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Originally Posted by russell b w
I have a circuit for one of these boxes. The coin-and-fee checking equipment consisted of an internal microphone which picked up the sound of the inserted coin 'dinging' a bell, and so the call would be allowed to go through.
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There was no coin and fee checking (C&FC) equipment associated with an AB box. The coins sat in a scale pan which required the correct number of pennies to be inserted before the dial was unlocked to allow dialling of all digits.
The microphone in the coin shute was for use on calls via the operator. The operator would say "Please insert x amount" and then listen to the tings and clanks as coins were inserted. There were different gongs for each coin slot and value of coin.
Most pre STD exchanges didn't have local call timers (LCTs) either, so a local call could go on forever without additional charge. Calls via the operator were charged by time and distance and when your credit was about to run out the operator would come on the line and ask you to insert more money.