11/02/2008

708B VHF FM backpack transceiver




7 series VHF FM backpacks were developed in late 60s to replace the heavy A series VHF FM backpacks. They are second generation VHF backpack radios developed 100% by China. They use all silicon transistors and much lighter and smaller than A series, and use much less power. However basic functionalities remain the same as A series. It has four radios, 705, 708, 709 and 714. They are basiclly the same radios but cover different frequency ranges (705 covers 36.0 – 46.1 MHz, 708 covers 28.0 – 36.6 MHz, 709 covers 21.5 – 28.5 MHz and 714 covers 20.0 – 26.0 MHz). Development started in late 60s; production started in early 70s. They were certified by military in 1973. Improved A, B and C versions were introduced later in mid 70s.



Unlike that of in USSR, the 7 series are mainly used by artillery units and infantry units who are responsible in coordinating with artillery units within PLA. 705 is for communication between artillery and infantry units, 708and 709 are for artillery commutation network of battalion and company level units and 714 is for communication between artillery and armored units.



My sample is a 708B made in 1978, which is a direct replacement for A211B. 708B covers 28.0 – 36.6 MHz in one single band. It has both voice and CW modes; however, voice is its primary mode. It has a simple telegraph key built on top of the radio. The output power is 1.5 Watt. It could use 1.5 meter fish bone whip antenna, 2.7 meters whip antenna (1.5 meter one plus 1.2 meter extension) and 40 meter directional antenna. In voice mode, with 1.5 meter antenna, the range is no less than 6 km; with 2.7 meter antenna, the range should be greter than 12 km; with 40 meter directional antenna, the range is about 15 to 25 km. In CW mode, the range could be extended around 10% accordingly.



708B weights 7 kg (without batteries and 10kg with two 12V unit batteries) with a dimension of 305x120x320 mm. Upper part is the radio body with a battery box attached to the bottom of the radio permanently. Two 12V unit batteries are used; or 20 D size batteries in place of the two unit batteries. Like Silicon 2 Watt, the radio power switch is on one side of the battery box.



Other than normal transceiver functionalities, this radio would also be used as a repeater. You need two radios to complete a repeater. These two radios connected together using a telephone line, and these two radios need to work on different frequencies, one for receiving, the other one for retransmitting. The radio also could be controlled by a field telephone up to 2 km away. These features are crucial to create an artillery wireless communication network. However, artillery wireless network is secondary to wired artillery communication network in PLA battle order before 90s.



My 708B is a NOS unit with full range of accessories. However, when I tried to test it the first time, this one was not working 100%. The receiving was fine without issue, the transmitting was an issue, it could transmit in CW mode, however, in voice mode, no voice could be transmitted. Luckily, the manual’s troubleshooting session gave me a clue. One of the transistors turned out to be bad. After it was replaced, the radio is functional 100%. Wow!