10 May
2017
10 May
'17
1:22 p.m.
The Model 33's I bought back when I was a teenager, were all ex-Telex use and had
exceedingly complicated wiring harnesses as well as built-in modems. They had paper tape
readers and punches with various auto-start/auto-stop relay options.
The exceedingly complex wiring harnesses were to add various options. The wiring harnesses
made the wiring of the unit look, literally, a thousand times more complex than it
actually was.
In the end all I ever did was find the magnet wire, and the keyboard contact wire, for the
main unit and the punch.
I usually also found a useful current loop supply in the base. Sometimes I would find
bipolar relays and 20mA/60mA conversion supplies. I was a little surprised at some of the
current loop supplies - some of them weighed 20 pounds and were obviously made for driving
exceedingly long lines (open-circuit voltage way over 100VDC). Seemed odd these were in
there considering the units had modems just a foot away from magnet and keyboard switch.
Tim N3QE
10 May
10 May
2:06 p.m.
On Wed, May 10, 2017 at 6:22 PM, Shoppa, Tim via cctalk
<cctalk at classiccmp.org> wrote:
I usually also found a useful current loop supply in
the base. Sometimes I would find
bipolar relays and 20mA/60mA conversion supplies. I was a little surprised at some
of the current loop supplies - some of them weighed 20 pounds and were obviously
made for driving exceedingly long lines (open-circuit voltage way over 100VDC).
Seemed odd these were in there considering the units had modems just a foot
away from magnet and keyboard switch.
Not necessarily for long lines.
Some teleprinters had the receiving magnet directly in the current loop. The
resistance of this was too small to limit the current to 20mA (or whatever) so
a series resistor was added.
Now the time constant of an LR series circuit is L/R. In other words a higher
resistor gives a shorter time constant. You get more rapid operation with a
higher voltage supply as you then have a higher resistance circuit.
-tony
2:22 p.m.
On May 10, 2017, at 2:06 PM, Tony Duell via cctalk
<cctalk at classiccmp.org> wrote:
On Wed, May 10, 2017 at 6:22 PM, Shoppa, Tim via cctalk
<cctalk at classiccmp.org> wrote:
I'd put it this way: For teletypes, the receiver is predominantly an inductive load.
And it wants a fairly short rise time, so you pick the time constant you need, which gives
you R, which in turn gives you the needed drive voltage. And that works out to be a
fairly high voltage in a lot of cases, especially with the older machines with 60 mA loop
current. The needed loop R is almost entirely the series R; the receiver coil adds very
little to the total.
paul
I usually also found a useful current loop supply
in the base. Sometimes I would find
bipolar relays and 20mA/60mA conversion supplies. I was a little surprised at some
of the current loop supplies - some of them weighed 20 pounds and were obviously
made for driving exceedingly long lines (open-circuit voltage way over 100VDC).
Seemed odd these were in there considering the units had modems just a foot
away from magnet and keyboard switch.
Not necessarily for long lines.
Some teleprinters had the receiving magnet directly in the current loop. The
resistance of this was too small to limit the current to 20mA (or whatever) so
a series resistor was added.
Now the time constant of an LR series circuit is L/R. In other words a higher
resistor gives a shorter time constant. You get more rapid operation with a
higher voltage supply as you then have a higher resistance circuit. 
3:12 p.m.
On Wed, May 10, 2017 at 2:22 PM, Paul Koning via cctalk
<cctalk at classiccmp.org> wrote:
I'd put it this way: For teletypes, the receiver
is predominantly an inductive load. And it wants a fairly short rise time, so you pick
the time constant you need, which gives you R, which in turn gives you the needed drive
voltage. And that works out to be a fairly high voltage in a lot of cases, especially
with the older machines with 60 mA loop current. The needed loop R is almost entirely the
series R; the receiver coil adds very little to the total.
I'm restoring a Model 19 teletype (5-level Baudot). The current loop
is 60mA, and uses 120VDC for the loop. Typical of the class.
-ethan
2:24 p.m.
On 05/10/2017 10:22 AM, Shoppa, Tim via cctalk wrote:
The Model 33's I bought back when I was a
teenager, were all ex-Telex
use and had exceedingly complicated wiring harnesses as well as
built-in modems. They had paper tape readers and punches with various
auto-start/auto-stop relay options.
Wasn't Telex 5-level, not 7? So they'd be Model 32s--or is my memory
faulty?
--Chuck
2:31 p.m.
On May 10, 2017, at 2:24 PM, Chuck Guzis via cctalk
<cctalk at classiccmp.org> wrote:
On 05/10/2017 10:22 AM, Shoppa, Tim via cctalk wrote:
Classically, yes. I wonder if ASCII was done at some point. Also, teleprinter service
generically comes in a number of flavors, including 6 bit codes (of many variants) for
newspaper wire services.
paul
The Model 33's I bought back when I was a
teenager, were all ex-Telex
use and had exceedingly complicated wiring harnesses as well as
built-in modems. They had paper tape readers and punches with various
auto-start/auto-stop relay options.
Wasn't Telex 5-level, not 7? So they'd be Model 32s--or is my memory
faulty?
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