[GreenKeys] constant current

[Tim Swarthout]

The QST article that Jim Haynes is trying to locate is "The Modern
Teleprinter Local Loop" by Frank Merritt, VE7AFJ and is in the January 1972
issue starting on page 40.

A short excerpt from that article follows

"It is interesting to note that the transistor offers a near perfect
solution to the keying problem in low-voltage circuits such as that of the
local loop. The loop power-supply voltage has been reduced from the 100- to
150-V dc range to 40 V dc. This is a significant  step. Authors of other
early articles developed low-voltage local loops with varying degrees of
success. In 1959, W2JAV showed a 30- to 35-V dc loop using an 2N270
transistor with the selector magnet connected for 20 mA operation. In 1962,
Van Brunt provided a good discussion of the constant-current technique in
the local loop using the Zener diode "in" the loop. In 1964, a demodulator
called El Simpatico I used a 24-V dc local loop. The lowest local-loop
voltage I have been able to discover was featured in the Triple "T"
Converter, in which the local-loop supply voltage was 15 V dc.

Don Stoner, W6TNS, developed a remarkable demodulator called the RT-1. The
local loop is slightly different and is presented in Fig. 4. It will be
noted that the constant-current section shown in this schematic is a bit
different for the circuit in Fig. 3. The important thing in this discussion
is that Q1 is a constant-current transistor and the loop supply voltage is
24 V dc. Q2 and Q3 provide a sophisticated transistorized switch in the
keying circuit. The first time I used this circuit I obtained a 5-point
increase in the range of my model-15 printer. This was an improvement over
any other quasi-constant-current circuit I had ever used. This circuit does
work!"

Another note to this story is that in the early 1960s, Teletype Corp made
and sold a constant current loop driver as the 177010. I have a manual for
this device. It is built to go in the LESU of the Model 28 machines. It
used 40 volts. Also, surprisingly, (maybe not for that era)  it used
germanium transistors!!!

In the early 1970s, I built a constant current loop driver of my own
design. It worked very well and used 24 volts driving the model 15 magnets
at 60 mA. I could send it RYs at full speed (60 WPM) with a computer, and
it never missed a beat. I didn't scope the coil wave form and I could have
(and now wish I would have!). A problem that still exists after you get
this going is that you still need that high voltage on the keyboard
contacts to keep them somewhat cleaned up. This may be one reason why
constant current drivers never caught on in amateur circles.

Tim Swarthout, WA5QEG