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STEU-6800 synthesizer
Analogue frequency synthesizer

STEU-6800 was an analogue frequency synthesizer, developed around 1970 by AEG Telefunken (Germany), for use in combination with the German SP-20 radio set. It was connected between the high-speed (burst) morse keyer and the S-6800 transmitter, and offered 1296 channels [1]. In practice, it was used as the driver stage (German: Steuer-Sender) for the S-6800 transmitter.

The device is housed in a metal enclosure that has the same external dimensions as the other parts of the SP-20. As digital PLL synthesizers 1 did not yet exist at the time, a collection of 72 carefully picked (different) crystals – was used to obtain the desired frequency by way of mixing.

This was done by organising the crystals in two matrices of 36 crystals each. A single crystal in each matrix was selected by two selectors with 6 positions each (6 x 6 = 36). For this, three rotary selectors are available at the front panel, whilst a push-button and 6 LEDs act as the fourth one.
  
STEU-6800 analogue synthesizer

Only a small number of STEU-6800 units were ever build, probably no more that 10 units or so. Operating the device must have been very complicated. As the selectors at the font panel have a limited range (1-6 rather than 0-9), it is not possible to enter the desired frequency directly. Instead, it was necessary to use large conversion tables to select any of the 1296 channels. As a result, the device had a short operational life and was abandoned after the first field tests [2].

It was decided to use the S-6800 transmitter with individual crystals (installed in the socket on its front panel) rather than using the analogue synthesizer. In the late 1970s, when the digital PLL had become available, the the crystals were eventually replaced by the KS-30 digital synthesizer.

We are indebted to Jim Meyer and Fritz Arends for providing backgrounds on the STE-6800, such as the name, the intended purpose and its development history. Fritz Arends was one of the key developers of the SP-20 at Telefunken in Ulm at the time. He was also responsible for the S-6800 transmitter. And Jim is a former user, who has helped us on numerous occasions before [1][2].

  1. The first papers about digital PLL frequency synthesizer appeared around 1974.  Wikipedia

STEU-6800 analogue synthesizer STEU-6800 analogue synthesizer Front panel Seen from the front Sliding off the case shell Interior with shielding panel fitted SP-20 analogue synthesizer Interior bottom view (carried board)
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STEU-6800 analogue synthesizer
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STEU-6800 analogue synthesizer
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Front panel
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Seen from the front
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Sliding off the case shell
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Interior with shielding panel fitted
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SP-20 analogue synthesizer
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Interior bottom view (carried board)

Features
All controls and connections of the STEU-6800 are at the front panel. At the left is a 20-pin socket for connection of an external keyer, or burst encoder, such as the Speicher or the MMP. At the right is a similar 20-pin socket for connection to the S-6800 transmitter. 12V DC power is provided by the transmitter via this socket. When connected, the keyer takes over the function of the transmitter's morse key. When the keyer is not connected, it is replaced by a terminator.


There are four selectors at the front panel, each with a setting between 1 and 6. Three of these are rotary selectors, whilst the fourth one consists of a push-button and 6 indicator lamps. The first two selectors (A and B) control one bank of 36 crystals (6 x 6), whilst the remaining two (C and T) control the other bank of 36 crystals. This gives a total of 36 x 36 = 1296 channels. Note that the channel selectors have no direct relation to the actual transmission frequency.


Block diagram
The diagram below shows how the circuit works, and how the two banks of 36 crystals each, are mixed to obtain the desired frequency. At the top left is a crystal oscillator with a fixed frequency of 2.6 MHz. It is the starting point of the frequency synthesis, and is also used for producing a high-speed morse code signal, by causing small frequency shifts (Frequency Shift Keying or FSK).


After amplification, the 2.6 MHz signal is mixed with the frequency from a single crystal of the rightmost crystal bank, that lies in the 27.375 to 27.651 MHz range. It causes small changes in the output frequency. The output of the first mixer is amplified and mixed with the frequency of a single crystal from the leftmost crystal bank, which lies in the 32.400 to 53.000 MHz range. This bank causes the bigger variation in the output frequency, which will eventually be ~ 2 - 24 MHz.


