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300 A   Angola
Short range 20W short wave transmitter

The 300-A was a valve-based short-wave spy radio transmitter for the 10-18 MHz frequency range, developed in Czechoslovakia in 1961 by Správa 6 1 for use in foreign espionage by the secret state police (StB) and by Správa 1 (espionage). The 20W transmitter was suitable for short-range communication and was first used in Angola in 1961. The transmitter could be converted for long-range communication by adding the external 300-B 200W power amplifier to the set.
The unit has a built-in power supply unit (PSU) and is powered directly by the 220V AC mains. It has five banana sockets at the front panel for connection of an automatic morse keyer. A manual morse key can be connected as well.

The transmitter produces an output power of 20W, which is suitable for short-range morse communication. At the left side of the unit is a PL519 coax socket for connection of a small antenna tuner (shown here on top of the device). The frequency is determined by a crystal that should be inserted into the socket at the right.
300-A transmitter with antenna matcher and morse keyed (1961)

The 300-A was probably produced in small quantities, as the unit has no project number and no engraved text on the front panel. Instead, the text and the model number are applied by means of Dymo™ labels. No documentation and/or circuit diagram is available at the moment. The unit was used in Angola (1961) and, as the 300-AB, in Congo (1961) and Kurdistan (1965-1968).
  1. Správa 6 refers to Government Department 6: Communication Technology.

300-A transmitter with antenna matcher and morse keyed (1961) 300-A transmitter (20W) Socket for connection of the automatic morse key Installing the automatic morse keyer Automatic morse keyer Connecting a conventional morse keyer Mains cable and fuses at the rear Simple antenna tuner for 300-A
300-A transmitter front panel (without keyer) 300-A transmitter front panel (with keyer) Right side Left side Crystal inserted in the socket at the right side Suitable crystal Another suitable crystal Rear view of the antenna tuner

The diagram below shows the location of the controls and connections on the 300A transmitter. Note the four wooden strips at the upper and lower sides which are necessary for providing sufficient cooling, especially when the 300B Power Amplifier is installed on top of it. The unit can be connected directly to the 220V AC mains and a filtered 220V outlet is available for the 300B. Power is applied in two stages: (1) the LT for the filaments, and after a while (2) the HT voltage.

An asymmetrical antenna can be connected to the PL519 socket at the left side. A symmetrical antenna, such as a long-wire or an inverted-V, can also be connected if the external antenna tuner is used. Like the transceiver, the matcher should be tuned for maximum antenna current.

Five green banana sockets are available on the front panel for connection of an automatic morse keyer. The keyer has a paddle sticking out at the front. Pushing the paddle to the left produces a series of dots, whilst pushing it to the right produces a series of dashes. The speed can be set with the speed-knob. A small button on top of the keyer is used for tuning the transmitter. If necessary, a standard manual morse key can be connected to the upper two banana sockets.

Three large knobs on the front panel are provided for tuning the transmitter. They are marked (from right to left) O (oscillator), G (grid) and P (plate). The O-knob is used for adjusting the crystal's overtone, whilst the G and P-knobs are used for tuning the transmitter's output power in combination with the meter. For this reason the P/G selector is available just below the meter.
200 Watt version   300 AB
For longe-range radio traffic, the transmitter could be enhanced with a 300B Power Amplifier, which increased the 20W from the transmitter to a massive 200W. It was placed on top of the transmitter and came with a dedicated 300C Antenna Tuner, as shown in this image.

The enhanced set was known as 300AB and was first used in Congo and later also in Kurdistan.

 More information
300-A transmitter, 300-B PA and 300-C antenna tuner

The 300-A transmitter is housed in a grey hammerite enclosure that measures 33 x 23 x 12.5 cm. The construction consists of an aluminium frame, a front panel, a rear panel, an upper case shell and a lower case shell. Both shells are made of perforated metal, so that the electronics are properly cooled. For this reason wooden rigs are provided at the edges of the case as spacers.
Despite its relatively boring exterior, the 300-A is extremely well built. It consists of no less than nine valves, three of which (ECC82) are used for the automatic morse keyer which also uses two large telephone relays for the dots and dashes.

The valve closest to the crystal socket is the oscillator, which the one next to it being the doubler or tripler. The output stage of the transmitter consists of two 6L41 valves in a balanced circuit, producing approx. 20W output. The output stage is shown in the image on the right, as seen from the rear of the device.
PA stage

The chassis roughly consists of three compartments: the electronics at the centre, the control panel at the front and the power supply at the rear. In the centre section, all HF valves are mounted at the top side, whilst the valves of the automatic keyer are mounted at the bottom.
Interior Interior Top view Bottom view Power lines (the red wires) Automatic morse keyer Oscillator PA stage
PSU detail PSU detail Wiring behind the front panel Wiring of the morse paddle connector Components behind the front panel Side view Coil Valves

Valves   tubes
  • EF80
  • 6L43
  • 2 x 6L41
  • 11TA31
    Voltage regulator
  • 14TA31
    Voltage regulator
  • 3 x ECC82
    Morse keyer
  • 300-A4
    Transmitter 20W
  • 300-B5
    Power Amplifier 200W
  • 300-C3
    Antenna tuner for 300-B
  • ?
    Antenna tuner for 300-A
The 300 A and the 300 AB were used in the following countries:
  • Angola
    1961 (300A)
  • Congo
    1961 (300AB)
  • Kurdistan
    1965-1968 (300AB)
  1. Anonymous, 300-A transmitter and accessories - THANKS!
    Transmitter kindly donated by anonymous former user. July 2015.

Further information

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Crypto Museum. Created: Wednesday 19 August 2015. Last changed: Thursday, 08 October 2015 - 20:47 CET.
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