H-250/U Handset & VCEB

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Ohmmeter Readings

  • A (gnd) to B (ear) = 1,000 Ohms (can be higher on the VCEB version)
  • A (gnd) to C (PTT) = normally open, 6 Ohms PTT pressed
  • A (gnd) to D (mike) = normally open, 150 Ohms PTT pressed
  • A (gnd) to E (External) = always open

Anti- Anti-eavesdropping

If an enemy modified a radio or telephone that used an handset by tapping into pins A (gnd) and D (mike) they could listen in to conversations near the handset even if the radio was off or the handset was on hook. To prevent this the H-250 mike wire goes through the PTT switch and so the mike is only active when PTT is pressed, preventing eavesdropping.

Speaker

Using an HP 204 Audio Oscillator and the TS-585 Audio Level Meter the AC impedance of the H-250 can be seen to be about 1,500 Ohms. It is common for the AC impedance to be higher than the DC resistance because of the inductance of the coil. Some of the early headphones used with crystal radios had an AC impedance up to 10,000 Ohms but had much lower DC resistance. A 0 dBm tone at 1 kHz into the earpiece is loud when the ear piece is held to the ear and can still be heard with the H-250 on the bench. A 4 kHz tone is loud with the H-250 on the bench.

Mike

Agilent 4395A spectrum analyzer 1 Hz to 5 kHz, PTT pressed, whistle into microphone. On the most sensitive range (-10 dB) of the TS-585C/U there is no meter movement with a loud whistle. A Fluke 87 DMM set to ACV will show about 1.2 mV with PTT pressed in a quiet environment. With a loud whistle the reading might get to 2.5 mVAC.

This mike will not drive a PC audio line level input. It needs amplification first.

Push To Talk Switch

H-250 PTT Switch

I have heard that there's a magnet inside the switch proper. Photo shows the switch separated from the handset.

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