[GreenKeys] MJE340 Loop Keying Transistors

[Lawless, Loveless & Childress]

   >>>having smoked my share of 340s!
   Needs  a  heat  sink  AND  a little diode/resistor/cap bypass to shunt
   spikes to GND. My Kleinschmidts used to consume coffee cans (it seems,
   now)  of  MJE340's  annually,  as every time the STOP relay cycled the
   Loop Keying transistor went "poof!"
   Eventually,  I  built  a  "Loop  Reader/Loop Keyer" with optoisolators
   firing  the  MJE340 and reading the Loop Voltage which gave me TTL and
   RS-232  levels  IN  and  OUT, keeping the nasty Loop Voltage out of my
   Altair and, later, my S-100 box. Also had a LOOP HOLD switch (or PRINT
   INHIBIT) to force the loop voltage HIGH manually.
   After  reading  a  Hoff  article  I got involved with using UART's for
   speed  up-conversion and Kluged this box to my semi-homebrew TU, based
   on 1458  opamps.
   Bypassing   components   and   the  slow  response  time  of  1975-era
   optoisolators  meant  that  the  maximum Baud rate was under 300. More
   modern  components and newer techniques would improve this to the 1200
   Baud level easily.
   Sorry.  After 31 years the schematic is buried in the detritus of five
   houses,  two condos and a divorce, so I can't give you exact component
   values,  but,  by description, it should be easy to noodle out on perf
   board.
   DE BUCK  W7LV