Sunday, September 29, 2013
Ringing an old wooden wall telephone
Today I took some time to give attention to an old wooden wall telephone that my father has been tinkering with. The goal is not so much to restore it to original shape, as it is to have a functional conversation piece on the wall of the garage shop. Actually, the real goal is to have fun playing with this stuff, but the cool old phone on the wall that actually works is fun too.
From what I can tell, the ringing signal is about 20Hz with an amplitude of roughly 50VRMS, after looking at the output of out OOMA box with the oscilloscope. So, I connected the old HP 201 signal generator to the bell (with it's 0.3uF capacitor in series) and started adding different springs, weight on the clapper arm, and changing the clapper position on the arm until I got the resonance into the 20Hz range. It seems that it will ring with +- 1 Hz excitation of so, so the Q of this pendulum resonator is about (20Hz/2Hz)=10. It was satisfying to actually hook it to the ooma box and call myself and actually have it ring, after a few attempts that did not ring.