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Kongo 50W
Long range 50W short wave transmitter

Kongo 1 was a compact valve-based short-wave 50W spy radio transmitter for the 12-22 MHz frequency range, developed in Czechoslovakia in 1960 by Správa 6 2, for use in international espionage by the secret state police (StB) and by Správa 1 (espionage). The transmitter was commonly used in combination with a Zenith 1000 receiver and debuted in 1960 in Congo.
The transmitter measures only 29 x 14 x 5.5 cm and came with a matching power supply unit (PSU) of similar size (which is missing here). The transmission frequency is determined by a crystal that is inserted into a socket at the right side. Also connected at the right side is a standard manual morse key, such as the one shown here, or an automatic key like Pivoňka.

The image on the right shows the Kongo 50W transmitter with a set of crystals and a small manual morse key. The cable is for connection to the PSU. It has a 8-pin Jones plug at the end.
Kongo transmitter with crystals and morse key

The Kongo 50W transmitter was usually supplied as part of a complete (spy) radio station, that consisted of the transmitter, a power supply unit (PSU), a suitable short-wave receiver, a set of crystals, a morse key or an automatic keyer, frequency tables and coding material (cryto). In order to obtains the best possible secrecy, the unbreakable One-Time Pad (OTP) was commonly used.
The image on the right shows a typical Kongo 50W transmitter (without its power supply unit) together with the complementary Zenith Royal 1000 receiver that was obtained in the free West.

The Zenith receiver offered a good alternative to the purpose built spy radio receivers of the era. It was one of the first commercially available short-wave receivers that was fully built with transistors, and it offered a good performance. As the scale is not accurate enough for narrow-band reception, it was commonly 'calibrated' by tuning it to the transmitter's crystal frequency.
Czechoslovakian Kongo transmitter with Zenith 1000 receiver

The Kongo transmitter was probably built in small quantities, as the unit does not have a project number. Although the radio produced an output power of 50W, it was not always suitable for long-range communication between Congo and Czechoslovakia. For this reason, the 300-AB set, with an output power of 200W, was introduced a year later (1961). At present we have no further information about this transmitter. If you have additional details, please contact us.
  1. The name Kongo is probably incorrect, but as this device does not have a project number or any other kind of identification, we have choosen it as a nickname. Kongo is the Czech name for Congo.
  2. Správa 6 refers to Government Department 6: Communication Technology.

Kongo 50W transmitter (1960) Kongo transmitter with crystals and morse key Inserting a crystal Crystal inserted at the right side Morse key connected to the right side Front panel Antenna sockets and power lead at the left side Front panel detail

The Kongo 50W transmitter is a rather simple device with only a few controls and connections. The black cable at the left connects the transmitter to the PSU, which provides the LT and HT voltages. At the right side are banana sockets for the crystal (shown below) and a morse key.

Controls and connections on the Kongo 50W transmitter

All controls are at the front. The crystal frequency can be doubled or tripled and the Oscillator tuning knob should be set to approx. the desired frequency, which the rightmost meter as an indicator for the oscillator current. Antenna coupling and tuning are adjusted with the two large knobs at the left. They should be adjusted for maximum PA current (leftmost meter).
The transmitter is housed in an aluminium frame with perforated panels at the top and bottom. These panels provide sufficient cooling for the valves, provided that they are not obstructed. The case can be opened by removing these two panels with no less than 16 screws at either side.
The device is well built and, despite its high age, it is still in very good condition. At the right are three unmarked valves, of which the rightmost one is the oscillator. The large valve at the left is a 6146B, made by RCA in the US. To the left of the large valve is a tuning capacitor, with a coil behind it. The coil is connected to the antenna.

Unfortunately, the circuit diagram of the transmitter is not available at the moment.
Kongo 50W transmitter interior
Kongo 50W transmitter interior Bottom view PA stage (top view) PA stage (bottom view) Oscillator, multiplier and modulator (top view) Oscillator, multiplier and modulator (bottom view)

Valves   tubes
  • 6146B
    50W Power Amplifier (RCA, USA)
  • 6L41 (2x)
    Oscillator, Frequency Doubler
  • ?
    Unmarked valve, stabilizer (75 or 150V)
  1. Anonymous, Kongo 50W transmitter - THANKS!
    Transmitter kindly donated by anonymous former user. July 2015.

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Crypto Museum. Last changed: Friday, 04 September 2015 - 21:29 CET.
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