[GreenKeys] Model 28 signal lines

Tue Oct 15 16:12:23 EDT 2019

Hi Barry - I was lucky when I got my first teletype that it already had
been wired with just two cables coming out of it - the power cord and the
signal line. Since then I have learned that there are 1352 variants of
Model 28 machines. Luckily, most all of them are wired in a similar
fashion. I will probably make a mistake (or mistakes) in this message but
someone will correct me I hope.

Do you have any model number info on your set? ID tags on the typing unit
LPxx? the keyboard base LAKxx? The cabinet? the Electrical Service Unit
(LESU)? If it is a Bell system unit the components will model numbers like
28D, 28G, etc. instead. Maybe send a photo of your LESU - that'll help us
help you.

Unless your unit has been completely rewired by some previous owner, you'll
see that the typing unit has connector R and keyboard base has connector F
whose multiwire cables run into the LESU. Then the LESU has cables that run
up to the terminal boards C1-C40. You'll find the R, F, and C numbers on
the wiring diagrams.

There are many wiring diagrams for various models here -
http://www.navy-radio.com/tty/tty-wd-list.htm

Here is a typical schematic wiring diagram for a 28KSR set
http://www.navy-radio.com/manuals/tty/jh/ttyman9/drawings/2875wd.pdf
The actual wiring diagram for this model is at
http://www.navy-radio.com/manuals/tty/jh/ttyman9/drawings/2874wd.pdf

You'll see that C5 and C15 are the terminals where the signal line attaches
(unless you have a signal line filter installed). The keyboard contacts and
the selector magnets are in series so there are no separate input and
output cables, just one happy loop - that way you always see what you are
typing. Note - A, B, D, E are terminal strips inside the LESU. You'll see
that the selector magnets are wired in parallel on the A strip. Just
measure the resistance across the signal line - 67 ohms if they are
parallel connected, 264 ohms if they are in series. Parallel was almost
always the rule for M28 gear. You must temporarily remove jumper C13-C11 to
make this measurement - otherwise the line shunt relay shorts the signal
line unless the AC power is ON.

If you have motor control, here is a typical schematic -
http://www.navy-radio.com/manuals/tty/wd/2893WD-6601.pdf

The models above have the selector magnets directly connected to the signal
line. It was also common to have  models with the selector magnets keyed by
a polar relay and an internal loop supply. The polar relay coil is then
operated by the external signal line. If your LESU has a line relay, a
transformer for the internal loop supply, and a polar relay, then you have
this model.
A typical schematic diagram is here
http://www.navy-radio.com/manuals/tty/wd/2879WD-6311.pdf
The actual wiring diagram is here -
http://www.navy-radio.com/manuals/tty/wd/2878WD-6203.pdf
Signal connections are still C5 (+) and C15 (-). Correct polarity is
important.

No, sorry, a 48v power supply will NOT work. The M28 requires a higher
voltage in order to make the selector magnets pull in fast enough to copy
60wpm signals. A 120vdc supply with a 2000 ohm resistor is the standard way
to do this.

Good luck and welcome to Model 28 teletypery!

Nick England K4NYW
www.navy-radio.com

On Tue, Oct 15, 2019 at 2:29 PM B M <iamvirtual at gmail.com> wrote:

> I took a look through the theory & operation manual for the Teletype Model
> 28 KSR (floor standing model), but I am still confused on where the signal
> lines in and out are located.  I do see the 2 terminal strips at the back
> of the cabinet with labels from 1-20 and 21-40.
>
> I have a few questions
> 1. which terminals are for the signal lines in?
> 2. which terminals are for the signal lines out?
> 3. how do I determine that the unit is still set to use 60ma loop?
> 4. will 48v @ 60ma be sufficient to drive the model 28 KSR?
>
> As you can see, I am not familiar with Teletypes.
>
> Any help would be greatly appreciated!
>
> Thanks
>
> --barrym
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