Local Battery Conversion
Converting a Local Battery Set to Common Battery
Local battery telephone sets are sets like a wooden wall telephone with a magneto that uses dry cell batteries. Some sets were made in metal and one set was made like a 500 type telephone but with a crank. There are hundreds of designs and thousands of configurations.
The dry cells in these sets, 2 or 3 cells, provided the talk power. The signaling to the Central Office was by turning the crank on the magneto. The magneto generated about 20 Hz AC, depending on how fast you cranked it, the other phones on the line would ring and the Operator would be notified you wanted to place a call.
Often these sets are in good condition, and if they are totally working you can easily convert them to work on a Common Battery line (thatís a normal phone line like you have at home now).
Some people rip out the insides and start over, some people leave the old parts in place and add new parts. This design lets you leave the inside parts in place and build an external circuit that will simulate the battery and use the old circuit and still let you use it on your home line, as well as provide total isolation.
Old phones had one side of the telephone line connected to the magneto and sometimes to the metal case of the transmitter. Also the bell or transmitter wiring may have been via the hinges, another point of contact with the telephone line. All of this could lead to problems as you can get a nasty shock if there is a ground connection to your body when you use the telephone. Unlikely you would be standing in a puddle of water when you used the phone, but why take chances?
The one disadvantage of this circuit is that an incoming call will NOT ring the old telephone set. See the lower part this page for that. This will require a slight modification to the telephone.
Dry Cells like the ones used in these telephones are quite expensive so we will substitute a small 9 Volt DC wall wart. This will provide power to the telephone as well as allow the use of a relay to determine when you go off hook and want to answer the phone. Sorry but without a dial there is no way to call out.
It is possible to add a rotary or touch tone dial, mount it in the battery compartment but the whole idea of this conversion is to make the set work like it did, local battery.
The Relay is any 5 or 6 volt relay with a resistance of around 200 ohms. This will allow about 30 to 40 Milliamps of current to flow in the battery circuit.
Filtering on the wall wart will be an additional 2200 MFD capacitor and this should be sufficient, do not use a switching type of supply.
A 2.2 MFD capacitor is connected across the battery leads in the phone, this make the phone think it is using dry cell batteries.
The relay contact acts like a hook switch contact and connects the repeat coil to the common battery line.
Two 100 Ohm resistors and a 2.2 MFD capacitor make the repeat coil look like a telephone set to the phone company (on your home line). If the volume is too loud on the phone, remove the 2.2 MFD capacitor across the resistors.
The repeat coil is a miniature one. Click here for information. It will provide total isolation, so you can stand in the puddle of water when you use the phone (just kidding!).
You will need to test your phone before you use this circuit. Make sure it works properly, use 2 flashlight batteries in series, for 3 Volts for the #6 dry cells and place a rotary dial phone across the line connections at the wooden wall set. You should be able to talk and listen on the wooden wall set, when you are off hook on the rotary phone (either an AE 80 or WE 500). On the rotary phone your voice should sound perfect. After this test, then you can build the circuit and all should work perfectly.
Very old receivers on the wooden wall sets may have lost their magnetism. If the sound is not good, the solution is to replace the complete receiver or remove the old magnet and coil and substitute any modern receiver element that will fit.
If you talking is weak or has lots of static, cleaning the carbon grains in the transmitter is a difficult task and I would not suggest it. You can remove the old transmitter and substitute a WE T1 element. Mounting this requires some careful thinking, but stuffing the transmitter cup with paper towels will hold it in place, You can also cut out a small section of the plastic cover on the T1 to get more sound into it from transmitter cup or horn.
Of course clean all the contacts on the hook switch and so on.
Here is the circuit, have fun: (Remember this works almost all the time, well 98% or so)
Making your Phone Ring
This will require a slight modification, the ringer wiring will need to be changed and a relay added to connect the ringer to the magneto when you turn the crank. There is a wide variation in ringers in old phones, some were of the low resistance variety, they may not work well on a standard telephone line. See my book on testing for ringer equivalents here. DO NOT use a ringer capacitor except for the 0.47 MFD shown in the drawing, or you may suck out the available ringing power in your phone line. Adjust the ringer mechanically to get the best sound.
Wires A&B are the wires that originally ran to the bell. Wires C&D are connections on the magneto that have power (ringing) when the magneto is cranked. Wires E&F are the terminals at the bell where you removed wires A&B from. Wires G&H go directly to the common battery telephone line, not the connections to the circuit in the above drawing. The capacitor connecting the bridge rectifier/relay to the magneto may not be necessary, try it without the capacitor first.
An alternative method (actually about the same but much more nicely illustrated) is at http://atcaonline.com/ringercontrol.html. Also Stan has a controller with the same functionally but done slightly different (also better illustrated) at http://atcaonline.com/controller.html. There is some great history stuff about payphones on that site also. Why not join where you are visiting the site? In my design I suggest an 0.4 MFD capacitor to keep the ringer load low.
This document will be revised with a complete book available on here .... check back