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RTP-8   SSB
Yugoslav spy radio set

RTP8-SSB, commonly abbreviated to RTP-8 was a single-piece spy radio set, developed around 1970 in the former Yugoslav Republic and built by the Institut za Elektroniku Telekomunikacije i Automatiku (IETA) 1 in Zagreb (Croatia). Both receiver and transmitter are crystal driven, and have a frequency span of 3-7 MHz, divided over two bands. The receiver is suitable for SSB and CW.

The radio was available as a single-piece all-in-one unit (RTP8-SSB), and as a three-piece variant (RTP8-SSB/3). The single-piece variant is shown here. It consists of a transmitter with an output power of 6-8W, a receiver with a 1µV sensitivity, and a built-in power supply unit that is suitable the 110/220V AC mains and a 12V DC source.

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.
  
RTP-8 seen from the front right

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 3 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. The company still exists today as Radio Industry Zegreb (RIZ).
  2. The Yugoslav Republic, also known as Yugoslavia, was a country in Southeastern Europe for most of the 20th century. Although it was a communist state behind the Iron Curtain, it was never part of the Warsaw Pact. After the Yugoslav Wars (1991-2001), it was dissoluted and split into six individual republics.

RTP-8 seen from the front left RTP-8 seen from the front right Front view Receiver section Crystals, meter and morse key Internal morse key Transmitter section Mains power cable
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RTP-8 seen from the front left
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RTP-8 seen from the front right
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Front view
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Receiver section
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Crystals, meter and morse key
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Internal morse key
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Transmitter section
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Mains power cable

Versions
  • RTP-8 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. The radio set featured here, is of this type.

  • RTP-8 SSB/3
    This is a three-piece variant of the above radio, consisting of a PSU, a transmitter and a receiver. Because 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.  More information
Controls
The image below provides an overview of the controls on the single-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.


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 the word ANTENNA in the top right corner.

RTP-8 seen from the front right RTP-8 seen from the front left Front panel Crystals, meter and morse key Operating the internal morse key Power input sockets Rear view Antenna terminals at the rear
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RTP-8 seen from the front right
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RTP-8 seen from the front left
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Front panel
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Crystals, meter and morse key
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Operating the internal morse key
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Power input sockets
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Rear view
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Antenna terminals at the rear

Operation
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 device can be powered directly from 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 transmitter crystal sockets at the front panel, left of the meter. 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, the typical radio noise should be heard through the headphones.
  
RTP-8 seen from the front right

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.

After connecting a suitable antenna to the terminals at the rear, 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.


Parts
A complete RTP8 radio station consists the following parts:

Single-piece RTP-8 spy radio set Headphones RX and TX Crystals External morse key
Key
110/220V AC power cable
Spy radio set   RTP-8
The RTP-8 shown in the image on the right is fully self-contained. It has a built-in mains power supply unit, but can also be powered by an external 12V DC source.

This version of the RTP-8 was typically used in fixed radio stations, or carried around as an ad-hoc clandestine station, for example built inside a common inconspicuous briefcase.
  
RTP-8 seen from the front right

Headphones
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

Crystals
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 radio set. 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

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

Power
Depending on the circumstances, the RTP8-SSB was powered by the built-in power supply unit 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 internal 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, switched – together with the antenna – by an internal relay.

A suitable wire antenna should be connected to the spring-loaded antenna terminals at the rear. When the transmitter is off (i.e. morse key up), the receiver is powered and the wire antenna is connected to the receiver. As soon as the morse key is pressed, the internal relay passes both power and antenna directly to the transmitter.
  
Power input sockets

The image above shows the power terminals in the rear left corner of the radio set. The square 3-pin socket at the edge accepts the mains power cable. The two banana sockets (red and black) are for connection of an external 12V DC power source. Above these terminals are the fuses.


Interior
The interior of the RTP8-SSB is easily accessible. By removing the top and bottom panels – each of which is held in place by four 3 mm screws – all internal parts can be reached, as shown below.

The image on the right shows the interior of the RTP-8, seen from the top, after both covers have been taken off. At the left is the receiver, whilst the transmitter is at the front right. The PSU is at the rear right, with the transformer just visible.

When comparing the RTP-8 shown here, with the three-piece variant, it becomes obvious that they are nearly identical. In fact, it is merely a metal frame with 3 individual units: the transmitter, the receiver and the power supply unit. The only difference is that in the single-piece version, the three units are connected by the internal wiring.
  
Interior - top

The radio's metal frame is roughly divided into three sections, that are separated by metal panels. The leftmost compartment holds the receiver, which consists of two PCB's mounted back-to-back, each slotted into a card-edge socket. The transmitter is located at the front right and consist of a single sided PCB, with the RX/TX relay mounted below it. The parts of the antenna tuner, are fitted between the transmitter board and the front panel. Further images below.

Interior - top Interior - top view Transmitter (top) PSU (top) Receiver (top) Power connections (interior) Rectifier diodes Antenna tuner (top)
Interior - bottom Interior - bottom view Receiver (bottom) PSU (bottom) RX/TX relay (bottom) Transmitter (bottom) Wiring of the receiver controls Relay wiring
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Interior - top
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Interior - top view
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Transmitter (top)
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PSU (top)
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Receiver (top)
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Power connections (interior)
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Rectifier diodes
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Antenna tuner (top)
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Interior - bottom
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Interior - bottom view
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Receiver (bottom)
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PSU (bottom)
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RX/TX relay (bottom)
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Transmitter (bottom)
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Wiring of the receiver controls
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Relay wiring

Technical specifications
Power supply
  • Input
    110V or 220V AC (switch selectable)
  • Output
    12V DC
  • Current
    2A max
Receiver
  • Frequency
    3 - 7 MHz (crystal operated)
  • Mode
    SSB, CW (morse)
  • Sensitivity
    1µV
  • Audio
    10 mW
  • Current
    40 mA
Transmitter
  • Range 1
    3-4 MHz (80 m)
  • Range 2
    4-7 MHz (40 m)
  • Output power
    6 to 8 Watt
  • Mode
    CW (morse)
Documentation
  1. Kratkovalni Radio Uredjaji RTP8-SSB i RTP8-SSB/3
    Shortwave Radios RTP8-SSB and RTP8-SSB/3.
    May 1970 (Croatian).
References
  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: Saturday, 03 February 2018 - 21:03 CET.
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