[GreenKeys] Loop current supplies

[Jim Haynes]

If you need to make a loop current supply, you might want to look at
the instruction book for the Northern Radio Type 238 current loop control.

http://www.virhistory.com/tmc/tmc_pages/NorthernRadio/ib_238-1-2_4_8_66.pdf

This provides a constant-current circuit so you don't have to adjust the
loop current as you switch machines into and out of a loop.

Another article with a similar design is in the January 1972 issue of QST,
"The Modern Teleprinter Local Loop" by Frank Merritt VE7AFJ.

jhhaynes at earthlink dot net