EEVblog #363 - Gold Phone Teardown - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 02, 2012, 10:47:47 pm
content: Dave.

author: steve30
author_rank: Frequent Contributor
timestamp: October 02, 2012, 11:52:25 pm
content: Interesting.Would be really cool if you could get it fully working.I wonder if all payphones use the same principle for detecting the coins or not. I rarely make telephone calls but when I used payphones years ago, I remember them always working, but when I came to use one about 3 years ago, there were lots of them around, but I struggled to find one that would actually accept my coins!

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 02, 2012, 11:53:36 pm
content: Interesting teardown.I believe the reason for the dual coils on the coin validator is a means of measuring not only the metal content, but the diameter of the coin, as well. As the coin rolls between the 2 coils, it acts like the core of a transformer, coupling a signal from one set of coils into the other. The length of the resulting output pulse would depend on the size of the coin, with a larger coin taking a longer time to roll past the coils, giving a longer output pulse.

author: krivx
author_rank: Frequent Contributor
timestamp: October 03, 2012, 02:45:20 am
content: The sliding mechanisms to allow servicing aren't too surprising, I've seen the same thing in automated payment machines in multi-story carparks. Obviously these devices can be large and heavy and every minute out of service costs money, so they need to be repairable quickly in the field.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 03, 2012, 05:05:39 am
content: Quote from: krivx on October 03, 2012, 02:45:20 amThese things were build and used at another time. Government-owned phone company, tenured government employees.This wasn't about costing money when being out of service. This was about doing the right thing - in the minds of the government employees. Cost was the least of their worries. Providing the citizens with what a government phone company deemed adequate service, following laws and regulations was one goal (laws and regulations they typically heavily influenced). Not exposing government tenured workers to unduly stress and difficulties when having to service such a phone was another goal.

author: notsob
author_rank: Frequent Contributor
timestamp: October 03, 2012, 05:35:59 am
content: I knew someone involved in designing one of the public phone series - not the gold one.They installed their test 'articles' in very rough city areas, to see what that local boys could do to the phones.The eventual product had hardened steel cases to make it more difficult for them to drill holes into the phone (to access switches with probes), teflon coated wires in the flexible steel hand piece cable to stop destruction via cigarette lighters, teflon coatings in the coin slot to stop super glued coins, coin detector to check if it was a coin or a metal washer, chute to let sand thru (someone liked pouring sand into the units to break them), mounting bolts with 2" wide heads (some units were being ripped out of the phone booths with ropes tied to vehicles). the list goes on. The eventual phone killer that they couldn't stop was someone resorted to pouring coke into the phones via the coin slot.So it is not only the electronics design and the mechanical design, it's making them vandal proof. (definitely s designers nightmare)

author: firewalker
author_rank: Super Contributor
timestamp: October 03, 2012, 08:42:53 am
content: So... No new Altium license?Alexander.

author: muris
author_rank: Contributor
timestamp: October 03, 2012, 10:08:07 am
content: Very interesting teardown, Dave!I especially love the coin accepting mechanism, it intrigues me very very much. I wonder what the signals looks like through those coils and how fast does the micro-controller must "match" it to the pre-programmed values in order to accept or reject the coin on it's way down.Best regards!

author: madires
author_rank: Super Contributor
timestamp: October 03, 2012, 11:53:39 am
content: Quote from: steve30 on October 02, 2012, 11:52:25 pmDave might need to provide a dial tone, not just DC power.

author: frenky
author_rank: Unknown
timestamp: October 03, 2012, 12:14:16 pm
content: It could be rejecting coins because the cassette is not inserted and micro switch is in open position.

author: SteveyG
author_rank: Unknown
timestamp: October 03, 2012, 12:25:21 pm
content: Quote from: frenky on October 03, 2012, 12:14:16 pmSame thoughts.

author: steve30
author_rank: Frequent Contributor
timestamp: October 03, 2012, 02:41:55 pm
content: Quote from: madires on October 03, 2012, 11:53:39 amPossibly.Is there any reason why it shouldn't just be connected straight to a telephone line?Also, just wondering, is the telephone's ringer a proper bell, or a speaker?

author: SeanB
author_rank: Super Contributor
timestamp: October 03, 2012, 03:19:53 pm
content: Public phones work off the standard -48V telephone supply, but need additional data delivered. One is a metering pulse of -60V applied between both lines and the earth connection of the phone. There is also a swap of line polarity when the call is connected to the other party, to tell the phone that billing has started and it must expect a billing pulse within a local unit time. If the pulse does not come and it is calling to a number not in the preprogrammed prefix table for freecall number blocks the phone will cut the call and go into tamper mode, where it disconnects, releases the line, tries to sieze the line then dials the self report number of the Telco and reports the tamper for repair. It will either stay in this mode and only receive calls, or will reset on the next successful meter pulse, depending on the programming.Older call boxes had a much simpler mechanism, where coin data was transferred by pulses to earth when you put coins in the mechanism, which had a coil for each unit, and gave a pulse for each. Thus a 5c piece would give one pulse, a 10c 2 and a 20c 4. Even worked via an operator connected call, where the operator would listen to the clicks and count them then connect your call. Hard to fool the operator with a long tikkie, as there was a written called number record to grill. Automatic exchanges however could be fooled by wet toilet paper across the 3 wires, and dial a 0 to give 10 pulses.......... Or a 4k7 resistor on 2 croc clips, not that I ever did this, of course, though one box had a large count on the meter and 5c in the coin box............. One guy did a 11 hour call to his girlfriend, and the conversation from his side was " Ja liefie.............Nee Liefie.................Rerig liefie" ( Yes dear, no dear, really dear) for the 11 hours..............

