[GreenKeys] RS232-Current loop converter troubleshooting

[Sarah Autumn]

Thanks John,

Yeah, I agree that the the phrase "worked fine at first" should be a dead
giveaway. I'm just also considering the possibility that it is still
working fine, but I just made some silly mistake.

The resistance of the selector magnet in my 15 is 55 ohms. I will take the
rest of the measurements after I get home from work tonight. Thanks again
for your help :)

On Mon, Jan 18, 2016 at 12:24 PM, John Nagle <nagle at animats.com> wrote:

> 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
>
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