Pay Phone Handset - Page 1
Forum section: electronics / beginners

author: Renegade
author_rank: Regular Contributor
timestamp: February 27, 2016, 03:06:49 am
content: A local hardware outlet store carries some crazy stuff, and I just picked up a pay phone handset for $3 CAD. However, I don't know what the typical characteristics are, or what would be safe to throw at it.I've looked, but can't seem to find much useful information.The wires are red/red/white/white in a 2x2 connector (like those breadboard jumpers that you'd use with an Arduino pinout) (rotate however):Code:red redwhite whiteIs it safe to experiment, or should I buy a couple more?

author: roffvald
author_rank: Unknown
timestamp: February 27, 2016, 04:21:18 am
content: I'd say one red/white pair for speaker and one for mic, measure the resistance, the higher one should be the mic.

author: Renegade
author_rank: Regular Contributor
timestamp: February 27, 2016, 04:40:13 am
content: Quote from: roffvald on February 27, 2016, 04:21:18 amErm...I get 119 ohms on the white/white and 137 ohms on the red/red. I wasn't expecting that.Would white/white and red/red make sense? I'm a tad surprised there.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 27, 2016, 04:54:35 am
content: You can unscrew the microphone and speaker covers to figure out where the wires are going.

author: Renegade
author_rank: Regular Contributor
timestamp: February 27, 2016, 05:03:39 am
content: Quote from: SkyMaster on February 27, 2016, 04:54:35 amUnfortunately, that would require a hammer. I tried. I think they're glued in place.

author: MSO
author_rank: Contributor
timestamp: February 27, 2016, 05:18:39 am
content: It sounds about right that a pay phone would have the element covers glued. Well, I have genuine dial phone so I unscrewed the elements and found the white/white wires connected to the speaker element and the red/black wires connected to the microphone.My phone is brand new in the original packaging, so all the wiring is still intact, handset to base. I measured 30ohms across the speaker element and 1.2k ohms across the microphone. I'm not sure how much help that is, but the white/white and red/red sounds plausible, but I'd say there's something amiss with the microphone element.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 27, 2016, 05:50:02 am
content: Depending on the age of your handset, you could have an old carbon microphone. Try banging the handset on the bench and see if either the red or white pair changes resistance (after it settles down of course). The speaker is just a coil of wire so it won't change. If the microphone is a carbon mic, it will be all over the map. I just tried it on some old carbon mics that I've got and I saw values from 30 - 200 ohms.The polarity doesn't matter on either the speaker or the microphone so it makes sense that one color would be for each.

author: Renegade
author_rank: Regular Contributor
timestamp: February 27, 2016, 06:07:24 am
content: Quote from: MSO on February 27, 2016, 05:18:39 amThanks for that. It will help.Quote from: edpalmer42 on February 27, 2016, 05:50:02 amBanging it around in both cases yielded nothing. It looks new, so I don't think that's a surprise.And thanks for the polarity hint.

author: SkyMaster
author_rank: Frequent Contributor
timestamp: February 27, 2016, 03:32:46 pm
content: My made in Canada by Northern Telecom, Model 500, dated 1981, as the following:white/white to the speaker (there is also a component soldered to the speaker terminals; without any marking)Red/Black to the microphoneJust like MSO phone

author: amyk
author_rank: Super Contributor
timestamp: February 27, 2016, 05:19:35 pm
content: Listen as you apply the ohmmeter. The current might be enough to cause some audible clicking of the speaker.

author: Audioguru
author_rank: Super Contributor
timestamp: February 27, 2016, 06:24:55 pm
content: What will you use the very old handsets for? They sound awful.Old telephones: "What did you say? What did you say?" Do you like to repeat everything you say so it can be understood? With the sounds poorly produced then the words "failing" and "sailing" sound the same.

