Coin Phone Tester
Often testers made for testing old telephones do not provide the line current or the voltage needed for testing 3 slot or single slot coin telephones. This one does. Line voltage is 48 Volts DC and line current, short circuit is 45 mA and with a coin telephone connected the line current is about 40 mA. Plug in 48 VDC supplies are not too easy to find, but the are around. If you cannot find one, use a 36 VAC transformer with a bridge rectifier and a 3300 MFD filter capacitor. If you cannot find a 36 VAC transformer, use 2 each 18 VAC transformers in series.
The relay is a 4PDT 24 VDC relay, two varieties are common, one coil is 650 Ohms and the other is 700 Ohms, either will work in this circuit. The relay contacts are not used.
One jack is for the coin telephone, the other jack is for any telephone to listen and talk to the coin telephone you are testing. I would suggest a cheap phone with a built in speaker phone. That way you leave the speaker phone on and listen for noise in the coin telephone or make adjustments or changes and listen for results.
Jameco is currently selling a 48 VDC Wall Wart for about $9, Part Number 215095, Brand ReliaPro, 48 VVDC 250 Ma and UL approved. If hum is heard you can add a filter capacitor rated at 2200 MFD 63 VDC across the output. A fuse rated at 1/4 Amp would be good also.
If you find a 30 VDC surplus wall transformer (typically from an old PC Printer), this can be used also, remove three of the 100 Ohm resistors in the Ring lead and change the 1K LED resistor to 220 Ohms. The 48 VDC supply is the best choice.
I used standard 24 Volt DC Relays in this design. It is not true battery feed, a double coil relay on a large frame would be required. The 24 VDC Relay works just fine and is an off the shelf item.
Theory of Operation
The Wall Wart provides a source of filtered 48 Volts DC. Depending on the power supply used, you may need to add additional filtering. Connecting a 2200 MFD or 3300 MFD capacitor rated at 63 Volts DC across the output of the wall wart will provide the filtering needed. Observe polarity of the capacitor and the + of the capacitor must be connected to the + of the power supply.
The LED and 4.7K resistor is for power on indication. The long lead of the LED is the + side of the LED.
Battery feed is by a relay and 4 each 100 Ohm resistors. The resistance of this combination provides current limiting to about 45 MA. There is some inductance in the relay to provide additional "resistance" to the voice signal. While this is an unbalanced line, battery feed resistance is only in one lead, it is not a problem. This is only a bench tester and not something that would feed battery over long distanced or in a length of twisted pair beyond a hundred feet or so. When you use a rotary dial, the relay will click when you dial.
Voltage drop across the 100 Ohm resistors in series is about 14 Volts or so. An LED and 1K (1,000 Ohms) resistor are an indicator that tells you the phone is off hook and current is flowing.
The two separate circuits, or battery feed lines, are connected so you can talk to each other. The 330 Ohm resistor and 2.2 MFD capacitor provide this path for voice. This is connected in such a way as to simulate several miles of cable.