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ASG-6800 →
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15W py radio transmitter

The S-6800 is a solid-state 15W 1 short-wave (SW) transmitter, developed and released around 1970 by AEG Telefunken in Germany, as part of the SP-20 spy radio set. The radio was intended for military use by Special Forces (SF), operating behind enemy lines, as well as for Stay-Behind Organisations. The S-6800 was always used in combination with the ASG-6800 antenna tuner.

Like the other modules of the SP-20 radio set, the transmitter measures 115 xc 105 x 32 mm and has all controls and connections at the front panel. A metal rail at the top surface allows the matching antenna tuner to be mounted on top.

The transmitter is powered by a 12V DC source that should be connected to two banana sockets at the front. Also at the front are two banana sockets for the connection of a morse key or an early burst encoder, such as the NATO RT-3. The set was originally crystal operated and was suitable for the 2...24 MHz frequency range.
S-6800 transmitter

In the late 1970s, the crystal was replaced by the KS-30 frequency synthesizer. The transmitter was designed for sending messages in morse code and operates in CW (A1), with an output power of ~ 15 Watts. When tuning the antenna however, an output power of 10 mW is used. The output of the transmitter is connected to the input of the ASG-6800 antenna tuner. When using the KS-30 frequency synthesizer, the transmitter has also be used in FSK (F1) mode, in which case the burst encoder should be connected to the peripheral socket of the KS-30 synthesizer.

  1. Below 10 MHz, the transmitter produces an output power of 20W.

S-6800 transmitter S-6800 transmitter (bottom) and ASG-6800 antenna tuner (top) S-6800 with quarz crystal Quarz crystal installed Without crystal (dummy plug installed) Crystal operated set with RT-3 burst encoder With KS-30 frequency synthesiser and RT-3 burst encoder Typical setup
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S-6800 transmitter
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S-6800 transmitter (bottom) and ASG-6800 antenna tuner (top)
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S-6800 with quarz crystal
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Quarz crystal installed
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Without crystal (dummy plug installed)
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Crystal operated set with RT-3 burst encoder
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With KS-30 frequency synthesiser and RT-3 burst encoder
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Typical setup

All controls and connections of the S-6800 are at the front panel, as shown in the diagram below. The unit is powered by a 10...15V DC power source (typically 12V from a car battery) that should be applied to the banana sockets at the center of the front panel. The unit is switch ON with the power switch to the left of the red banana socket. The battery voltage can be checked by briefly pressing the Battery Test button along the lower edge. The indicator should be in the green area.

When operating the transmitter with a crystal, the crystal should be inserted into the appropriate socket, which is shown here with a protective plastic cap installed. When used in combination with the KS-30 synthesizer, the latter should be connected to the peripheral socket at the left.

A long-wire antenna and a suitable counterpoise can be connected directly to the banana sockets at the right, but these sockets should only be used if no antenna tuner is available. In normal use, the ASG-6800 tuner should be connected to the SMB socket at the right center. When tuning the antenna, the Tune-switch should be pressed, which results in a 10mW output signal. The controls on the ASG-6800 tuner should then be adjusted for a maximum reading of the indicator.

Antenna tuner
The ASG-6800 antenna tuner was designed for use in combination with the S-6800 transmitter. Like the transmitter, it was manufactured by AEG Telefunken in the mid 1970s (1975-1977).

The tuner was connected to the transmitter by means of a short coaxial cable with an SMB-connector at both ends. At the bottom it has two rigs that allows it to be slotted onto the S-6800 transmitter (see above).

 More information
ASG-6800 Antenna Tuner

Analogue synthesizer   STEU-6800
Along with the S-6800 transmitter, a complex analogue frequency synthesizer was developed, that allowed the set to be operated on any of 1296 front panel selectable channels.

The analogue synthesizer was too complex in operation however, as the channel number did not have a direct relation with the corresponding frequency, and the device was abandoned in favour of individual crystals.

 More information

STEU-6800 analogue synthesizer

Digital synthesizer   KS-30
Initially, the S-6800 transmitter was crystal operated. Later, an external synthesizer unit (KS-30) was developed by Pfitzner. This was probably early in 1983. The sythesizer allows a frequency coverage of 2-24MHz in 1kHz steps.