The diagram above shows how the two crystal banks work. The 5 lines from two of the front panel selectors (1-6) are used to select a row and a column of an electronic matrix. As a result, one of the 36 crystals is selected. In practice, the crystals of each matrix are organised in 6 banks of 6 crystals each. The banks are connected in parallel and the row signal is used to select the appropriate bank, whilst the column signal is used to select one of the 6 crystals in that bank.




Interior
Like most other components of the SP-20 product line, the interior of the device can be accessed simply by removing a single recessed screw from the centre of the rear side, and sliding off the case shell. Note that it is a tight fit that may bind, so be careful not to damage any internal parts.

Inside the unit is a rectangular frame with a large carrier board at the bottom. All internal circuits are mounted and connected to this board. This board is protected against short-circuits with the case shell, by means of a thin pertinax sheet.

The top of the compartmented frame is shielded by a metal panel that is held in place by three recessed screws. After removing the shield, the individual circuits of the unit are exposed. The interior can roughly be divided into two sections: the rear half that holds two large crystal banks, and the front half with mixers and oscillators.
  
Interior

The image above shows a top view of the frame, with the two crystal banks removed from their sockets. At the front half, the signal goes from right to left. At the far right are the keyer circuit and the 2.6 MHz oscillator. Next are the mixers that combine the signals from the crystal banks.

Each mixer consists of a crystal oscillator, an amplifier, the necessary filters and a finally the actual mixer circuit. Each crystal oscillator is connected to a single crystal from its crystal bank, as selected by the front panel controls.

The image on the right shows a close-up of the rightmost crystal bank. At the front right are the 13 pins of the socket through which it connects to the carrier board at the bottom. The crystals are arranged in 6 blocks of 6 crystals each, and one side of each crystal is connected to ground. The other side is fed to an electronic matrix.
  
Rightmost crystal bank. At the front right is the connector.

Each block of 6 crystals has its own matrix switch, which is housed in a rectangular gold-plated enclosure that has 10 pins at either side. The entire crystal bank is constructed as a so-called cordwood structure, with the components mounted between two parallel printed circuit boards.

The cordwood construction method 1 was a very popular space saver in the 1960s, but makes it very difficult to service or repair a broken unit.

At the far left of the frame, is a 4-stage output filter, followed by an amplifier that delivers an output power of approx. 20 mW, which should be sufficient to drive the transmitter. The output amplifier is constructed as a minature circuit and - like the crystal banks - it is housed in a gold-plated enclosure of which the 10 contacts at the top are unused. The image on the right shows the output amplifier, located behind the filters.
  
Output filter and amplifier

The overall mechanical and electronic construction of the device is excellent, with just a few modificiations in the circuits. The modular crystal banks are well-constructed, but it must have been a nightmare to calculate the correct frequencies for each of the 72 individual crystals.

The control panel on the other hand, seems to be overly complicated. It is attached to the main carrier board by means of a 32-pin header at the bottom. It holds the sockets for connection of the external keyer and the S-6800 transmitter, plus the four selectors for the two crystal banks.

In principle, the front panel should have had four rotary selectors: two for each crystal bank, but it was decided to replace the rightmost selector by a push-button and 6 lamps, probably due to lack of space. As a result, a complex circuit with three ICs had to be developed to do the same.
  
Front panel PCB with damaged IC (left)

The image above shows the front panel detached from the frame, with its PCB facing upwards. Here we see the contacts of the three rotary dials, and the ICs that are used for the push-button selection. One IC (towards the right) is sticking out of the PCB. The one at the far left has been damaged – probably due to falling – and had to be replaced. At the bottom is the 32-pin header.

  1. Examples of cordwood structures are the American FLYBALL modules that were used in the KW-7 cipher machine, and the Philips Circuit Blocks that were used in a variety of devices.

