[GreenKeys] Snubber tests

[Peter Gottlieb]

I just ran a few tests.  The simplest snubber which still absolutely 
protects the MJE13005 consists of a diode (I use 1N4007's for just about 
everything) in series with a 100K 2W carbon resistor across the selector 
magnet.  The voltage on opening goes up to around 500 volts then shows a 
nice exponential decay.  The resistor gets mildly warm after steady 
copy, probably a 1 watt would do fine.

So now I hooked up an optocoupler as the input from the low level data 
line.  I first tried a TIL111 because I have a bunch of them, no good, 
not enough current transfer.  The best I could find I scrounged from an 
old switching power supply board and its datasheet says it has a 200% 
CTR (Current Transfer Ratio) yet I am not seeing anywhere near that and 
it is marginal.  I must be missing something.  I am driving at 20 mA and 
only need 17 mA on the output.  I think I am expecting it should go into 
saturation and it isn't designed to do that.  I may need additional 
components, maybe make it like a darlington or use a tiny FET (I used to 
use the 2N7000 a lot, must have some still around somewhere).

In any case, I did get data from the Dovetron through to the 2B.  There 
was no QRM generated at all, at least not that I could detect even on 
very weak signals.  However, I was unable to get good copy on any 
signals.  One problem was that many signals were not 22 msec timing (ie, 
not 45.45 bauds).  One that was, still no joy, and I note the 2B is not 
spacing evenly.  It sometimes doesn't space properly, or at all, when 
cold so I have to get into that part of the mechanism and lubricate, 
clean and adjust it as necessary. 

Getting there.  Once the opto problem is resolved I need to put one in 
for the other direction so the keyboard runs the Dovetron.  Then add a 
full/half duplex switch (hmmm, I wonder if the Dovetron switch does 
that?) and build the final loop supply/signal converter into a suitably 
boatanchory looking box.  I did note that the Dovetron "standby" switch 
does nicely enable/disable the printing.

Peter