# Landline telephone in seriel

#### christiannielsen

Joined Jun 30, 2019
221
Hi,

I have connected two old land line telephones in seriel with a 9 volt battery.
When I lift both handset a line opens between the two phones. But in only the one of them am I able to hear the other person in the other end.

I dont have another telephone to switch with, and I think I must try to check wether its the microphone in the handset or a connection in the telephone.

But does anyone know about this issue? Is it suppose to be that way? or is it more likely a faulty telephone or handset?

#### Dodgydave

Joined Jun 22, 2012
9,209
Swap microphones or handsets to prove it..

#### Externet

Joined Nov 29, 2005
1,520
Landline telephones can yes, work that way. The 48VDC is not for the ringer.
But try more than 9 V in series. Perhaps two 9V in series so both telephones get more than 4.5V each. Or 12V. And yes, swapping handsets as told can diagnose a bad microphone.

(-)12V(+)-----------------------------red--phone1--green-----------------------------red--phone2--green------------------------------(-)

#### marcf

Joined Dec 29, 2014
259
Hello

Actually phones connected in the POTS (Plane old Telephone System) are connected in parallel with the battery source from the CO (Central Office)

The CO voltage is typically 48V. (4 lead acid batteries in series). The voltage is limited to 48v as it is considered 'intrinsically safe' as you cannot be electrocuted with 48VDC.

With this in mind the maximum loop length for a POTS is 3 miles, assuming 22AWG wire with a resistance of 16.5 ohms / 1000 ft. ( 54 ohms / Km).

The DC resistance of a POTS telephone appliance is 180 OHMS.
The normal loop current in a POTS is 12ma to 80ma.

The range of voltage drop across a POTS subscriber appliance (a telephone) is 4 to 15 Volts.

So two phones connected to a 9Volt battery should be OK as 9v/150ohms = 50mA. You might consider putting some resistance between the battery and the connected phones to lower the loop current down to 15mA or so, however, as this would extend the life of the battery.

This resistance would also be handy if you wished to feed the audio into an amplifier (PA).

#### christiannielsen

Joined Jun 30, 2019
221
I will swap the handset to check and I will put in a 300 ohm resistor.

I just opened the phone I think might be the problem and saw dark areas on the pcb. Maybe you can issue something from it and tell me what I should measure to diagnose.

#### christiannielsen

Joined Jun 30, 2019
221
okay so I have checked the handset and they are both working properly.

As far as I can tell the problem is on the pcb in the picture in the section that handles the microphone. That would be the white and brown wire.

#### Externet

Joined Nov 29, 2005
1,520
...So two phones connected to a 9Volt battery should be OK as 9v/150ohms = 50mA. You might consider putting some resistance between the battery and the connected phones to lower the loop current down to 15mA or so,...
TWO telephones in series with battery are around 400 Ohms total resistance. Not "9v/150ohms = 50mA" as above.

Did you try with 12V ? That will yield a better 30mA current.
I have seen a telephone that was polarity sensitive to ring and tip. Try reversing connections.
Try to find another telephone in known good operating condition and come back with findings...

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,108
Reversing polarity is what I would suggest. Most phones are insensitive to polarity. A phone company one with touch tone would not generate tones if the polarity was wrong.

I was surprised recently when a service guy that teaches telephone repair didn't know that the wall cables should be twisted. Patch cables are not.

The older phones (hardwired and 4-prong plugs) could keep the colors straight.

Modular plugs attached to a wire, would require the wires to be reversed. e.g. pin 1 to pin 6.

It has an interesting side effect. The wire colors inside the phone don't change colors for tip and ring etc.

For the few times I wanted to service a telephone, I used a power supply and an LM317T set up in current mode (about 40 mA) and a 600:600 transformer.

Your piture is a picture of switch contacts. I had a hard time recognizing it.

#### christiannielsen

Joined Jun 30, 2019
221
Please notice that there IS a one-way communication meaning the One phone does work (I can hear the person in the other end clearly)
Does your advice still stand regarding polarity and resistor? (so should I use a 400 ohm instead of 300 ohm?)

#### christiannielsen

Joined Jun 30, 2019
221
TWO telephones in series with battery are around 400 Ohms total resistance. Not "9v/150ohms = 50mA" as above.

Did you try with 12V ? That will yield a better 30mA current.
I have seen a telephone that was polarity sensitive to ring and tip. Try reversing connections.
Try to find another telephone in known good operating condition and come back with findings...
I havent yet. Do I need to ensure/limit to 30mA with 9 volt battery? How do I do that?

