all cases the current is set to 60 mils by variable resistors. I have so many places to measure loops, I buy 100 milliamp panel meters three at a time.

By the way, 120 is the most common voltage.

But the current MUST be 60 mils for a model 15!

If you have your loop connected to the selector magnet, it should be pulled in already.If you can still push the armature in, then there is something very wrong with you loop supply. Harold is correct - everything in the loop is in series. If you want two machines to talk to each other, both keyboards and both selectors are all in series across the loop supply.

I have machines in three different locations here and everything is in series.

Tom N3CRK