[GreenKeys] Re: Model 15s with holding magnets...

[Don Robert House]

Robert,
A an Illinois Bell Teletype Serviceman I personally did not see a  
holding magnet version in service. Of course by 1966 we did not have  
many 15s or 19s in service.  However, I am familiar with the use of 20  
milliamp and 10 milliamp telegraph circuits.  These were usually set  
up with the use of hubs for broadcast of weather information.  Many of  
the loops were long and difficult to support 62.5 milliamps; so 20  
milliamps was a second choice.  In the Chicago area there were many  
weather machines... some shared the same loop in which case the two  
machines each used a special selector assembly that operated on 10  
milliamps.  I wonder if anyone on this list has any experience with 10  
millamp machines?

I worked in the suburban area.  We did not have any machines set up to  
run on either 20 or 10 milliamp loops.  All of our machines used 62.5  
milliamps -130 vdc.  At least two of our machines (we had over 600 to  
maintain) had long loops.  Rather than use 20 milliamp selectors we  
used both the positive and negative supplies with two cable pairs tied  
in parallel to reduce resistance.  NO ONE got near those pairs with  
260 volts 62.5 milliamps.  If you bumped that voltage it could put  
your shoulder out of joint.  All places along the cable route had  
special red rubber insulator caps warning of the high potential.   
Later on the circuits were redesigned to run on -130 62.5 mA.

Ah memories.

Don
K9TTY



On 20 Nov 2007, at 10:48 PM, WA5CAB at cs.com wrote:

The M14/15/19 sets were available with two types of selector assemblies,
pulling and holding. Pulling magnet selectors only operate on 60 ma  
(magnet coilss
are round or cylindrical). Holding magnet assemblies can operate on  
either 20
or 60 ma (magnet coils are smaller than pulling magnet coils and are  
square
with radiused corners). Most machines were equipped with pulling magnet
selectors. Some of the Bell or WU system refugees could probably shed  
light on what
service usually used holding magnet machines. I know that they are  
much rarer
than the pulling magnet ones. And I don't recall ever seeing holding  
magnets on
a military set.

Other than the coils, there are mechanical differences . If a machine  
with a
pulling magnet selector runs open, you will see that the selector  
armature
doesn't move. If one with a holding magnet selector runs open, the  
armature moves
rapidly one cycle per bit. With current flowing through the holding  
magnet,
the armature is held against the magnet poles during the bit set  
period and a
MARK gets set. With no current, it isn't held and a SPACE gets set.  
The pulling
magnet has to pull the armature against its return spring across an  
air gap,
which takes a significantly stronger magnet field.

There are hundreds of M15 variants and I haven't worked on any since  
1973 so
I couldn't hazard a guess as to what the resistor might be for.

In a message dated 11/20/2007 9:08:37 PM Central Standard Time,
RLAAG at PACBELL.NET writes:
> ONE MORE THING, THE SELECTOR MAGNET CURRENT... I SUPPOSE THE MAGNETS
> WIRE IN SERIES FOR 20 MA LIKE THE 28 AND PARALLEL FOR 60 MA... RIGHT
> NOW THERE IS ALSO A STRANGE RESISTOR IN THAT AREA ON A BRACKET... IN  
> MY
> APPLICATION I DOUBT IF ONE IS NEEDED THERE BUT AM NOT SURE WHAT IT WAS
> FOR... I WAS WONDERING IF THE SEL MAGNETS NEED TO BE IN PARALLEL LIKE
> IN THE 28 OR IF THEY JUST RAN NORMALLY ON ONLY 60 MA???


Robert & Susan Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
<wa5cab at cs.com> (Primary email)
<wa5cab at comcast.net> (Backup email)

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