[GreenKeys] AP Model 15 information

[Dave Emery]

On Mon, Feb 23, 2004 at 09:20:22PM -0500, sdaitch@ibb.gov wrote:
> To the best of my knowledge and feeble memory, the DC loop from
> the telco was 60 ma, not 20 ma.

	That may be correct.   I know that later tty equipment I used 
interfaced to minicomputers was all 20 ma polar, but I am not sure I
ever attempted to determine what the loop current was in our college
radio UPI machine DC telegraph circuit.

	It was a polar loop (differential) rather than an on/off
keyed loop such as that used with the selector magnets on the machine
and there was a honking old grey cylindrical cased WE polar relay to key
the actual machine magnet loop from it.     I remember the polar
relay slowly went bad and we had to have the techs in to readjust it
back to set points that provided low distortion as copy on the machine
began to get more and more garbled.



> The subscriber units were a single channel tone decoder, for one of the service channels, and contained a DC loop supply.  There were two neon bulbs on the
> front of the unit, actually a recessed panel, one was lit if there was an audio signal on the incoming VF phone line.  The other lamp keyed with the loop current.  

	That matches what the UPI used in our area.   I know it was Lenkurt
and 25A sound right.   It had two orange neon indicators as you describe,
one for carrier and one for the actual data (that flashed on and off
as the machine printed).

> I don't think the signal lamp acutally was a test of a valid tone pack signal, but triggered by any audio on the phone line, and a noisy phone line would give a false good signal.

	My impression was that it was driven off an envelope detector on
the output of the narrow bandpass filter for the specific audio FSK
channel used for that wire.   So if that tone dropped out it would go out.

	Of course random line noise might well contain enough energy to
light it.

	I also saw some of the VFT gear used in the early 80s and it
was considerably smaller (the Lenkurt 25A was rather large for a FSK
receiver) and looked rather like a small external modem of the 90s
with LEDs on it.

-- 
   Dave Emery N1PRE,  die@dieconsulting.com  DIE Consulting, Weston, Mass 02493