author: NewBeginner
author_rank: Regular Contributor
timestamp: October 03, 2012, 03:46:20 pm
content: Dave, it seems to me that, after all this time, you still hold a grudge on Altium. Move on my friend

author: G7PSK
author_rank: Super Contributor
timestamp: October 03, 2012, 04:10:35 pm
content: Would the reason that the phone did not accept the coins be that the cash drawer was not in place and holding down the micro switch, also are the coins today identical in all ways to the coins used when that phone was made. I have noticed that many older coins will not work in vending machines and that some vending machines will no take the latest coins due to dis similar metals and slight differences in weight or size.

author: reagle
author_rank: Unknown
timestamp: October 07, 2012, 03:15:05 am
content: As Dave was saying "Vandal Proof", I kept looking at the cable to the headset. That looked very easy to cut/rip. The phones I've dealt with in the former USSR, had a metal spiral like shroud covering the wires. Then the rotary disk would get melted, so they started using vandal proof metal keypads

author: SeanB
author_rank: Super Contributor
timestamp: October 07, 2012, 05:28:08 am
content: These were typically used inside a shop or such, where there is some supervision. The cutting of the cable would be noticed at least. Outdoor phones have the steel spiral wrapped cordsets here ( where you can still find one) and the locks have been upgraded to a very secure and very hard to damage style as well. Cabling is always in steel conduit as well, or buried in a plastic conduit under plaster. The phone booths have evolved from the old close door type to a GRP eggshell to the latest variants which are a steel column with a steel rooflet and a steel bar forming the side.

author: T4P
author_rank: Super Contributor
timestamp: October 07, 2012, 06:57:09 am
content: Basically what the engineers have been thinking, "Use steel where possible"

author: SeanB
author_rank: Super Contributor
timestamp: October 07, 2012, 07:05:37 am
content: Concrete takes too long to cure............ The railings on pedestrian walkways used to be aluminium, but it all grew legs. The buggers even stole the stainless steel fastenings off a bridge, causing it to collapse. For that I put the blame squarely on the contractors renovating it, as they are only accessible with a swing scaffold and hanging down 5m above a river known to have crocodiles.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 07, 2012, 01:09:14 pm
content: ABBA - Ring Ring

author: SeanB
author_rank: Super Contributor
timestamp: October 07, 2012, 01:54:37 pm
content: Ed, you gotta find videos that are not restricted...........

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 07, 2012, 04:10:22 pm
content: Quote from: SeanB on October 07, 2012, 01:54:37 pmErr sorry didn't realise. What's wrong you cant see it?

author: SeanB
author_rank: Super Contributor
timestamp: October 07, 2012, 05:12:13 pm
content: Have to watch on YT, not embeddable in my country......... Nice side recommendations though.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 08, 2012, 07:54:51 am
content: Quote from: SeanB on October 07, 2012, 05:12:13 pmNo worries.

author: reagle
author_rank: Unknown
timestamp: October 08, 2012, 04:21:25 pm
content: Speaking of bastards taking bridge apart. How about somebody walking away with copper from a live HV transmission line? That's impressive

author: Cranky
author_rank: Newbie
timestamp: May 18, 2014, 12:38:18 pm
content: Wow. This brought back some memories.I serviced these for about 14 years.A bit of info...These phones were operated by customers leasing them from Telecom (at that time) CT4 = customer telephone 4.They were obviously line powered (48V positive earth) as you have discovered and the super caps on the bottom of the mainboard would run the electronics during a call.They were also polarity sensitive. They would power up and accept coins when connected in either polarity but would drop the call on answer if the polarity was incorrect.Speaking of which, in normal operation the phone would accept coins into the mech' whenever the display was active. I am guessing the coin mech' in the video was faulty as it should have accepted coins when they were inserted.It would collect coins into the cash box on answer, and then collect the set amount every time it would receive a meter pulse (for multi-metered aka long distance calls).On answer the line polarity reversed, then within 250mS it should receive a 50Hz longitudinal meter pulse of 250mS duration. The meter pulse was the same amplitude and phase on both 'legs' of the line and was referenced to earth. (The goldphone required an earth to operate.)The lift up mechanism allowed the service tech to test the coin mechanism by making a test call to designated test numbers and the coins collected would drop into the refund chute, not into the cash box. (Calls to these numbers weren't charged to the lessee of the phone.)The coin mech' would use the coils to measure the thickness, diameter and material of the coin to determine its value.The little kicker solenoid wouldn't pull in acceptance of a coin, just depower enough so that the weight of the coin would operate the kicker arm and allow the coin to run into the coin holding area.Just a bit of trivia: The same coin mech' was used in the green and grey coin and card payphone used in the street at the time.The tariff unit at the top of the mainboard was able to be modded to adjust the tariff. The left side for the tariff on answer and the right side for each subsequent collection.The three diode positions on each side would set the tariff. from the top down they were 10c, 20c, and 40c, so you could set the tariff from 10c t0 70c just by adding or removing diodes.The board in the video is set for 50c.Later revisions of the board had DIP switches to set the tariff.I still have the maintenance key for the rear lock and quite a few spares parts laying around. I also have 2 complete gold phones as well.Don