Re: Model 33 wiring complexity

[Ethan Dicks via cctalk]

On Wed, May 10, 2017 at 2:22 PM, Paul Koning via cctalk
 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

Re: Model 33 wiring complexity

[Paul Koning via cctalk]

> On May 10, 2017, at 2:24 PM, Chuck Guzis via cctalk  
> wrote:
> 
> 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?

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

Re: Model 33 wiring complexity

[Chuck Guzis via cctalk]

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

Re: Model 33 wiring complexity

[Paul Koning via cctalk]

> On May 10, 2017, at 2:06 PM, Tony Duell via cctalk  
> wrote:
> 
> On Wed, May 10, 2017 at 6:22 PM, Shoppa, Tim via cctalk
>  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.

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

Re: Model 33 wiring complexity

[Tony Duell via cctalk]

On Wed, May 10, 2017 at 6:22 PM, Shoppa, Tim via cctalk
 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