sundance-communications.com/forum Forums General General Telephone & Systems Information Inside the central office

The DACS system (pair-gain units) used here in England are 140V. The old telex circuits had voltages swinging positive and negative which were certainly high enough to give a bite as well.

Didn't the old American coin-phone circuits -- at least within the Bell System -- use +/-120V pulses for coin collect and return?

Yes, Paul. Coin phones used 130 volts DC, either applied from tip or ring to ground to either accept or return the coin on prepay circuits. Most coin phones here in this area are "smart" phones now that have all of the thinking going on inside the set.

Like Bill said, silence is very bad. Went into a very small SXS office many years ago and heard total silence. A contractor had cut 25 pair outside plant cable miles away from the C.O. Locked up to whole office. Once the coils were pulled on the 25 pair count the switches dropped down and call processing resumed.

Notifier’s concern that some of these voltages could be present is a valid one. Any form of carrier or range extension uses voltages in the DC 130 plus range. If things are done correctly any pair that has this type of voltage or is a special circuit should be recapped or red ringed at any terminal where it appears.

Wow. I did not know how much voltage that stuff carries! Glad I know now

N-carrier, 130 positive on one side 130 negative on the other, would knock the crap out of ya.

Total silence: Now that's a good one! Back in my days as an electrician, one of my pals worked for the company that maintained lighting in most of Bell Atlantic's central offices. He told me of a time where he clipped a wire from a ballast in a fluoresent light that managed to trickle it's way into a bank of #5 crossbar equipment in the Springfield, VA office. According to his reccolection of what happened, "the buzzing just stopped and I ran to the bathroom to act like nothing ever happened".

Thanks for all the great info! I've been inside three central offices within the past month, definitely a "don't touch unless you know what your doing" environment. One was an original CO from back in the day, even in the historic part of town. Still has that nondescript brick building look.

I did a some telephone work years ago, mostly office phone systems and residential, aware of your standard 48-52 volts and ringer voltages (that tingles!) but wasn't aware of anything around 148 volts. I'll just make sure I stand clear of anything capped red!

One other question about the mic setup. Is that to talk between CO to CO, or just within the same CO? Why couldn't you just accomplish the same with a butt set? Or is that just two different animals!

RedTail explained that pretty good, it was local testboard. Which of course nowadays nothings local it's all remote. I think Ed said something about the dial in talk back speakers that replaced the mics. It was the testboard to the frame. So test board would say, I need shoes on cable pair, whatever, and frameman would go to the location, push the button and let the testboard know the shoe was in for testing. (also called sticks)

How were the phones wired to accept the signals? Did they use something like cold-cathode tubes to provide segregation of +/- for collect/return and to keep the impedance of the solenoids to ground from unbalancing the line?

The old pre-pay coin phones here weren't so sophisticated. Coin collect or return was purely mechanical with the user being instructed to push button "A" (collect) or button "B" (return).

Those phones are all long gone, as are the post-payment types which superseded them during the 1960s.

These days it's the same as you describe; self-contained microprocessor controlled units with all the "thinking" going on in the phone itself.

It’s been a while. We just wired red to ring, green to tip, and yellow to ground. The C.O. had a coin appliqué circuit that send the collect and refund voltages. A coin relay in the pay phone operated one way to dump the coin into the box and the other way to drop the coin into the refund chute. The coin relay and all of the components inside the phone have been replaced with electronic circuitry