[GreenKeys] RS232-Current loop converter troubleshooting

[John Nagle]

> From: Sarah Autumn<neverether at gmail.com>
> To: Greenkeys<greenkeys at mailman.qth.net>
> Subject: [GreenKeys] RS232-Current loop converter troubleshooting
 >
> I finally got around to building the loop to serial converter that everyone
> knows and loves (http://www.aetherltd.com/connecting.html). It worked fine
> at first, but now I'm getting this weird behavior.
>
> When there is no teletype connected, on the pins for the selector magnet
> (J1 5&6) have voltage. When I connect the teletype to the pins, the voltage
> drops to almost nothing, and the magnet doesn't operate. If I remove the
> teletype from pins 5&6, the voltage jumps back up to 100V. When I hook the
> teletype directly to my loop supply, it runs fine.
>
> I have also tried 2 different power supplies: One is an acoustic coupler
> modem used for the deaf, and another is a REC-50.
>
> I have checked the values of all my resistors and capacitors. What simple,
> easy thing am I doing wrong?

     "Worked fine at first" indicates something has failed.

      When the Teletype is hooked up, power is applied,
and it's in a MARK condition (selector magnet should be pulled in),
but you're reading a low voltage at the output, please measure
voltages between the following points:

    Pins 4 and 5 of IC2: (Optocoupler in) - expect about 5V
    Pins 1 and 2 of IC2: (Optocoupler out) - expect near zero, because 
switch is closed.
    Across R7 (the big ballast resistor) - expect about 5-10V less than 
100V input voltage.
    Pins 1 and 2 of J3 (120V in) - expect about 100V
    Pins 5 and 6 of J1 (Selector magnet out) - expect about 5-10V.

With power off, measure the resistance of the selector magnets
unplugged from the converter board - expect 55 or 220 ohms.

The steady-state voltage of the selector magnet is supposed to
be low.  That's the purpose of the the ballast resistor R7.
Remember, this is a constant-current system, not a constant
voltage one.  After a SPACE to MARK transition, the huge inductance
of the selector magnets prevents much current flow at first.  So
there's low current in the loop, but high voltage across the selector
magnet.  Then, over the next 3-4 milliseconds, current through
the selector magnet increases until DC equilibrium is reached.
At that point, you have a 2K resistor and a 55 ohm selector
magnet in series.  Steady state voltage across the selector
magnet in that case is only 3.3V.

If you have a scope, you can watch this decay.

				John Nagle
				nagle at aetherltd.com