author: MSO
author_rank: Contributor
timestamp: February 27, 2016, 07:01:01 pm
content: Quote from: Audioguru on February 27, 2016, 06:24:55 pmYou're probably thinking back to the days when telephone poles were wired like this every 300 feet or so:http://www.sandman.com/images/oldpolewires1.jpgIt's been my experience that the old dial phones with coil speaker and microphone elements sound far superior to any mobile, cell or wireless phone that I've ever used.

author: Renegade
author_rank: Regular Contributor
timestamp: February 28, 2016, 06:57:54 pm
content: Quote from: amyk on February 27, 2016, 05:19:35 pmHA! BINGO!White is the mic, and red is the speaker. Both click.Thanks for that!Quote from: Audioguru on February 27, 2016, 06:24:55 pmI don't know if it's the quality of the parts, or something in sending/encoding the signal, but I've always found that telephony audio is really low quality sound. Talking over Skype is far, far better with much higher quality sound. I don't care what anyone says, but that frequency range above and below regular telephony ranges really makes a huge difference.But, this isn't so much about the actual quality of the sound as it is about me learning. Like, just above I learned a new trick to help identify mics/speakers through listening for clicks while testing resistance. That's one more trick in my trouble-shooting arsenal, and that's what this is really about.

author: SeanB
author_rank: Super Contributor
timestamp: February 28, 2016, 07:47:13 pm
content: Telephone is bandwidth limited, 300Hz to 3.5kHz, with a roll off above and below. This is to limit noise, and to allow multiplexing of multiple signals using a simple codec. The instrument itself however is often capable of better performance, typically 150Hz to 10kHz, but this is only really usable on a short direct link that does not go through an exchange where it will be limited by the line interface cards. Connect 2 phones together using only a local 12V battery supply in series, and the audio is pretty good.

author: Audioguru
author_rank: Super Contributor
timestamp: February 28, 2016, 10:55:01 pm
content: When Canada allowed competition with Bell in 1980 then other telephone systems were beginning to compete with Bell and the audio company I worked for began selling business telephone systems.The customers and I complained that the sound was awful so I transmitted tones on one telephone line at work and received them on the next line. 3kHz was down to -12dB so I complained to Bell that their telephone lines had not enough 3kHz. They said it meets spec because it is better than their -15dB limit (which isreallybad).Then I designed and made a transmit equalizer that boosted 3kHz by +10dB and every time I demo'd it then it was sold.Polycom sells speaker phones that user two telephone lines to double the audio bandwidth and they sound great.

author: amyk
author_rank: Super Contributor
timestamp: February 29, 2016, 01:15:08 am
content: Quote from: Renegade on February 28, 2016, 06:57:54 pmThat suggests the mic is not carbon, but moving-coil or moving magnet type.

author: helius
author_rank: Super Contributor
timestamp: February 29, 2016, 03:42:20 am
content: A carbon microphone can have a wire inserted through the holes in the handset to make an electrical connection. This was sometimes an issue with payphones and persons trying to defraud the phone company by faking the effect of putting coins in the box.There is a short clip from the film "Wargames" that demonstrates the technique: On many payphones, in addition to making a short beep for each coin, the line is actually shorted through the metal of the coin for an instant. This deters playing back recordings of the beeps, since the expected DC pulse will not be detected at the central office. But by shorting the carbon microphone to a ground, such as the front of the coin box, it makes a similar pulse.

author: Renegade
author_rank: Regular Contributor
timestamp: February 29, 2016, 05:32:42 am
content: Quote from: SeanB on February 28, 2016, 07:47:13 pmIt's been a few years for me to deal with telephony audio, so I'm rusty. But that all makes sense.Quote from: Audioguru on February 28, 2016, 10:55:01 pmInteresting. I'll keep that in mind.Quote from: amyk on February 29, 2016, 01:15:08 amNow, I wouldn't have ever thought of that as it's out of my knowledge scope. Thanks!Quote from: helius on February 29, 2016, 03:42:20 amI'll try to make sure that whatever I make doesn't lead to a thermonuclear Armageddon.