[GreenKeys] Model 28 Typing Unit activated immediately at power-up. What's stuck?

[Tim Griffin]

Hello Nick!

My many thanks for your response and explanation. Yes, this is good news
for me, for starting out, and I will get sorted on figuring out a loop
project. I will, of course, ultimately want to source a Baudot feed via a
connected computer, as I don't expect to have a second teletype anytime
soon!

Thanks for the note on the paper/platen size. I was really scratching my
head trying to correlate the chart Page 3-70 of Guide 1149B that shows the
table of paper widths and platens. It doesn't look like I'd find a platen
to fit regular fanfold computer paper anyway as what I have measures 9.5"
wide (full edge-to-edge), with 9" between holes (center to center). But,
the maximum listed in the table is 9" overall width.

So, I will hunt for narrower paper!

All the best and thank you for your amazing resources at www.navy-radio.com.

Tim
Ottawa, Canada


On Thu, Aug 26, 2021 at 1:00 PM Nick England <navy.radio at gmail.com> wrote:

> Oh yeah, normal sprocket feed teletypes are 8" between holes and 8.5"
> overall - so the usual fanfold with 8.5" between holes won't work. The
> narrower paper is available however - check the archives.
>
> But teletype made every sprocket size option known to man so check
> carefully.
>
> Nick England K4NYW
> www.navy-radio.com
>

Teletypes operate via a 60ma current loop - if no loop current, then the
machine "runs open" as yours is doing. So that's normal behavior.
In nrmal "idle" there is current (MARK) in the signal loop which pulls in
the selector magnet armature, disengaging the clutch on the main shaft -
the motor runs but nothing else happens.
When a SPACE occurs (no current), that is the start pulse of a character
and the clutch engages, At the end of the character, the the stop pulse is
a MARK (current flowing) and the clutch disengages until the next start
pulse comes along.
So your machine thinks it is constantly getting START but never a STOP.

Loop current is usually provided by a 120v power supply with 2K resistor
(10-20 watts) in series. The high voltage is necessary to overcome the
inductance of the selector magnet - at low voltage the magnet armature
won't move fast enough.

The manuals give lots of detail, but here's a good overview that may be
more helpful -
http://www.samhallas.co.uk/repository/telegraph/introduction_to_RTTY.pdf
Also check out
https://www.navy-radio.com/manuals/tty/Principles_of_Telegraphy.pdf

Welcome aboard - teletypes are amazing, fun, and addictive!
Cheers,
Nick England K4NYW
www.navy-radio.com
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