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Old 1st Nov 2017, 4:19 pm   #6
russell_w_b
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Location: Penrith, Cumbria, UK.
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Default Re: Telephone cradle switch(es)

Quote:
Originally Posted by Sparks View Post
Thank you Russell and Tony.
You're welcome. Your question got me thinking and I looked into a tele 706 with the regulator strapped out as was done on long lines.

The equivalent d.c. resistance shunted across the capacitor via the second gravity switch, the mic, the receiver, the bridge resistor and the ASTIC is typically 830 ohms, although this may go higher or lower depending on the state and position of the carbon mic. As this is in series with the bell-bobbins there would be a d.c. resistance across the line of 1830 Ohms, which would sink 27mA and probably not seize the line. The continuous passage of this current through the carbon mic would eventually 'fry' it. So this second switch contact is also an indirect break to keep d.c. out of the speech circuit.

If the mic resistance dropped to, say, 230 Ohms, then it might just seize the line.

I'm sure I've seen some continental instruments where the gravity switches are more simply arranged though.
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