Sliding off the case shell Top shielding removed Interior with top shield Bottom view SP-20 analogue synthesizer Interior - top view Front panel detached 2.6 MHz oscillator and keyer
2nd mixer Connections between the crystal banks and the mixers Output filter and amplifier Miniature IC on the front panel board (SN5492) Interior Rightmost crystal bank. At the front right is the connector. Side view of a crystal bank. The 'golden' parts are the electronic matrix switches Installing the leftmost crystal bank
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Sliding off the case shell
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Top shielding removed
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Interior with top shield
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Bottom view
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SP-20 analogue synthesizer
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Interior - top view
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Front panel detached
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2.6 MHz oscillator and keyer
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2nd mixer
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Connections between the crystal banks and the mixers
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Output filter and amplifier
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Miniature IC on the front panel board (SN5492)
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Interior
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Rightmost crystal bank. At the front right is the connector.
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Side view of a crystal bank. The 'golden' parts are the electronic matrix switches
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Installing the leftmost crystal bank

Restoration
When we received the synthesizer featured on this page, it was assumed that it was probably some kind of burst encoder, similar to the so-called Speicher morse keyer. After all, it was identified as such in Louis Meulstee's excellent book Wireless for the Warrior, Volume 4 [3].

At the first inspection we noticed that the knob of the rightmost rotary selector at the front panel had been damaged, probably from falling. We will deal with this part later. After opening the unit however, we noticed that it was neither a high-speed keyer, nor a memory (German: Speicher), but a full blown analogue synthesizer.

This was a great discovery, as it was hitherto not publicly known that an analogue synthesizer for the SP-20 had ever existed. It was a bit worrying however, that one of the ICs on the front panel circuit board appeared to be damaged badly.
  
Damaged IC (SN74156) on the front panel

The image above shows the damaged IC, of which the bare chip has become visible. This IC is part of the front panel push-button selector (with the 6 red indicator lamps), and controls the least-significant unit of the channel selection. Luckily, based on the wiring pattern of the PCB and the surrounding components, we were able to deduce that it was an SN74145 decoder/driver [4].

As it appeared to be very difficult to remove the PCB from the front panel, the replacement IC was soldered onto the remains of the old one's legs. It had to be as close to the PCB as possible, as otherwise it wouldn't fit in the available space.

In the meantime, the damaged edges of the front panel — probably also caused by falling — were restored and painted, just like the knob of the rightmost rotary selector, that had completely lost its original shape. With some metal/epoxy filler and a lot of patience, the knob got most of its former glory back, as shown in this picture.
  
Front panel with repaired knob

Once the cosmetic issues were solved, it was time to reassemble the device and give it a try. This was done by connecting it to the S-6800 transmitter, using the interconnection cable of the later KS-30 synthesizer. It was assumed that the latter had the same wiring as the STEU-6800. This assumption turned out to be correct, and the STEU-6800 showed a sign of live immediately after a 12V power source was connected to the transmitter. The first of the six red LEDs (1) became lit.

Further tests showed that it will be difficult to determine how the channels are assigned, partly because some of the crystals are no longer working (like the other damages, probably caused by falling), and partly because of the poor suppression of spurious signals generated by the device.

The following items had to be restored:

  • Knob of rightmost rotary selector damaged
  • Damaged IC on the front panel PCB
  • Damaged paint on the edges of the front panel
Front panel PCB with damaged IC (left) Damaged IC (SN74156) on the front panel Broken IC removed from PCB Repaired from panel unit Broken IC replaced by new SN74125 Front panel with damaged knob Front panel paint restored partly repaired knob (filled with epoxy/metal glue)
Front panel Front panel with repaired knob
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Front panel PCB with damaged IC (left)
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Damaged IC (SN74156) on the front panel
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Broken IC removed from PCB
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Repaired from panel unit
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Broken IC replaced by new SN74125
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Front panel with damaged knob
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Front panel paint restored
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partly repaired knob (filled with epoxy/metal glue)
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Front panel
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Front panel with repaired knob

Connections
Left
The table below shows the pinout of the leftmost socket as seen from the front of the synthesizer. This socket is used for connection of peripherals, such as a morse key or a high speed morse burst transmitter, like the the Speicher or the MMP. If the socket is unused, a terminator should be installed.

  1. n.c.
  2. Loop 1 1
  3. Loop 1 1
  4. n.c.
  5. GND
  6. Loop1 1
  7. GND
  8. Loop 2 1
  9. Loop 2 1
  10. 10...15V
  11. n.c.
  12. Loop 2 1
  13. 10...15V
  14. Tastung 3
  15. n.c.
  16. n.c.
  17. Auftastung 2
  18. Tastung 3
  19. n.c.
  20. Auftastung 2
Right
The table below shows the pinout of the rightmost socket as seen from the front of the synthesizer. This socket is used for connection to the transmitter. The loop wires shown in the drawing reflect the internal wiring of the socket, just like in the later KS-30 synthesizer.

