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Yugoslav spy radio set

RTP8-SSB and RTP8-SSB/3 were two nearly identical solid state spy radio sets, developed in the former Yugoslav Republic around 1970 and built by Institut za Elektroniku Telekomunikacije i Automatiku (IETA) 1 in Zagreb (Croatia). Both receiver and transmitter are crystal driven, with a frequency span of 3-7 MHz (divided over two bands). The receiver suitable for SSB and CW.
The set was available as a single-piece all-in-one unit (RTP8-SSB) and as a three-piece unit (RTP8-SSB/3). The latter is shown on the right. It consists of a transmitter (TX) with an output power of 6 - 8W, a receiver (RX) with an input sensitivity of 1µV, and a power supply unit (PSU) that was suitable for 12V DC and 110/220V AC.

The three-piece version was used for true agent communications and is probably the rarer one of the two. During the 70s and 80s, it was used for intelligence gathering inside Yugoslav Republic 2 and for espionage in the neighbouring countries.
Crystals and headphones fitted

The single-piece version measures 23.5 x 21 x 5.5 cm and weights just 3 kg. It is predominantly built with parts from the US, Germany and The Netherlands, made by companies such as Philips, RCA and Siemens. The three-piece version is slightly heavier and each piece has a different size, but is otherwise identical. The units are connected together by means of short power cables.

RTP8-SSB and RTP8-SSB/3 radios are extremely rare as, because of the Yugoslav Wars (1991-2001) [1], not many of them have survived. The one shown here, was used by the Croatian intelligence service in the years before the Yugoslav Wars and has miraculously survived. It is still in operational condition and has been used for amateur radio communication until recently.
  1. Institute for Electronics, Telecommunications and Automation. This company also built the Collins PRC-515 and many other radios under licence of the original manufactuers, and still exists today as Radio Industry Zegreb (RIZ).

Complete RTP8-SSB/3 set Three-piece version RTP8-SSB/3 Receiver and Transmitter Rear view of the three units with power cables in place Morse key and output power indicator RTP8-SSB/3 Crystals and headphones fitted Complete set, ready for use

  • RTP8-SSB
    This is the all-in-one version of the radio set. The front panel is comprised of the receiver (left) and the transmitter (right), whilst the PSU is fitted at the rear end of the transmitter. The internals are identical to the 3-piece version, but the wiring is somewhat different.

  • RTP8-SSB/3
    This is a three-piece variant of the above radio, consisting of a PSU, a transmitter and a receiver. Bacause of the smaller size of the individial pieces, it can be hidden more easily. The parts are connected together by means of short power cables.

The image below provides a quick overview of the controls on the 3-piece variant. Both transmit and receive frequency are controlled by crystals that are plugged into a socket at the front panel. Note that the receive crystal should by 9 MHz higher than the actual frequency. Two knobs on the receiver can be pulled-out to enable Attenuator and the Beat Frequency Oscillator (BFO). Audio output is available on a 3.5 mm jack socket to which the stethoscope headphones are connected.

The transmitter has two knobs that are tuned for maximum power output, using the meter as a power indicator. An external morse key should be connected to the 3.5 mm jack socket at the bottom left. Alternatively, the built-in morse key, mounted just below the meter, can be used. The radio is powered by a 12V DC source, either from the 110/220V AC PSU, or from an external battery. In the latter case, a push-button on the front panel of the transmitter can be used to check the voltage. As long as the button is depressed, the meter shows the DC power voltage.

Receiver and transmitter front panel

All text on the front panels of the units is in English, which was quite common for any Cold War spy radio set, regardless its origin. This was done for two reasons: first of all to fool the local police in the guest country, making them believe it is a British or American device, and secondly because the operator was often a foreign national who did not speak the language of the originating country. Note the incorrect spelling of ANTENNA on the front panel of the transmitter.
Three-piece version RTP8-SSB/3 PSU Receiver Receiver front panel Receiver rear panel Transmitter front panel Transmitter Transmitter rear panel

Operation of the RTP8 is pretty straightforward and similar to other spy radio sets. Unlike most other sets of the era however, the receiver does not have a VFO and can therefore not be freely tuned over the frequency bands. Instead it needs a dedicated crystal for the selected channel.
The receiver and transmitter should be wired to the PSU and the PSU should be wired to the AC mains. After selecting the appropriate channel, probably from a predetermined list or schedule, the crystals for that channel should be inserted into the receiver and the transmitter. Note that the matched TX and RX crystals are not identical.

Suitable headphones should be connected to the PHONE socket of the receiver. The receiver may now be turned ON by rotating the VOLUME knob clockwise. After a short delay, common radio noise should be heard through the headphones.

