[GreenKeys] TTY 33 rangefinder puzzle

[Jones, Douglas W]

I've been trying to adjust the rangefinder on my 33 and I've found that it's extremely finicky.

Rangefinder settings that give me almost reliable text reception give me only 50% reliability on control characters, and if I tune it to reliably receive LF, I get about 5% misreceived text.  It seems to work better at typing speed than at full speed with paper tape, so context seems to matter.

So, I looked at the current through the selector magnet on the face of a scope.  The current rises to full current in about 1/2 of a bit time.  That seems awfully slow.  I measured the current by hooking the scope across R5, but perhaps I should have used R7?  With reference to Teletype schematic 181821 dated 4-28-67 (that one seems to describe the board I have in my CCU).

It is sort of interesting that you can see the selector armature closing on the waveform as the current rises to about 4/5 of the peak current.  The moving selector armature induces a counter EMF that stops the current rise until the armature move is done.

Also, the waveform across R5 shows instant turnoff because the decay current in the selector magnet is shunted elsewhere (via CR3, C1 and R9) so it doesn't show up.  Had I used R7 to take my measurements, I'd have seen the decay current.

Any hypotheses about what would slow the current rise that much (and possibly the current fall?)

          Doug

PS:  the fact that the local-mode loop is entirely floating with respect to ground lets you do evil things with a scope probe, grounding any part of the circuit.  If it were not a floating loop, I'd have to use a two-channel scope in differential mode.