  1. HF
  2. Loop 1 1
  3. Loop 1 1
  4. HF
  5. GND
  6. Loop 1 1
  7. GND
  8. Loop 2 1
  9. Loop 2 1
  10. 10...15V
  11. n.c.
  12. Loop 2 1
  13. 10...15V
  14. n.c.
  15. GND
  16. n.c.
  17. Auftastung 2
  18. n.c.
  19. n.c.
  20. Auftastung 2
  1. These lines are 'cold' links between the left and right sockets. They run straight from the peripheral to the transmitter and are not connected to anything inside the synthesizer.
  2. In A1 mode, Auftastung is used for the connection of a morse key or a medium-speed burst encoder. The line Tastung should be grounded, by means of a terminator plug.
  3. In F1 mode, Auftastung is used to enable the transmitter, whilst Tastung is used for keying the data with a high-speed burst encoder. The line Tastung is only available on the leftmost socket.

Connecting to the transmitter
For connection between synthesizer and transmitter, a 16-way shielded cable is used, with a 20-pin SME20P connector 1 at either end. Two pins of this connector are used for connection of the shield and two other pins remain unconnected. The wiring 2 of this connector, when looking into the female sockets on the devices, is given below. Both plugs are wired identically (1:1).

Wiring of the multi cable bewtween synthesizer and transmitter

A suitable cable with a length of 1 metre was supplied with the military version of the SP-20. This cable is also known as NSN 5995-12-188-3633. A small connector block was generally used with the (grey) Stay-Behind version of the radio set. The latter requires the synthesizer to be placed adjacent (to the left of) the transmitter, so that the connector block can be fitted to both sockets.

  1. This is a military connector made by Winchester Electronics [3].
  2. This is the same as the solder side of the male plugs.

Cable between synthesizer and transmitter Close-up of SME20P plug Looking into the pints of the male plug Disassembled plug Close-up of the wiring of the SME20P plug Short SMB cable Terminator plug
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Cable between synthesizer and transmitter
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Close-up of SME20P plug
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Looking into the pints of the male plug
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Disassembled plug
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Close-up of the wiring of the SME20P plug
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Short SMB cable
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Terminator plug

Connecting the RT-3 burst encoder
When using the RT-3 burst encoder, it should be connected directly to the morse key sockets at the front panel of the transmitter (not to the synthesizer). In this case, the terminator plug should be installed in the leftmost socket of the synthesizer.

Connecting the Speicher burst encoder
In order to support very high speed burst transmitters, such as the Speicher or the MMP, the STEU-6800 uses FSK modulation. The keyer should be connected to the leftmost socket on the synthesizer, rather than directly to the transmitter. The diagram below shows the wiring of the cable between Speicher and the synthesizer, when looking into the sockets of the devices.

  1. GND
  2. Tastung
  3. Auftastung
Connecting the MMP burst encoder
Connection to the later MMP burst encoder is similar to Speicher, but the cable has a LEMO plug at the MMP-end. The diagram below shows the wiring of the cable that is used between the MMP and the STEU-6800, when looking into the sockets on the devices. The line Auftastung is driven by the MMP just before sending the message. It enables the transmitter. Once the transmitter is stable, the actual message is sent via the Tastung line in FSK.

  1. GND
  2. Tastung
  3. Auftastung
Terminator
Note that the leftmost 20-pin socket on the synthesizer is not used, but that a terminator plug should be present in this socket for correct operation of the set. Suitable terminator plugs were supplied with each SP-20 radio set. The pinout of the terminator is given above, when looking into the leftmost female socket on the synthesizer. Pins E, H, R and V are connected together.

Terminator plug on the leftmost socket of the synthesizer


References
  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence, December 2017.

  2. Fritz Arend, Background information on the development of STEU-6800
    Personal communication via [1]. December 2017.

  3. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

  4. Texas Instruments, SN741456 BCD-to-Decimal Deceder/Driver
    Datasheet, 1998. Retreived December 2017.
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Crypto Museum. Created: Sunday 10 December 2017. Last changed: Thursday, 01 March 2018 - 08:55 CET.
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