[GreenKeys] Model 15 Wiring

[ad7i]

Greetings Greenkeys Members --

I've spent the weekend going through my Model 15 with a continuity tester
and an ohm meter, to understand how it's wired.  I've attached a PDF of the
schematic that I've created for my particular M15.  It only includes the
active wiring, and not all the dead, unconnected and unused wiring within
the TTY.  With all that completed, I still have some open issues.

I'd like to ask Green Keys members for their insights and suggestions for a
few issues regarding the model 15.

In my M15, on the back, left side, near where the line relay would mount,
mounted to the base is a power power to control current to the motor.  The
side of the relay is marked "TP178306".  Does anyone know the electrical
characteristics of the coil in this relay?  AC or DC coil?  Typical voltage
applied to the coil?

There is a leaf spring contact set on the base that is normally closed when
the typing unit is removed.  These contacts, when made, connect Terminal 46
(one side of the selector magnets) to Terminal 34 (one side of the
keyboard).  These contacts become open (open circuit) when the typing unit
is seated on the base.  What is the purpose of cross connecting the magnets
to the keyboard when the typing unit is removed from this base?

On one Teletype schematic I found for the M15, it shows that each selector
magnet is 105 ohms and the two are connected in series.  In my M15 each
magnet is about 195 ohms and the two are connected in parallel, presenting
a resistance of about 95 ohms between Terminals 45 and 46.  Is 195 ohms per
magnet a typical number for a M15 typing unit.?

On my M15 the power comes in from terminal 21, through the power switch and
then to the shell of the edison fuse holder (even though the Teletype
schematic shows Terminal 21 going first to the fuse, as one might expect),
and then the fuse connecting to the switch).  Wouldn't it be more typical
for a normal electrical circuit that is protected by an Edison fuse to have
the "hot" connect to the pin and "load" connect to the shell?  I don't know
how 1950s homes were wired with Edison fuse holders, but common sense would
seem to suggest that the shell (and not the pin) would be used for the load.

I'd be grateful for any advice or comments offered.

Paul Newland, ad7i
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