Using military handset with the IC-705
Home-made mic-amp for dynamic microphone
When the cicadas sing loudly during the steamy hot summer season in Japan, the noise forces one to tightly push the speaker-microphone onto ear to listen when operating outdoors. And, when there are other people around even in a quiet place, one will try turning down the sound volume and listen to the speaker held close to ear.
Most amateur radios do not require speaker-microphones, namely a “handset”. However, most common radios for the military use a handset type device, and “H-250 handset” has been used by the US military since the Vietnam war, it is to say that the H-250 has a highly approved performance quality. I therefore have a few of them in my collection since I appreciate its high reliability and simple construction.
This article is written about my investigation of the internal structure of the H-250 handset that I studied by disassembling one that is in my collection, as well to refer to a home-made a conversion cable for connecting to an amateur radio.
Required functions that written on specifications of the H-250
The H-250 has been produced by various manufacturers, and delivered to the US military, so the products must be produced according to required specifications and pass a functional inspection.
Part of a document published online that shows the H-250 handset specifications required by the US Department of Defense (published in 1977)
Required performance specifications of the H-250 (excerpt)
Calculated microphone frequency characteristic
Vibration and drop test to confirm performance quality specifications, including electric characteristics, are mandatory.
Calculating physical strength is also mandatory.
Other than that, a heat-resisting test, as well as a spray test to confirm durability are also required.
Moreover, ecologic recyclable materials that are environmentally friendly are also required.
Environmentally friendly and recyclable materials are required.
Disassembling the H-250
The whole device
Except for the PTT switch, the only way to disassemble the whole device is to break them by force, so I used a saw to cut all components apart.
The H-250 handset is constructed as a main body, speaker, microphone (dynamic microphone) and PTT switch. The speaker and microphone are water-resistant, and the PTT switch is also covered by a rubber gasket, so it is also waterproof.
The PTT switch is secured by screws, the cover for speaker and microphone are attached on the main device body so it is inseparable. It seems that if any damage or malfunction occurred on the speaker or the microphone, there is no way to repair them but replace with a brand-new handset. However, the whole device is apparently glued together by a kind of super adhesive, and so it does not look fragile nor easy to break.
Exploded drawing of an H-250.
A simple structured hook attached at the back of the device, to use to hang on a carrying handle or on a backpack.
A hook is on the back of the headset.
Furthermore, the hook can hang on the strap of helmet and places the speaker close to ear to use as a headset.
Soldier wearing a Fritz helmet with an attached H-250 handset ©PH1(SW) J. ELLIOTT, USN
The insertion requires to be deeply stroked in or transmitting will not function. Once pressed PTT switch, MIC line connects radio, and the PTT line drops to GND then switches the device to the transmit mode.
Inside the PTT switch. Left: when the PTT switch is released. Right: when pressed down, the MIC line is activated to transmit signals.
What interesting is that the switch uses a magnet instead of a spring? When the switch is released, the magnet pulls itself to the metal frame to turn the switch back to its original position. This design enables the switch to achieve a durability of over 2 million actuations, unlike a normal spring whose tension fatigue occurs sooner.
The action of a PTT switch not using a spring. The magnet pulls the metal contact to return the switch back to its original position.
The switch is a bipolar single-throw switch which the result of PTT and MIC lines worked together. When the switch is pressed, MIC line connects radio first, then the PTT line drops on GND. The slightly different timing in action is caused by the different sizes of the pair of prominences at the connecting point.
Closeup of the bipolar single-throw PTT switch. Different sizes of the two points makes the action timing slightly different.
A 4.5cm in diameter, 1cm in thickness, small speaker is used. However its impedance is pretty high at 1000Ω. The cone paper is sealed by a water-resistant resin.
A 2.7cm in diameter, 1.3 cm in thickness, dynamic microphone is used. The impedance is low at 150Ω. It is sealed by a transparent film.
Another interesting feature that the H-250 has is that it can reduce surrounding noise to a certain level, the function of a noise canceller.
The speaking side of the microphone (left) and its back phase (right). Both phases with sound receiving holes.
