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  • EEVblog #363 – Gold Phone Teardown

    Posted on October 3rd, 2012 EEVblog 14 comments


    Forum Topic HERE
    Teardown Tuesday
    What’s inside a Telstra CT4 “Gold Phone” Payphone?
    And how do electronic coin validators work?

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    • John

      Ey Dave, what you’re using for de-interlacing? The interleave artifacts are slicing the picture apart pretty badly whenever there is movement, and the fact that the phone is sitting on top of carousel platform ain’t exactly helping…

      • http://www.eevblog.com EEVblog

        Yes, I accidentally shot this one in 50i mode. I didn’t want to redo the renders to fix it.

    • Jay Ts

      Hi Dave — that was fun. At first, I was wondering if this payphone would be like the old style here in the USA. They don’t (didn’t?) even have 2 wires connected to them; they just used the Earth (the actual planet Earth) as the ground return. Sometimes, ground is really the ground! The POTS (Plain Old Telephone System) here uses about 48 volts in a current loop, with about 90 volts supplied for the ringer, which of course was originally a solenoid driving the clapper on a bell. Think 1920s electrical technology!

      The signaling for depositing (counting) coins and releasing them was even stranger. The operator could tell the phone to give a refund, and I think that also used pulses of 90 volts. I don’t remember exactly. (Anyone??)

      I used to be friends with a guy who was a lineman with various US phone companies for many years and he told me fun stories about servicing payphones all over the US. One of their basic methods for diagnosing an errant payphone was to find a water hose and a spigot, and soak the ground around the phone to see if it worked afterward. Dry ground doesn’t conduct electricity so well, and that is a common problem! Imagine, you are somewhere “out in the field”, and you are asking anyone nearby how long has it been since it rained, and was that unusual? :) If they answered yes, it explained why the phone was working ok, but stopped working recently.

      • http://www.eevblog.com EEVblog

        Wow, earth return!

      • Worf

        In North America, it’s rarer for businesses to own a payphone (this gold phone looks like it would be owned by a business and kept inside for public use but in a more secure location). Here, they’re owned an operated by the telephone company, so they’re more rugged because they’re out in the open accessible and often unmonitored (if it was on premises, someone would probably notice someone trying to break into it).

        So our North American ones are known as fortress phones, with anti-vandal cabling and everything because they often stood alone.

        The coin signalling was, like long distance signalling, in-band. One could use a special device known as a “red box” to simulate coins being inserted (just a DTMF tone) and the phone company would recognize how many coins by the number of tones put on the line.

        For coin return, the other end used a “green box” to signal the payphone to return the money, which is how the operators used to return money as well. The payphone recognized the tons and gave you the refund. Of course, only the called party can signal it.

    • Ted

      Did try with coin drawer in place? That microswitch is there for a reason.

      • http://www.eevblog.com EEVblog

        Yeah, no joy either way.

    • Bob

      Interesting that your payphones don’t use a vandal-resistant cord on the handset, considering all the other “hardening” measures that were taken on this thing.

      Stainless steel armored cords were standard issue on payphones here in the US.

      • http://www.eevblog.com EEVblog

        The ones that are found in phone booth have all the armoured plating etc. These ones are design for use in clubs and pubs etc.

    • pertinax

      I remember to have started my hacking career in sixties. I used to go to phonebooth near our house and stuffed my haircomb into the coin slot. Then i dialled 020 and when the phone went off hook i banged that machine once with my fist. That disturbed the phone so that i could listen “Top 10 Hits” for free.

    • Salas

      Less shades and glare with the new lights.

    • Nik

      The manuals for this phone are at the following location:

      http://zlh.halcon.tv/files/Phreak/Payphones/CT/

      CT4 is the model number.

    • gordon

      Perhaps it’s my inner 13 year old speaking, but I didn’t immediately think of the phone’s ringer when you said “anti-tinkle module”. I wonder what happens to these phones when you … uh … get them wet …

      • gordon

        By the way, add my vote to the idea of digging into the coin validator in greater detail in some future video.