[GreenKeys] M28 Selector Magnets

[DR HOUSE]

The machines have more range and print more reliably at 62.5 Milliamps.
For RTTY and ITTY applications there is not a lot of concern about  
loop characteristics since the loop length is usually less than 15 feet.
Telegraph transmission takes on more variables when the loop length  
gets longer for land line private wire.
Inductance and capacitance cause distortion.  With longer loops and  
several machines on the loop it gets more difficult to keep the loop  
current at 60 milliamps.  One of the applications where this became  
problematic was weather service.  Many weather machines on private  
wire were optioned for 20 milliamp operation, and in some large cities  
special selector magnets were used designed to operate on 10 milliamps.

The lower the operating current the less range for accurate copy.   
Also in the equation is the baud rate.  If the machine is printing at  
60wpm there is a lower chance of garble.  Increase to 100wpm and  
everything needs to be just right to have accurate copy.

I once installed a 28 ASR temporarily on a very long loop.  Used both  
+130 and -130 VDC to get the 62.5 milliamps.  Also had to use a wave- 
shaper to balance all of the cable capacity.  A wave-shaper is a set  
of multi-tapped coils (inductors) to get the signal optimized for the  
selector magnets.  Voltage on telephone company loops is limited to  
less than 200 volts so the circuit had to be re-designed and cut over  
in less than a couple of days.  You did not want to get your hands on  
that loop..!

Don
K9TTY