Pay Phone Handset

[Renegade]

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: [Select]

red        red
white     white
Is it safe to experiment, or should I buy a couple more?

[roffvald]

I'd say one red/white pair for speaker and one for mic, measure the resistance, the higher one should be the mic.

[Renegade]

I'd say one red/white pair for speaker and one for mic, measure the resistance, the higher one should be the mic.

Erm...

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.

[SkyMaster]

You can unscrew the microphone and speaker covers to figure out where the wires are going.

[Renegade]

You can unscrew the microphone and speaker covers to figure out where the wires are going.

Unfortunately, that would require a hammer. I tried. I think they're glued in place.

[MSO]

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.

[edpalmer42]

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.

[Renegade]

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.

Thanks for that. It will help.

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.

Banging 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.

[SkyMaster]

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 microphone


Just like MSO phone

[amyk]

Listen as you apply the ohmmeter. The current might be enough to cause some audible clicking of the speaker.

[Audioguru]

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.

[MSO]

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.

You'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.jpg

It'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.

[Renegade]

Listen as you apply the ohmmeter. The current might be enough to cause some audible clicking of the speaker.

HA! BINGO!

White is the mic, and red is the speaker. Both click.

Thanks for that!

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.

I 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.

[SeanB]

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.

[Audioguru]

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 is really bad).

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.

[amyk]

White is the mic, and red is the speaker. Both click.

That suggests the mic is not carbon, but moving-coil or moving magnet type.

[helius]

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.

[Renegade]

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.

It's been a few years for me to deal with telephony audio, so I'm rusty. But that all makes sense.

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 is really bad).

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.

Interesting. I'll keep that in mind.

White is the mic, and red is the speaker. Both click.

That suggests the mic is not carbon, but moving-coil or moving magnet type.

Now, I wouldn't have ever thought of that as it's out of my knowledge scope. Thanks!

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.

I'll try to make sure that whatever I make doesn't lead to a thermonuclear Armageddon.