Help identify this mystery component!

The vintage telephone speaker I used for my Bluetooth Handset project has a mysterious component wired across it’s terminals, as shown above (it’s the black cylinder with two leads on the upper left).  The component measures as an open circuit on my DMM, but obviously it has some hidden function.

So far my guesses are:

  • A lightning arrestor/spark gap or back to back diodes that protect the person using the phone from strikes or other high voltage on the line
  • Some sort of lowpass filter to keep you from hearing otherwise audible tones used for signaling
  • A device that keeps the high voltage ring signal from damaging the speaker

I found a schematic at the Bell System Memorial that shows a symbol for the device, but I don’t recognize it.

Does anyone know what this thing is and what it’s for?

We’re also having a discussion about it on flickr.

9 thoughts on “Help identify this mystery component!”

  1. It is, as you guessed:
    A back to back diodes that protect the person using the phone from strikes or other high voltage on the line.

  2. In vintage telephone sets in Germany i found a similar thing which is named “ear protection rectifier”, consisting of two antiparallel normal si diodes.

  3. I agree – probably a two anode zener. What this does is to restrict any voltage to 0.6V + the voltage of the individual Zener. And it is non-polarized.

    Also common on phones is a Voltage Dependent Resistor. Not sure how they worked, but with pulse dialing they were used to stop other phones in the house ringing.

  4. Diac or two-anode zener is used to protect speaker from the ring voltage in case you pick up the receiver when phone is ringing.

  5. An axial capacitor would measure as an open here….Does your DMM have a capacitance meter? I think they use to put 10pF caps across the leads for, as you stated, a filter.

    Other than filtering signaling out, it could also be used to keep 60hz buzz under control too.

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