US Payphones wiki by payphones.telesfor.org

COCOT phones most of the times connect the coin relay with 3 wires:

• red - green: normally open, closes when plastic flag is up (happens when a coin is deposited)
• red - white: should always measure around 1 kOhm. Connected via the spring contacts to the coil, or - when the solenoid is engaged - to a 1k resistor.

Typical faults:

• Protel: always returns, clicks twice. Check the flag contact operation, should go up with a coin and reset when coil operates (Elcotel ignores the flag input)
• Any model: collects and/or return works intermittently, relay tries to operate but too vague. Check red-white wires, if shows about 2k (double the normal) - clean the contacts or adjust the spring. It's the one between where red and white wires are soldered.
• doesn't operate: red-white measures open. Check the coil resistance (by white wire and the opposite side, where the resistor is soldered). If OK, check / adjust contacts, otherwise coil wire is damaged, need a new unit.

Western phones use two-wire connection. Circuit is open when flag is down (no coins deposited). Otherwise the same - 1 kOhm either via the coil or the resistor, once the coil is operating.

Faults outside the coin relay:

• Protel: check the NiCad battery (4.8V)
• Elcotel: check the line voltage on-hook: min. 44V, check the current off-hook with the payphone: min. 20mA. Relay voltage is taken from the line and multiplied 2x.

• 2.1 Elcotel 5 - low frequency hum when off-hook, except when voice announcements are delivered. Happens especially with boards installed in the heat (Arizona). Cause - as expected, some electrolytic capacitors went bad. It's tricky though, I replaced all of them and they all measured correct capacitance. Must be ESR issue. Because I was replacing them in small groups, I can't tell clearly which one was bad. Hum was gone after replacing 3x 220uF/25V (C22, C112, C61) and 10uF/100V (C44). See the layout.