If no noise is heard, turn the RF GAIN knob fully clockwise and turn off the attenuator (ATT) by pushing-in the RF GAIN knob. Adjust the RX TUNE knob for maximum noise. This tells us that the RX front-end is optimally tuned for the selected frequency. The receiver is now ready for use.

Connect a suitable antenna and counterpoise directly to the terminals of the receiver or, if the receiver is powered via the transmitter, to the transmitter. In the former case, the transmitter needs its own set of antenna wires. Install a suitable crystal at the front of the transmitter.

When an suitable antenna is connected to the transmitter, press the morse key and adjust the TUNE and LOAD adjustments for a maximum meter reading. The transmitter is now ready for use and messages can be sent using either the built-in morse key (just below the meter) or an externally connected one (recommended). Although no external keys were supplied with the radio set, operators were encouraged to select the key of their choice and connect it to the transmitter.
A complete RTP8 radio station consists of the RTP8-SSB transceiver or, in case of the three-piece variant, of a transmitter, a receiver and a power supply unit (PSU), plus a number of accessories. Below is an overview of the parts that were usually supplied with the three-piece RTP8-SSB/3.
Power Supply Unit (PSU) 110/220V AC - 12V DC
Headphones RX and TX Crystals External morse key
12V DC power cables 110/220V AC power cable

Power Supply Unit   PSU
The Power Supply Unit (PSU) measures 16 x 6.5 x 5.5 cm and is powered either by the 110V or 220V AC mains. The correct voltage has to be selected with a slide switch at the bottom.

The PSU provides a secondary DC voltage between 12 and 13.5V, to power both the transmitter and the receiver. At the back are two identical DC outlets. The transmitter and receiver can be powered individually (by using both sockets), but the receiver can also be powered via the transmitter.

Receiver   RX
The receiver measures 23.5 x 8.5 x 5.5 cm and is powered either directly from the PSU or via the transmitter. In the latter case, a relay inside the transmitter controls the power to the receiver and also supplies it with a signal from the TX antenna. In practice it was commonly powered separately.

The receiver is crystal driven, which means that a suitable crystal has to be inserted into the socket at the front right. The frequency of the crystal has to be exactly 9 MHz higher than the desired receive frequency.

Transmitter   TX
The transmitter measures 18 x 12.5 x 5.5 cm and provides an output power between 5 and 8 Watt. Like the receiver it is crystal-driven, which means that a suitable crystal has to be present in the cystal socket at the front left. Ensure that the band selector (3-4 and 4-7) is in the correct position for the selected frequency.

After connecting a suitable antenna and inserting a crystal for the desired frequency, the transmitter has to be matched to the antenna. This is done by keeping the morse key depressed and using the TUNE selector in combination with the LOAD adjustement, to adjust the meter for a maximum reading.

The receiver of the RTP-8 delivers its audio output onto a 3.5 mm jack socket at the front panel. This is suitable for the connection of a standard earpiece from a portable radio.

In an operational context, the RTP8 came with the Sennheiser stethoscope-style headset shown in the image on the right.
Sennheiser headphones

In an operational context, the RTP8 was suppied with several pairs of crystals. Each pair consisted of a crystal for the transmitter (TX) and another one for the receiver (RX). Note that the frequency of the RX crystal has to be 9 MHz higher than the actual frequency, in order to match the IF. 1

The crystals were manufactured by Institut Mihailo Pupin (IMP) in Belgrade (Serbia), and were packed in small carton boxes, bundled in pairs (TX/RX) by means of cellotape.
Crystals supplied with the RTP-8

  1. Note that both crystals are marked with the TX frequency, followed by the letter T (transmit) or R (receive), e.g. 5833T and 5833R. In this example, the RX crystal is actually for 14833 kHz, whilst the TX crystal is for 5833 kHz. The RX crystal is always 9 MHz higher than the TX crystal.

Morse key
The RTP8 was generally used in combination with an external morse key that was connected to the 3.5 mm mono jack at the front panel of the transmitter. The choice of key was free, so that the operator could pick the one with the best hand. Quite often, old German Wehrmacht WWII keys were used, like the one shown here.

If the operator had forgotten his key, or when the external key was broken, the small internal key, below the meter on the front panel of the transmitter, was used as a temporary solution.
External morse key

Power cables
Two short (low voltage) power cables are supplied with the three-piece version of the set. They allow the transmitter and receiver to be connected to the power supply unit. In addition, the receiver can be powered via the transmitter.

The two cables are wired identically and each have 3 wires: two for the 12V DC voltage (plus and minus) and one for the connecting the TX antenna to the receiver. The wiring of the cables is given below. The RTP8-SSB all-in-one version was wired internally.
12V DC power cables

Mains power cable
The PSU can be connected to the AC mains (110 or 220V) via the 3-pin Hirschmann socket at the rear panel. A suitable cable that fitted the most common wall sockets in Europe, was supplied with the set, such as the one shown here.

