[GreenKeys] device for reshaping TTY signal

[Jim Haynes]

On Sat, 15 Jun 2019, Nick England wrote:

> One mechanical example is described here -
> http://www.navy-radio.com/manuals/tty/155b-3702.pdf
> 
> There are some electronic versions (there was a card for the M35?)

The card for the M35 that I'm aware of is not exactly a regenerative 
repeater.  It is a circuit for more accurately timing the keyboard 
signals, using a contact on the transmitting shaft that closes on the bit 
edges.  The good ol' Model 28 signal generator wasn't so good at 110 baud, 
so a circuit was designed to retime the bits more accurately from a 
cam-operated contact.  There's a funny story connected with this. The 
Model 35 keyboard regenerator was always an option; and most of the 
machines shipped didn't have it.  Then AT&T decreed that as of a certain 
date they wanted the regenerators included in all the machines they bought 
from Teletype.  No problem, it was already designed in and just had to be 
manufactured and installed.  And then disaster struck - the machines with 
regenerators often generated completely wrong characters! While R&D was 
working on the problem unshippable machines were piling up in the factory, 
being stuck in every available space.  The cause of the problem should 
have been familiar - the timing contact would get contaminated with an oil 
film and then it didn't make reliable contact at the current and voltage 
levels applied to it.

I don't remember what solution was eventually chosen, whether it was using
gold-plated contacts and trying to keep them clean of oil, or whether it 
was increasing the current and voltage applied to the contacts until
it would burn through the oil film.  But the problem was solved and 
shipping resumed.

As noted earlier Teletype did once manufacture an electromechanical
regenerator for Bell System use.  I remember a solid-state regenerator
being designed, but was never put into production.  Instead Bell Labs
designed an electronic regenerator using tubes.  This is shown in the
1953 edition of "Principles of Electricity Applied to Telephone and
Telegraph Work" which is available online and also from used book sellers.
This design is probably that of U.S. patent 2,785,225 but there are
others.

A neat thing about digital communication is that by using regenerative
repeaters there is no limit to transmission distance.  Just space the
repeaters closely enough so that one practically never makes an error.

In the 1970s I dabbled in buying government surplus.  One thing I brought
home was a set of three relay racks, two of which were filled with
transistorized regenerative repeaters and the other was full of TD
stepping pulse generators.  I assume this was surplus from Air Force/
Western Union Plan 55 message switching system after it had been replaced
by AUTODIN.

Today you could easily make a regenerator with a UART chip.

The 1930s Teletype mechanical regenerator has the problem that like
the printers and reperforators of the day the mechanical selector is
subject to false starts and missing stop pulses.  If the stop pulse
is missing at a critical time the selector clutch fails to latch up and
the selector cam keeps going around out of sync with the signal.  So
a one bit error causes several character errors until the selector gets
back to where it stops on the stop pulse.  This kind of error can happen
a lot in radio work, so it's always advisable to put some kind of 
electronic regenerator between the demodulator and the printer.