SE-408: FIX

Radio Station FIX, SE-408, original US designation SCR-508; manufactured by div. U.S. manufacturers.

To renew the equipment of the Swiss signal troops after the ye ars of World War II, in 1946/49 a large amount of signal equipment was purchased from American surplus stocks.

The U.S. station SCR-608 was acquired in this context; with this set, the VHF range was for the first time in Switzerland used for military communication. At the same moment, frequency modulation was also used for the first time in a military radio set. The device got the designation „FIX“ resp. after the introduction of the new naming system the designation SE-400.

In 1951, the Swiss Army acquired a series of franch AMX tanks, the radio set SE-508 from french licence production was already built in. It is technically almost identical however, the frequency coverage is significantly lower. In the SE-408 the upper shortwave range from 20 - 27.9 MHz is covered.

Specifications

Principle: radio station consisting of one Transmitter and two Receivers
- Transmitter: BC-604 ; Frequency Range: 20 - 27.9 MHz, 10 preset channels with 100 kHz channel spacing (channel numbers 200 - 279).
- Receiver: two BC-603 ; Frequency range: 20 - 27.9 MHz, 10 preset channels per receiver, which can be tunes continuously, Single conversion, IF 2650 kHz
Modes: FM (F3)
Frequency Range: 20 - 27.9 MHz
Sensitivity: FM (F3) / Selectivity: 1 μV
Output power: FM (F3) 30 Watts

Power Supply

Accumulator / Batteries: from vehicle battery 12 V, the plate voltages are generated by means of a dynamotor (for 12 V operation, in the receiver a DM-34 and in the transmitter a DM-35 are used).
Generator: a generator „Homelite“ with a two-stroke engine (operated from a petrol / oil mixture) can be used to recharge the backup batteries.

Dimensions

Transmitter BC-604: 457 x 292 x 230 mm; 30.4 kg
Two receivers BC-603: 171 x 292 x 287 mm; 31.75 kg
Mounting frame FT-237: 908 x 387 mm; 20 kg; total weight of the complete station 82 kg

Accessories

The standard vehicle antenna is a three - element 2.7 m whip antenna. (Mast base MP-48-A, mast segments MS-51, MS-52 and MS-53).
The transmission crystals are stored in the crystal box BX-40
Remote operation is possible with the remote control equipment RM-29-D.
Measurements on the set for service purposes are possible with the adapter FT-384.

Operation

The radio station SE-408 or the corresponding original U.S. variant SCR-508 consists of two receivers BC-603 and a transmitter BC-604 mounted on a mounting frame FT-237. By default, the device is operated with 12V DC from the vehicle battery, a dynamotor generates the necessary plate voltages.

The device covers 10 crystal controlled channels in the range of 20 - 27.9 MHz. In the SCR-608 variant set, the frequency tripler stage was wired before the frequency doubler stage, so the twenty channels came in the 27 - 38.9 MHz frequency range; in Switzerland, this set got the designation SE-400.

The receiver has a large built-in switchable speaker, the VOLUME knob which is located almost in the middle of the front panel. The ten channel buttons on the top right are used to select the operating frequencies. By pressing the small PUSH to TUNE button on the left of the dial window, the receiver can also be continuously tuned using the knurled wheel below the station buttons.

In a bottom row of controls, the (auxiliary) antenna and ground connectors, the switch to tune and operate the set, the switch of the onboard intercom in a tank and the squelch control are located; the squelch threshold is set with the SENSITIVITY control.

On the Transmitter front panel, the controls are located on the right in a frame-protected area. On its left side, the ten fixed frequency buttons are located below the connectors for the (auxiliary) antenna and ground. The quartz crystals are inserted in the crystal holders located behind the cover on the left. The switch below the channel buttons switches from tune to operation.

At the right below the instrument, which can be set to tune and display the antenna current, the main switch and the switch for transceive - onboard intercom operation are located.

The transmitter is equipped with seven valves 1619 and one 1624 as a output valve. Some spare tubes are housed inside the cabinet.

Technical Principle

Receiver BC-603: The RF signal from the antenna is amplified (V1, 6AC7) and mixed with the signal of the RF oscillator (V3, 6J5) in the mixer stage (V2, 6AC7) to generate the intermediate frequency of 2.65 MHz. The IF is amplified in two IF amplifier stages (V4 and V5, two 12SG7) and has to pass a limiter stage (V6, 6AC7). The demodulation is done in the discriminator stage (V7, 6H6), the AF signal is amplified in the AF preamp valve (V10, 6SL7GT), which also acts as IF oscillator) and the AF output stage (V8, 6V6GT). Tube V9 (6SL7GT) acts as AVC and squelch.

Transmitter BC-604: With the pushbutton unit, the corresponding transmitter crystal (operation frequency is 54 times the crystal frequency) is connected to the transmitting oscillator (V107, 1619). After an RF amplifier stage (V101, 1619), the RF signal is amplified along with the audio signal, coming from the microphone and amplified in two AF amplifier stages (V105, V106, two 1619), and then fed to the modulator coil. The signal rich of harmonics is rectified (V102, 1619) and its twelfth harmonic is filtered out. After a frequency doubler stage (V108, 1619) followed by a frequency tripler stage (V103, 1619), the transmitter output stage tube (V104, 1624) is driven. The operations frequency corresponds to 54 times the transmitter crystal frequency and the output power reaches 30 watts. The signal is fed to the antenna output socket over one of ten pretuned antenna circuits selected by the pushbutton unit.

Tube Layout

Receiver BC-683-A: V1 (6AC7, RF amplifier); V2 (6AC7, (mixer); V3 (6J5, RF oscillator); V4 and V5 (12SG7, first and second IF amplifiers); V6 (6AC7, limiter); V7 (6H6, discriminator); V8 (6V6GT, AF final stage); V9 (6SL7GT, AVC, Squelch); V10 (6SL7GT, AF pre amplifier and IF oscillator); Dynamotor DM-34.

Transmitter BC-684-A: V101 (1619, 1st RF amplifier stage); V102 (1619, rectifier); V103 (1619, frequency tripler stage); V104 (1624, transmitter final amplifier); V105 (1619, 1st AF amplifier stage); V106 (1619, 2nd AF amplifier stage); V107 (1619, transmitter oscillator); V108 (1619, frequency doubler stage); Dynamotor DM-35.

Development

In a joint development project, the Link Radio Corporation, supported by Western Electric and Bell Laboratories, developed a frequency-modulated VHF transceiver which was ready for production in 1940/41. Due to the use of frequency modulation, the set was less prone to sparks and soon became the standard vehicle set of the American armored corps. With a modified frequency range, the device was adapted to the needs of the artillery in 1943/44.

After the end of the Second World War, many of these sets were for sale in the surplus market. After successful trials, the Swiss Army acquired a number of the American VHF radio, which gave the impetus for the shift of the tactical military communication into the VHF frequency band.

The parallel development SCR-608 working on the slightly lower frequency band was also introduced as SE-400.

Field Use

In 1951, Switzerland bought French AMX 13 tanks, these were introduced as „Light Tank 51“ in the Swiss army. This tank was factory-equipped with SCR-508 built unter French license. For this reason, the radio stations with the designation SE-408 have French front plate lettering.

In the late 1950s, the AMX tanks were modified and got an upgraded tank gun. As a result of this upgrade, there was no more space for the SE-408 radio in its original location and the tank was converted to a remote-controlled Autophon SE-407 radio.

The devices found another use with the border protection brigades until 1975 and were liquidated in 1976.