[GreenKeys] TTY-Connect Grounded Loops

[Bob Camp]

Hi

TTY machines were set up a lot of different ways to do a lot of 
different things. A few random examples:

1) Keyboards set up to do polar keying. Feed the stationary contacts 
with +/- 60 volts and have the moving contact flip between them. This 
is a very common model 28 setup.

2) Keyboards set up for shielded operation. One part of the three wire 
plug is ground the other two have the contacts on them. A lot of hams 
set them up this way.

3) Magnets set up for shielded operation. Same three wire plug issue as 
the keyboard same ham setup.

4) Line / Local switching built into the machine. Generally this stuff 
*assumed* a specific loop setup. This is a common item on a model 15/19 
setup.

5) Quite a few configurations of the model 28 have a filter in the 
keyboard contact line. If you hook the keyboard up in series with the 
magnets you put a .1 uf cap across the contacts and a couple of .01's 
to chassis ground.

Not all of these will smoke a TTY-Connect board all by them selves. 
Various combinations of them will do all sorts of odd things. Some of 
them are dangerous if you get them wrong.

This is not just a TTY-Connect issue. It's a problem any time you go to 
hook up one of these old machines and depend on a wiring diagram in a 
book. We get so used to running low voltage stuff  these days that we 
forget just how much high voltage can do when it gets loose. The worst 
thing in the world is to turn a digital tech loose on high voltage 
gear. He'll fry himself in no time flat ....

	Take Care!

		Bob Camp
		KB8TQ