|Noise Canceling mikes on back and end
This dynamic noise canceling mike type handset is the standard for many PRC-nnn series radios. The noise canceling is accomplished by two design features:
- The diaphram is stiffer than a normal microphone like the H-189 (non noise canceling) (taping the holes on the back will not make much improvement),
- There are holes behind the diaphram so sound arrives from both sides.
IMPORTANT - Many people don't realize how good the noise canceling feature is. They hold the mike 1/2 to 1 1/2" from their mouth, then complain that the handset has low output or that the radio has weak modulation. That's because of the noise canceling.
YOU MUST HAVE THE MIKE TOUCHING YOUR LIP!
This handset was designed for use under combat conditions which includes explosions, heavy trucks or tanks driving nearby, etc.
Label on Handset:Handset H-250/U
- A (gnd) to B (ear) = 1,000 Ohms
- 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
SpeakerUsing 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.
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
I have heard that there's a magnet inside the switch proper.
Photo shows the switch separated from the handset.
The H-138 was issued with the early PRC-25 radios. It was the first of the noise cancelling microphones, but some voice cancelling was experienced. It would be interesting to know if that was real or just the result of the operator's not being trained to use the handset with their lips touching the mike? It was replaced with the H-189 that is noise canceling and repairable. The H-250 has the mike element and speaker sealed so they are not replaceable.
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Page created 27 March 2001.