It was added to all existing SP-20 stations and connected to the transmitter via a multi-pin connector on the left. A rather long cable was supplied to connect the KS-30 to the S-6800.

 More information
KS-30 synthesizer

Connection block
With the military variant of the SP-20, it was common practice to connect the synthesizer to the transmitter by means of a one meter cable.

With the stay-behind version of the set however, it was more common to place the synthesizer to the left of the transmitter and connect the two units by means of the small connection block shown in the image on the right [1].
Connection block for fitting the KS-30 synthesizer directly to the S-6800 transmitter. Photograph by Jim Meyer [1].

The highly compact S-6800 transmitter is beautifully built. The interior can be accessed by removing a single screw from the rear end, and sliding off the case shell. The entire construction is built on a metal frame that is mounted to the front panel, just like with the other modules.

The image on the right shows the top side of the interior, with the large heat sink taking up most of the space. A total of seven transistors are mounted to this heatsink, two of which are used in the PA-stage. The other transistors are used in the driver stages. At the front left is a relay.

At the bottom side in the actual transmitter PCB, plus a narrow PCB that is covered by a metal panel. This PCB contains two hybrid building blocks marked KS 411 B, which is the oscillator, and KSB 408 B, which is the oscillator amplifier. Information about these parts is available below.
S-6800 interior

The build quality of the transmitter is extremely high, and only first class components are used from a wide variety of manufacturers. When looking at the interior, one can't help but noticing the enormous experience of the Telefunken engineers in creating robust and reliable equipment.

S-6800 outside its case shell Interior - top view S-6800 interior Detail Detail Relay Top view 2N3866 transistor
Detail Bottom side Bottom view Removing the oscillator board Removing the oscillator board Oscillator board with two building blocks Bottom side detail Bottom side close-up
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S-6800 outside its case shell
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Interior - top view
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S-6800 interior
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Top view
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2N3866 transistor
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Bottom side
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Bottom view
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Removing the oscillator board
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Removing the oscillator board
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Oscillator board with two building blocks
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Bottom side detail
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Bottom side close-up

Development of the S-6800 transmitter at Telefunken started in 1966 and took until approx. 1970. During that time, various designs were tried and much time was spent on miniaturization, as the circuits had to fit inside the cramped space of what would become the standard enclosure.

The image above shows an early prototype of the Kleinsender (miniature transmitter) S-6800 that was developed in 1966 by Fritz Arends, one of Telefunken's key developers. The functional specification was drawn in June 1966 and the first prototype was finished in November of that year. It delivered an output power of 15W.

Immediately after the prototype was released, development of the final design of the S-6800 was started. The requirements had meanwhile been adjusted by the first customer. It had to produce an output power of ~15W and it had to fit inside the smaller (now familiar) rectangular enclosure. The design of the upgraded S-6800 transmitter was finished in or around 1968.
Fritz Arends, developer of the S-6800 transmitter, with an SP-20 under his arm. Photograph via Jim Meyer [1]. Reproduced here by kind permission.

By 1970, the transmitter was ready to be rolled-out and a year later, in 1971, the official circuit diagrams were released [A]. Note that the circuit diagrams were updated in 1976, after several small modifications had been processed. The image above shows developer Fritz Arends in later years, holding one of 'his' beloved SP-20 spy radio sets under his arm. The image was taken at the Telefunken museum in Ulm (Germany). Photograph via Jim Meyer [1].

  1. Sender S-6800, full circuit diagram
    AEG Telefunken, 1971-1976. 11 pages. 1

  2. KS-411-B, geregelter Quarzoszillator (part of A)
    Regulated Quarz Oscillator (building block). Internal circuit diagram.
    51.6801.026.00 LV. Telefunken.

  3. KS-408-B, Oszillatorverstärker (part of A)
    Oscillator Amplifier (building block). Internal circuit diagram.
    51.6801.027.00 LV. Telefunken.
  1. Document kindly supplied by Jim Meyer [1].

  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004.
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Crypto Museum. Created: Monday 29 August 2016. Last changed: Thursday, 01 March 2018 - 08:51 CET.
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