#### christiannielsen

Joined Jun 30, 2019
221
Am I suppose to measure something at the big green block in the picture? dont know what it is...

Joined Feb 20, 2016
3,572
Put both phones in parallel and run the 9V via an audio choke.
The telephone exchanges are powered with 48V but it is fed to the phones via relay coils that block the audio and detect when the phone is off hook. So you can use a relay too.
The ring signal is AC that is switched onto the called line. I forget just what but 20Hz at 90V seem familiar. It is around 50 years since I was involved in telephones so I am no longer sure of the values.

Some of the solder joints look pretty poor.
And as you have 2 boards, compare the resistance readings across all the parts. And the voltages when powered.
The green things are capacitors.

Those switch fingers look to be a place that could cause problems to me.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,108
9 V/600 ohms is 15 mA

2 batteries in series might put the current where it's supposed to be. 30-40 mA. Know someone with a land line that you can try the phone on?

With the major components selected, you have to come up with values for 30 mA. You could build a box with 2 modular jacks and a DC jack.

You also don't know the minimum voltage the phone needs.

You can also use a regulated DC adapter.

#### christiannielsen

Joined Jun 30, 2019
221
I am afraid I am having a little trouble selecting what to do as there are various suggestions.

I have bought another phone and replaced the damaged one in the series which now looks like this:
(-)9V batt(+)-----------------------------red--phone1--green--------------red--phone2--green------------------(-)

No resistor.
And these two phones with this setup works just fine.

So what I understand is that I have to put in a resistor to limit the current and make the battery last longer right?
I have lots of resistors so can someone please help me with the calculation to select the right resistor? I read lots of different numbers in this thread and dont know which to go with.

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#### Externet

Joined Nov 29, 2005
1,520
Good you followed the 'schematic' and suggestion to try other telephone...

No, no added resistor needed at all.
The equivalent of your 'schematic' is
(-)9V batt(+)-----------------------------200Ohms--------------200Ohms---------------------(-)
Being the 200 Ohms the internal resistance of each telephone.
Which makes each phone work with around 4.5VDC across each. A low, but enough voltage, and a current of 22mA; low but enough current
----> 9V/400Ohm = 0.022A.
A spent battery will at a certain point stop working. A second 9V battery in series to yield 18VDC will extend working life.

Only when you go to higher series voltages like 24, 36, 48V..., then you may need an additional resistor in series to the telephones in series (not parallel) to bring the current down to typical 30-40mA. Example:
36V/1000Ohm = 36mA

"So what I understand is that I have to put in a resistor to limit the current and make the battery last longer right?
Not precisely. The resistor limits the current the battery voltage pushes in the circuit loop to a typical value suggested by telephone manufacturers of around 35mA for their typical manufactured internal resistance of around 200 Ohms.

Note nothing here is related to make ringer work.

I believe if a LED limited to a couple of mA is added in parallel to each telephone, an indication of "intended communication" would tell the operator someone lifted the handset at the far telephone. Have not tried it yet.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,108
The current regulator and increasing the voltage is probably the best way to increase the lifetime. The current regulator will drop some minimal voltage (e.g. 3V or more depending on the set current. You would need to find the lowest current that the devices operate reliably at.

An LT6700 https://www.analog.com/en/products/lt6700.html# type of part could implement a battery test feature.
You probably also could use it to monitor the line voltage and light a LED indicating a ring. Take a look at the line voltage with one or both phones off hook when using a current source.

#### Yaakov

Joined Jan 27, 2019
1,612
Telephones don't work that way. Landline telephones usually use a 48VDC circuit, in order to be able to operate the ringer, usually provided by the incumbent. You also want to learn about tip & ring and what it means.

This might help--

https://en.wikipedia.org/wiki/Tip_and_ring
The on-hook voltage from the CO is 48V but off-hook is ~9V. The ringer is 90VAC.

You can, in fact, use 9VDC to power a loop between two phones, it is a common thing to do.

#### BobaMosfet

Joined Jul 1, 2009
1,124
Landline telephones can yes, work that way. The 48VDC is not for the ringer.
But try more than 9 V in series. Perhaps two 9V in series so both telephones get more than 4.5V each. Or 12V. And yes, swapping handsets as told can diagnose a bad microphone.

(-)12V(+)-----------------------------red--phone1--green-----------------------------red--phone2--green------------------------------(-)
You can connect wires serially and communicate, but there's a bit more to it than that if you want something other than tin-can comm. Where do you think the 90VAC comes from? ILECs originally used a 48VDC driven motor to drive a magneto to superimpose a ring signal, like those produced by Holtzer-Cabot. That's how you make a dry-line (POTS in this case) ring.

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