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Sherlock Ohms

The Singing Telephone Switch

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stn564
User Rank
Bronze
Re: Truly Sherlock.
stn564   10/22/2014 4:02:11 PM
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I trained on Strowger in the mid-70's in Zimbabwe (where it was still in use in quite a few exchanges).  I later lived in a small town there which had a Strowger 1000 line exchange - this was in 2002.  I got the tech to take me in there a couple of times and it was great to hear the noise of the exchange again.  Not sure if it's still there - I wouldn't be surprised....

kartwump
User Rank
Iron
Re: Truly Sherlock.
kartwump   10/22/2014 12:27:58 PM
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I too cut my teeth on Strowger. As you say - the noise was deafening! A great improvement for me when I went to work on the much quieter mechanical cross bar switches! All ectronic switches are not nearly as interesting to be around. Where's the fun in not being able to hear a call being set up?!

 

 

kartwump
User Rank
Iron
Re: Truly Sherlock.
kartwump   10/22/2014 12:24:30 PM
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Thank you for the kind comments. As Ratsky indicated, as technology changed we have to maintain service by adding the new in with the old. The reason the old line cards did not exhibit the problem is that they have a passive line interface: when the subscriber goes off hook, it bridges the line and operates a Hal effect relay on the line card to indicate request for service. With the electronic interface there is a SLIC (Subscriber Line Interface Circuit) which does the same function as the relay. This is the source of the constant power to the line.

 

bpenfold
User Rank
Gold
damping
bpenfold   10/22/2014 11:32:25 AM
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This has hurt me before too. Classic LC filter circuit has a resonant point; bad. Stick in a resistor and now you have a damping RLC circuit. I had to fix power supply noise once by replacing the output cap with one that had a higher ESR! It needed that damping R.

Ratsky
User Rank
Platinum
Re: Truly Sherlock.
Ratsky   10/21/2014 1:51:46 PM
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The Strowgers were a two-axis stepper.  First digit selected the correct radial row along the vertical axis (like the single-layer steppers, but with 10 banks of radial selectors); the second rotated the shaft to select the right contact set from the bank selected by the first digit.  This fed a second Strowger switch that handled the third and fourth digits.  In a large CO, the noise was deafening during the business day!  My first encounter with these was in the original main CO serving Chicago's Loop in the mid-'70s (still in service then, one of the last along with Wall St in NYC).

vandamme
User Rank
Silver
Re: Truly Sherlock.
vandamme   10/21/2014 12:58:52 PM
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I remember those stepper switches from when I worked for Western Electric in 1966 in downtown Detroit. Clever contraptions.

Ratsky
User Rank
Platinum
Re: Truly Sherlock.
Ratsky   10/21/2014 10:14:59 AM
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@naperlou: "What was more interesting was the ability to mix the two in the same frame."

That's generally an absolute requirement in the telephone central office business.  Overall service must NOT be disrupted in routine maintenance procedures (like replacing or adding a line card or even a trunk card).  Obviously, frame replacement would impact ALL customers served by that frame, so that's a no-no.  When a frame is installed, the expectation (given by artificially long depreciation times dicated by the rate regulators) is for at least a 20-year service life!  It used to be worse: electromechanical ones had FORTY year minimum life requirements!  The only way to replace an entire frame is to build up the entire replacement frame, fully configure it, and set up a "cut-over switch" (essentially a double-pole switch for EACH line and trunk with a master control), and wire that to both systems (disrupting only one circuit at a time for rewiring the old switch).  When the new switch has been fully validated and tested, a "flash cut" is performed during the least-busy time for the switch, with all lines and trunks being transferred simultaneously using the cut-over switch.  A more sophisticated system would use "smart relays" for the cutover switch that would only transfer each line when idle.  Obviously, these requirements impose a need to allow mix and match for all generations of line, trunk, and service cards for the rated life of the frame!

This is why (for example) some parts of lower Manhattan were the LAST COs to replace their Strowger-switch (Google this marvel of electromechanical design by a funeral director!) COs in the '70s about 4 generations after they were declared obsolete!  That included much of the Wall St. area.... where one would expect a demand for the best telephone service!

 

naperlou
User Rank
Blogger
Re: Truly Sherlock.
naperlou   10/20/2014 12:40:43 PM
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Yes, Barry, this was truly Sherlock.  Your methodical pursuit of the problem is inspiring. 

It is interesting that the older type of card had no effect.  What was more interesting was the ability to mix the two in the same frame. 

Daniyal_Ali
User Rank
Platinum
Truly Sherlock.
Daniyal_Ali   10/20/2014 6:55:57 AM
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Thanks for sharing the story Barry. I loved the way you handled the situation by going through the setup systematically. The solution was very simple in the end, but these simple things are very hard to find. The thing i love about handling customer issues is you always learn something unique and new and it becomes a part of your experience for the rest of your life.

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