Both, the front, and the back of the microphone have sound input holes. Since the position of the front and the back are opposite, noise is input into both sides, which will cancel each other. The audio will not be cancelled since it is only input into the speaking side of the microphone. Theoretically, in an environment with a lot of noise that is input into both sides of the microphone at the same time, the noise will be cancelled. However, when voice audio is only input into front side of the microphone, it will not be cancelled, and it will also be easy to be picked up.
An example of noise cancellation.
The U-229 is the type of connector used for the H-250. It has been used by the US military for a long time. This connector is very easy to connect and disconnect, even operating with thick gloves on.
The U-229 connector. It is bigger and heavier than the metal connector (front) that is used in amateur radios.
Pin array. Pins are marked “A～E “.
Pin array and wiring diagram.
Socket and connector of the U-229 (front). To lock, twist right while pressing down.
The cord of the H-250 also wider than cords used on amateur radios. It seems to be heavily shielded to prevent RF interference. The outside cover is very thick and has a high tolerance against temperature changes and chemical substances. The H-250 that I disassembled today is one of 13 in my collection. I will not run out the study samples, even if this one gets damaged.
Comparing to the curled cord of a speaker microphone a Ham radio (front), the thicker H-250 cord (back).
Old version handset (H-189)
The H-189 is the previous version of the H-250. It is heavier than the H-250, and the noise cancellation function is not included. The hook on the back of the handset is rather short and does not seem practical in usage. The whole device has a round shape with a retro feel. Wiring and connecting are compatible with the H-250. Apparently, the H-189 and the H-250 were used together during the Vietnam war.
The H-189 handset. It has a retro feel.
New version of (H-250 VCEB: Volume Control Ear Bud)
The new version of the H-250 has a volume control knob and an earphone jack to prevent sound missing, it will not mute the audio, even if you turn the volume control all the way down. The earphone is corresponding with a monaural jack and accepts an ear-hook type earphone, a supplied accessory.
“H-250 VCEB” has a volume control and an ear-phone jack. The earphone construction turned out to be unexpectedly poor in quality.
Using the H-250 on the IC-705
I was curious to use the H-250 on an amateur radio, so I home-made a conversion cable to connect it to the IC-705. The reason that I choose the IC-705 is because the IC-705 is good for outdoor operation which is also suitable for the H-250.
The IC-705 I possessed is attached with a carrying handle, it is perfect for equipping with the H-250.
The U-229 connector that is used on the H-250 is too big and heavy, so I changed to an 8-pin metal connector instead. On top of that, to connect with the IC-705 I used an 8-pin relay connector with 2.5mm 4-pole plug and 3.5mm stereo plug.
Left: home-made conversion cable using an 8-pin metal connector Right: switching the H-250’s original connector to an 8-pin metal connector
Unlike the condenser microphone used with the IC-705, the H-250 handset uses a dynamic microphone. Because a dynamic microphone’s output level is lower than a condenser microphone, the modulation is also weaker, so I made a built-in mic-amp inside the conversion cable that is similar to the old version circuit of the IC-HM7.
I made a built-in mic-amp circuit similar to the HM-7.
Mic-amp circuit (slightly changed from the IC-HM7)
For the transistor, I used what I had on hand which is a common 2SC2712 amplifier. I also used chip components to make a built-in circuit inside the 8-pin metal relay connector. It seemed to be fine to use a plastic material case to install the circuit.
The wiring diagram of a 2.5mm 4-pole plug, a 3.5mm stereo plug and an 8-pin metal connector.
Chip components on a universal circuit board
Using chip parts to tightly assemble the circuit and store into 8-pin relay connector.
Mic-amp circuit installed into a male metal connector
Connected to the IC-705
Impressions after use
I have used the H-250 while operating the IC-705 for quite a few times. Once, I switched from the HM-243 to the H-250 during a QSO and asked the other station if any audio quality changed. I was told that the audio through the H-250 was softer.
H-250 was designed in the beginning particularly to use in noisy environments like a war zone with engine and gun-shot noises around. Also, when switching between transmit and receive, you save the hassle of switching between speaking and listening, back-and-forward with the speaker microphone. However, the PTT switch is a bit stiff to press, compared the ones that are used on amateur radios. This can cause a tired thumb when pressing for a long period of time.
Now, you may find some military surplus H-250s in the flea markets at some amateur radio events, or on an Internet auction website. Depending on the condition, the selling price is set between Yen 3,000 to 5,000 if you are interested in trying it out.