VHF Receiver E-628
VHF Receiver E-628, E-628; developed and produced by Autophon AG, Solothurn.
In parallel to their shortwave receiver E-627, Autophon did develop the VHF-receiver E-628 with coverage of the VHF Air band in AM in the middle of the fifties. This receiver was based on a project of a receiver similar to the E44.
This receiver did not only exist in it's military version as E-628, but was sold to civilian users too: The E-77 was used as a surveillance receiver by the Swiss PTT and exported to Finland and South America for Air traffic surveillance.
- Frequency range: 20 - 180 MHz in six ranges (20 - 29 MHz / 29 - 42 MHz / 42 - 61 MHz / 61 - 85 MHz / 85 - 122 MHz / 122 - 180 MHz), direct readout analog dial
- 510 x 275 x 260 mm, 20 kg
- Standard antenna was a special VHF antenna made by Autophon
The receiver comes in a army olive green - gray metal cabinet, a protective front cover will cover up the whole front panel during transports and can be attached to the back, when the receiver is in operation.
The front panel's width is 44 cm, with both carrying handles at it's sides, the complete receiver has a width of 51cm, height of 27cm, depth of 26cm and a weight of 20 kg. There are two more carrying handles mounted to the front panel giving additional protection.
It's worth having a look in the small compartment at the rear of the set: usually some headphones and the mains cable are stored there, You might be lucky and find some spare tubes still in their boxes with military seals.
The left side of the front panel can be divided in three sections: on the top the antenna / earth connectors and the signal strength meter, in the middle the dial, the bandswitch and the main tuning knob and a bottom row with all other necessary controls.
On the right hand, You find the speaker protected by a metal speaker grille, the mains voltage selector and the mains cable connector. A separate connector allows using the Z627/1 vibrator pack 6/12V DV power supply.
The antenna connector is of SO-239 type. Next to the antenna and earth connectors and some fuses, You find a round signal stregth meter switchable to display the plate / B+ voltage. A round knob allows to dim the dial lights.
Below, You find the dial of the turret tuning assembly. Due to the construction of the set, the analog dial is not linear, so on higher frequencies, the lines are much nearer then in the low band segments - thus dial accuracy is better low frequencies. Dial calibration has been made at the manufacturer using a crystal controlled calibrator providing a spectrum of known frequencies, and individual engraving of the frequency marks on each set's dial. The big rotary control at the left acts as bandswitch and operates the turret tuner, the right knob is the main tuning control, that can be locked mechanically on a tuned frequency.
The controls below are used for RF and AF signal processing: The switch „Anti - Fading“ activates the automatic gain control (AGC), the RF-Gain control (called „Empfindlichkeit / Sensibilité“) should be turned fully clockwise for standard operation with AGC on. The following knob „Laustärke/ Puissance“ (in german/french language) is the volume control. The next control will switch the receiver's FM deemphasis from none / 75 / 280 usec depending whether You listen to a commercial FM broadcast or a narrow band utility signal. The Squelch with adjustable level is active only on FM, the next control will switch the bandwidth from 8 kHz (AM mode) to 25 (FM narrow) and 75 kHz (acceptable for pleasent FM broadcasting listening, well suited for FM DXing). The AM / FM mode switch is situated a little bit above left to the station's clock.
The military sets have been equipped with a seven day movement Revue mechanical station clock. Quite often, only three little holes below the speaker grille witness that the clock has found a new owner. Caution: the clock fingers are painted with Radium containing flourescent colour, this can cause trouble when a set is sent abroad and the clock indicates radiation on a Geiger counter.
For FM broadcast reception, the modes switch should be set to FM, the AVC / „Antifading“ should be activated and the Sensitivity control should be turned fully clockwise on the „10“. Now look for Your favourite FM broadcaster in the 85 - 122 MHz segment, Squelch should be set to „0“, Bandwidth 75 kHz and Deemphasis should be set to 75 usec. The Deemphasis control will reduce the level of the higher parts of the audio spectrum to give You a more natural audio impression. On standard FM transmitters, the higher frequencies are „emphasized“, i.e. they are transmitted with higher amplification gain.
For the reception of military or commercial communication in the VHF band, switch to 25 or 75 kHz bandwidth and Deemphasis 280 usec. Be careful not to share any information You inadvertedly have picked up while tuning in these frequency segments - in many countries monitoring military and police communications is forbidden.
In the Air Band of 118 - 136 MHz usual amplitude modulation (AM) is used, so switch to Am to pick up the Volmet weather informations from Zurich International airport.
This fifty year old Autophon set offers very pleasent reception in the FM broadcast as well as the Air band - and usually this sets work very reliably, what You would expect from „Swiss Made“ quality. Not to mention that Swiss Army gear is usually heavy, rock-solid and is designed and built to last centuries…
The antenna signal from the coxial 70 Ohms socket has to pass two RF preamplifier stages; with the signal of the oscillator (ECC81) it is converted in the mixer stage (EF80) to the intermediate frequency of 10.7 MHz. This intermediate frequency is amplified by two IF amplifier stages (EF85, EF85), here, the IF filters ZF I, ZF II¹ und ZF III¹ are active.
If the narrow bandwidths of ± 25 kHz and ± 8 kHz mainly used for AM-reception are used, the valve V5 (EF85) acts now as 2nd mixer stage and converts the signal with the oscillator signal coming from Osz.2 (9,1 MHz) to generate the second IF of 1,6 MHz, now the IF filters ZF II² and ZF III³ are in use. On this lower intermediate frequency, filters with better selectivity can constructed more easily.
For AM demodulation valve V11 (EBF80) is used, this acts also as meter amplifier. For FM demodulation, the signal first has to pass a limiter stage (6BN6, Limiter) and will be fed to separate ratio detectors active on the IF of 10,7 and 1,6 MHz, in both of them, two OA72 semiconductor diodes are used.
V1 (ECC84, 1. RF stage); V2 (EF85, 2. RF stage); V3 (EF80, mixer); V4 (ECC81, 1. oscillator); V5 (EF85, 1. IF amplifier); V6 (EF85, 2. IF amplifier); V7 (6BN6, FM limiter); V8 (EF86, AF preamplifier); V9 (EL91, AF final stage); V10 (EBF80, noise limiter); V11 (EBF80, meter amplifier); V12 (EF85, 2. oscillator); D1-D4 (OA72, FM diskriminator.
V13, V14 (EZ80, two mains rectifier valves), V13 stabilisator 150C1.
The VHF receiver E-628 has been developed in 1955 as a successor to Autophon's first VHF receiver E46 which never went into mass production. It was also sold for civilian users and PTT (Swiss post telegraph telephone company) as E77 in parallel to the similar shortwave receiver E-627, the civilian version of this was called E76.
In the years 1955/58 some 174 sets has been delivered to the army. The E-628 was used as surveillance receiver of the signal and airbourne signal units. The sets have been faded out in 1985 and sold as surplus in 1991 and can be found at ham flea markets and online auctions from time to time.