For use on other networks, a separate cable had to be supplied or an alternative mains plug had to be fitted to the existing cable. Alternatively a common travel adapter could be used as well.
Mains power cable

Depending on the circumstances, the RTP8-SSB could be powered by the (internal) power supply unit (PSU) or by an external 12V DV voltage, such as the battery of a car. When used in the field, battery operation would be recommended, whilst in an urban environment the PSU was used.
The PSU allows the radio to be operated from the most common 110V and 220V AC networks. It produces a stabilized 12V DC voltage that drives both the transmitter and the receiver by wiring it up as shown in the image on the right by means of the two (identical) short 5-pin power cables.

Note that the PSU has two power outlets that are wired identically, but that we are using only one of them to connect the PSU to the transmitter's SUPP socket. The receiver's power socket is then connected to the transmitter's RX socket. This is done to let the receiver share the TX antenna.
Rear view of the three units with power cables in place

When the set is wired up this way, only the TX antenna has to be connected. An relay inside the transmitter, switches both power and antenna. When the transmitter is off (i.e. morse key up), the receiver is powered and the TX antenna is connected to the receiver via the 3-wire power cable. Using the morse key, turns the receiver off and connects the antenna directly to the transmitter.
The image on the right shows the internal wiring of the two power sockets at the rear panel of the transmitter. Two two sockets, here shown at the top left, are clearly wired differently with a black coaxial cable connected to the one closest to the edge. It should be connected to the receiver.

It is also possible to use separate transmit and receive antennas. In that case, the power socket of the receiver should not be connected to the RX socket of the transmitter, but to the spare outlet on the PSU. The receiver is then powered permanently and requires a separate antenna.
Transmitter rear panel wiring

This setup can be useful for full duplex traffic, or for split-frequency operation. The receiver can then be used to monitor a common calling frequency, whilst transmitting on a dedicated channel.
Another advantage of powering the receiver permanently, is that you can hear your own morse code when sending a message (sidetone). This is why it is the preferred configuration.

The image on the right shows the RTP8-SSB/3 in the preferred power setup. Note that both the receiver and the transmitter are directly wired to the PSU. In this case, transmitter and receiver each need their own antenna and counterpoise, and the receive antenna must have some distance from the transmit antenna to avoid damage to the receiver's front-end circuitry.
Recommended power setup

The three-piece version can also be powered directly from a 12V DC source, such as the battery of a car. In that case, a different set of cables is needed (not shown here).
Mains power cable 12V DC power cables Close-up of 12V DC power plugs Power cables connected between the three units Rear view of the three units with power cables in place Power socket
PSU Bottom view of the PSU Mains voltage selector External morse key Operating the external morse key Recommended power setup Close-up of recommended power setup Connections at the rear of the transmitter

All parts of the RTP8-SSB are easily accessible. Although we are showing the three-piece version of the radio here (RTP8-SSB/3), the circuitry inside the single-piece version is identical. The only differences are the connectors to the outside world and the internal wiring between the circuits.
PSU interior
The interior of the PSU can be accessed after removing the top panel by releasing just two bolts. At the heart of the PSU is a compact transformer that is visible at the centre. Looking inside the unit from the top, we can see the four rectifying diodes on one side of the transformer and a stabilizing circuit at the other side.

The bottom side just shows the 110/220V AC mains voltage selector and part of the compact transformer. The PSU produces a single 12V DC stabilized voltage.
PSU interior - top view

PSU interior - top view PSU interior - top view Rectifier Rear panel wiring detail PSU circuit Top view Bottom view Close-up of 110/220V AC voltage selector

Receiver interior
The receiver is neatly built and has circuits at both sides, each consisting of a single PCB that is slotted into a card-edge socket. The image below shows the top section that contains the RF stages and the crystal oscillator. Note that first-grade components have been used in the design.
The design has been kept simple by omitting a Variable Frequency Oscillator (VFO). As a result, the receiver can not be tuned freely over the entire frequency range, but needs a crystal to be inserted into the socket at the front panel. Note that the crystal frequency has to be 9 MHz higher than the desired actual RX frequency.

The image on the right clearly shows the high build quality of the unit, with an epoxy PCB and first-class components from West-European and American manufacturers, such as Siemens, RCA and Philips. Note the use of styroflex capacitors.
PCB in top section

The crystal oscillator is visible at the front left. It is wired to the crystal socket on the front panel. Note the use of a typical type of tunable ferrite coils or transformers as part of the tuned circuits of the receiver. The ferrites, with a blue plastic tip, are most likely manufactured by Siemens.
At the bottom are the IF stage, the crystal filter, the BFO the detector and the audio section, with CA3018 and CA3020 ICs from RCA. The quarz filter is made by KVG in Germany [2] and has a centre frequency of 9 MHz. The receiver delivers its audio output to a pair of Sennheiser head­phones that is connected at the front panel.

