|Noise Canceling mikes on back and end
Volume Control Ear Bud (VCEB)
note: the H-250 needs to be clipped in a location relativity
close the the ear where the ear bud will be used since the
ear bud cord is not that long.
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 diaphragm 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 diaphragm 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
This is a photo of 5 Handsets, not all of which are form fit and function the same as the H-250.
This is important since the non H-250 units will not work on radios that expect the H-250.
So, be sure to read the marking and be sure the handset you are using is really one of the H-250 type if that's what your radio needs.
Left to Right
H-350/U Electro-Voice Inc.
H-207 A/VRC Sonetronics
H-189/GR (broken clip)
98230, ON273827, Sonetronics (See: KY-68 secure telephone)
H-350/U Electro-Voice Inc.
This is a U-229 Audio connection.
- 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
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.
This is a version of the H-250 that adds a volume control and 2.5mm jack for an earpiece, like the one used with the Thales Speaker Mike.
The volume control effects both the handset and ear bud.
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.
Sonetronics - H-250 HandsetBack to Brooke's Photo of Military Audio Accessories, U-229 Audio Accessories, Military Audio, PRC68.COM, Squad Radio, Military Information, Home
Page created 27 March 2001.