[GreenKeys] Newbee Questions

[Jack]

Jim et al,

The "BREAK" key on a TTY harkens back to the old telegraph days.
Most of us are aware that a traditional telegraph circuit consisted of
a key and sounder on one end, and a key and sounder on the other
end. They were wired in series with a DC voltage which went to ground
closing the loop (get it ..."loop"?).
Each key had a shorting switch. If that switch is closed, then the
other end could send because the loop would be completed only through
the sender's key. If the receiving end opened the switch, breaking the line
("break"...get it?), then the sender would close his switch and standby for
a comment, request for fill-in, etc.

TTY's were put in place of the key and sounder and the BREAK key
served a similar function...when the BREAK key is operated, it opens the
loop and causes both machines to run open. In the Model 28's the open
circuit is "decoded" in the stuntbox and the KEYBOARD LOCK key is
operated, essentially disabling the keyboard and letting the typist
know the other end has a question, etc. The older machines would just
run open, a good enough indictation to the sender to standby. As with
a lot of other things, the 28 line "refined" the process!!

Your LESU has a motor control relay that probably is not hooked up. It
used to operate off the stuntbox when a FIGS "H" was received, shutting
down the motor. A subsequent BREAK would turn the TTY back on.
The odd looking relay has both a 110VAC coil and a set of coils that are
identical to the selector coils. The 110VAC coil is operated by the
FIGS "H" contact, and the selector-type coils are in series with
the loop.

The plate with the small transformer and capacitor is a loop
supply. If the transformer is about three inches high, the
supply is good for up to 300 ma. If the transformer is fairly small,
then it is rated for about 150 ma.

I would recommend that you plug the thing into a loop and see
what happens. My guess is that the potentiometer on the LESU might set the
internal or external loop current. In fact, try shorting the plug
first...maybe the machine is using the internal loop supply. Otherwise,
try anything from about 24VDC up to 150VDC, with some current
limiting, to get the machine to close up on the loop. Monitor the loop
current and be sure the selector coils are not warm. Be careful with
the high voltage.

By now you have probably figured out that the LESU serves no real
purpose for the average TTY person these days, except to terminate
the keyboard and printer connectors.

Yes...you do have to remove the spacebar to get the keyplate off. Have fun
getting
the keys back in!!!!!!

Jack WA2HWJ