Re: Payphone 500 / 600 handsets
Despite drilling out the rivet, the receiver does not want to open. There's definately something in there other than a standard 4T. I suspect possibly an amplifier and coil for hearing impaired users.
It clicks quite loudly with one polarity from a PP3, but only faintly with reverse polarity, and gives extremely (unusably) low volume on the field telephone circuit (which works nicely on standard handsets).
The circuit also, incidentally, gives quite astonishing volume into a 330 ohm (30 watt 100 volt line) PA horn speaker.
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