The Zellweger Uster receiver E-646 has been introduced as alarm receiver to be used with the air attack alarm network in connection with the air attack alarm transmitter S-510.
To adress parts of the troops directly with alarm transmissions, some antiaircraft units did transmitt alarm messages on fixed shortwave frequencies covering the whole country. A "call signal" preceeded the voice message and did open a "squelch- like" circuit muting the receiver when no alrm messages are broadcast.
All receivers come with a preprogrammed "Group address" (GR) and eight switchable "Rufadresse" marked with letters A - H. The selective call signal transmitted from the alarm network transmitter S-510 will open the muted alarm receivers on the correct frequency with the correct adress A - H. The selective call signal consists of whistle tone transmitted in the upper sideband of the frequency channel which is frequency modulated and have binary coding. In standard selective call mode (RN), it's duration is 0 - 9 seconds, in short call mode (RK) only 0 - 3 seconds.
After the voice message is transmitted, a end signal (1,5 seconds) will mute the receivers tuned to the air attack warning channel.
For selective call operation, the modes switch has to bet on RN or RK. In "normal selective call mode RN", the receiver is activated only every few seconds to save battery power, that's why the selective call signal is transmitted much longer. One set of batteries will give the receiver 7 days of continuous operation. In the "short selective call mode RK" as well as in all other reception modes, the receiver is active continously and batteries are exhausted quickly. The modes A3, A3JU and A3JO stand for normal AM and LSB / USB reception. It's good to know, that in German language U stands for lower and O stands for upper sideband.

Some receivers carrying a sticker "K" on the frontpanel are equipped with an additional circuit board which will activate the set on calls on all GR group addresses and all "Rufadressen" A - H for testing purposes.

In contrast to the prototype, the speaker grill has been moved to the left part of the frontpanel. In the upper row of controls, You find the modes switch to activate A3/A3JU and A3JO as well es the selective call modes RN (with short monitor periods and low power consumption and RK with continuous monitoring for coded transmissions, four decadic switches to select the reception frequency from 2'000 - 11'999 kHz and the "Rufadressen" switch.
In the lower row of controls, You find the main switch, the colume control, the connectors for headphones, external speaker, 24 V DC power and the BNC antenna socket.

The receiver E-646 can be powered from an external mains power supply, four ALN 6135-265-1751 7,5 Volt - batteries or by use of an adaptor 16 UM-1 mono cell batteries at 1,5 Volts each.
An external matching speaker can be used at a maxiumum distance of 25 meters, it has a separate volume control and a headphones jack which will switch off the speaker when active.

In a accessory aerial bag, You find everything to construct a long wire antenna to a 6 m mast, a 33 m coaxial cable to connect the antenna to the receiver and - as usual in Swiss Army equipment nearly everything to erect the antenna, even a hammer to put the tent pegs into the ground and a weight, which can be used the throw an antenna wire up on a tree or a building.

The air attack service on fixed shortwave frequencies with it's unique selective call system is not active anymore, the receiver can be used a a decent shortwave receiver to monitor Your favourite international shortwave broadcasters. It's well suited for fixed frequency operation, scanning throug a shortwave band is cumbersome as the set has only decadic frequency switches and no tuning knob. The set comes without electronic frequency memories and all the goodies to improve reception under dufficult conditions as switchable I.F. bandwidths, notch filter, variable AGC speeds and so on.
But if You use it for that, what it was made for, the E-646 is an excellent rugged stable fixed frequency single sideband receiver.

© Martin Bösch 24.7.2009 / 3.9.2010