At the rear is the power socket and two press-terminals for the antenna and counterpoise. The power socket should be wired either to the PSU or to the transmitter. In the latter case, the antenna signal is supplied by the transmitter.
Receiver interior - bottom view

In the former case however, the receiver needs its own antenna and counterpoise. According to the manual [A], a wire antenna of approx. 8 metres should be suitable. It is connected to the red terminal. A suitable counterpoise (i.e. ground or earth) should be connected to the black terminal. A wire length of 8 metres is recommended for the counterpoise as well.
Receiver interior - top view Receiver interior - top view Receiver interior - top view Behind front panel PCB in top section Rear connections Receiver crystal oscillator Receiver detail
Receiver interior - bottom view Bottom view Bottom view of front panel wiring Bottom section detail Audio section Crystal filter - manufactured in week 47 of 1983 Power socket wiring detail

Transmitter interior
The transmitter is shorter than the receiver, but roughly twice as wide. The interior can be accessed by removing both the top and the bottom panels. Inside the transmitter is a metal sub frame that holds the PCB, the relay and a power circuit. The rest is mounted to the font panel.
The image on the right shows the top section that holds the actual transmitter on a single epoxy PCB. It is neatly built with most capacitors made by Philips and transistors from a variety of brands. Also note the four high-quality toriod transformers that are bolted to the PCB, and the two power transistors mounted to a sub frame.

The antenna matching circuit is mounted directly to the front panel and consists of a multi-tap toroid coil mounted to a 12-position rotary switch, and a variable capacitor for fine tuning, using the small meter at the left as an indicator.
Transmitter interior

The meter can also be used to check the voltage of the power source, with is particularly useful when driving the transmitter from a battery. As long as the push-button on the front panel is pressed, the meter shows the battery voltage. Mounted below the meter is the internal morse key.
The image on the right shows the bottom section of the transmitter, with the internal morse key clearly visible in the corner at the top. It is wired in parallel to the socket for the external key at the front panel.

A large electric relay is visible in the rear section. It is encapsulated in a piece of foam in order to reduce noise when sending a message in morse code. The relay enables the transmitter, disbles the receiver and switches the antenna between receiver and transmitter at the same time. In this case, only one set of antenna wires is needed.
Bottom rear view

According to the manual [A], two wires with a length of 8 metres each, are recommended for the antenna and the counterpoise. When the receiver is powered via the transmitter, only one set of antenna wires is required. If the receiver is powered directly from the PSU, a separate set of antenna wires is needed. They should not be placed too close to the transmitter's antenna wires.
Transmitter interior Transmitter interior - top view Transmitter PCB Antenna matching coil and load adjustment Power transistors Transmitter PCB Power transistor
Transmitter interior - bottom view Transmitter interior - bottom view Bottom rear view Morse key Power detector Power circuit Antenna and power relay Relay detail

Power socket   PSU
The image below shows the wiring of the power sockets, when looking into the sockets from the rear of the equipment. The minus-terminal of the battery or PSU should be connected to the two leftmost pins, which are also connected to the chassis (GND). The two rightmost pins carry +12V.

Power socket   RX
The RX socket on the transmitter and the SUPP socket on the receiver are wired the same as the sockets shown above, but have an extra wire connected to the middle pin that carries the antenna signal from the transmiter to the receiver. In the transmitter a coaxial cable is used for this:

Power cable
The radio comes with two short black power cables, each with a 5-hole plug at either end. The plugs mate with any of the power sockets on the PSU, the transmitter and the receiver. All plugs are wired identically, so that the cables can be swapped freely. The wiring is as follows:

Wiring of a power plug, when looking at the solder side.

Technical specifications
Power supply
  • Input
    110V or 220V AC (switch selectable)
  • Output
    12V DC
  • Current
    2A max
  • Frequency
    3 - 7 MHz (crystal operated)
  • Mode
    SSB, CW (morse)
  • Sensitivity
  • Audio
    10 mW
  • Current
    40 mA
  • Range 1
    3-4 MHz (80 m)
  • Range 2
    4-7 MHz (40 m)
  • Output power
    6 to 8 Watt
  • Mode
    CW (morse)
  1. Kratkovalni Radio Uredjaji RTP8-SSB i RTP8-SSB/3
    Shortwave Radios RTP8-SSB and RTP8-SSB/3.
    May 1970 (Croatian).

  1. Wikipedia, Yugoslav Wars
    Retrieved April 2015.

  2. KVG, Quartz Crystal Technology GmbH
    Retrieved April 2016.

Further information

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Crypto Museum. Created: Tuesday 29 March 2016. Last changed: Thursday, 21 April 2016 - 11:33 CET.
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