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Philips ZO-47
Compact spy radio set

ZO-47 is a compact modular valve-based spy radio set, developed by Philips/NSF 1 shortly after WWII, around 1947, for use by the Dutch stay-behind organisation O&I. The first 100 sets were delivered in early 1948 [1], followed by an improved model — ZO-47/01 — in 1952. It is the only spy radio set ever developed by Philips. In the 1950s, ZO-47 was succeeded by American sets.

A full set consists of three same-size units: a transmitter, a receiver and a power supply unit, each housed in a black metal container with a lid that is held in place by a leather strap. A fourth container holds the spare parts and accessories.

The crystal-operated transmitter is suitable for CW (morse) in the 2.5-9 MHz frequency range, and delivers an output power of 4 to 4.5 Watts. It is operated either with the built-in morse key, or an external one. The receiver has a a VFO, and is adjustable from 3.2 to 7.2 MHz. It is suitable for A3 signals (AM, phone) and A1 (CW, telegraphy).
  
ZO-47 receiver, transmitter, PSU and accessories

The three units are interconnected by means of short cables that are fixed to one of the units. The set is powered from the 110/220V AC mains, or from a 6V DC source, such as a car battery. The ZO-47 was used by the Dutch stay-behind organisation O&I for the transmission of text-based messages — containing only the 26 letters of the Latin alphabet — that were encrypted with the so-called One-Time Pad (OTP). When used correctly, the OTP cipher is unbreakable.

The first 100 sets were delivered to O&I early in 1948. A second batch of the slightly improved ZO-47/01 was delivered a few years later. The ZO-47 is probably the only Dutch spy radio set ever developed. In the late 1950s, it was succeeded by American and later also by German radio sets. The existence of the ZO-47 remained unknown to the public until two complete sets were found in a 'forgotten cache' 2 in Utrecht (Netherlands) in 1992. After the incident, all remaining sets were collected and destroyed. The set shown here is one of the very few surviving ones. 3

  1. NSF was a wholly-owned Philips subsidary.  More
  2. A cache is a secret storage place of weapons, ammunition and (radio) equipment that can be accessed by its controllers in case of an emergency. Caches are often related to clandestine activities.
  3. Only two complete sets are known to have survived.

Four identical cases ZO-47 receiver, transmitter, PSU and accessories ZO-47/01 spy radio set ZO-47/01 spy radio set ZO-47/01 spy radio set ZO-47/01 spy radio set Aceesories and spare parts Low-impedance headphones
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Four identical cases
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ZO-47 receiver, transmitter, PSU and accessories
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ZO-47/01 spy radio set
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ZO-47/01 spy radio set
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ZO-47/01 spy radio set
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ZO-47/01 spy radio set
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Aceesories and spare parts
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Low-impedance headphones

Crypto Museum would like to thank the Dutch Army's Signals Service Historical Collection for supplying the documentation of the ZO-47 and the ZO-47/01, and allowing us to make them available on this web page. We are also indebted to Wim Kramer — who had been able to research a ZO-47 from a cache find in 1992 — for sharing his experiences, publications and photographs, and for allowing us to reproduce them here [2].
Features
The ZO-47 consisted of four equally sized metal boxes with a lid on top. The lid was held in place by a leather strap that ran through a 'gutter' at the centre of the lid, and had a so-called Click button at the end. The four boxes were finished with black wrinkle paint and contained a power supply unit (PSU), transmitter (TX), receiver (RX) and a collection of accessories and spare parts.

Click to see more

The diagram above shows the various features of the complete ZO-47 radio set. At the right is the Power Supply Unit (PSU) which should be connected either to the AC mains (127 or 230V) or a 6V DC source. The PSU is connected to the transmitter by means of a short power cable and the transmitter in turn powers the receiver via another short cable, which is visible at the lower left.

Metal container with lid and leather belt Leather belt with lock metal buckle with leather belt Crystal fitted in the crystal socket Internal morse key with adjustments AC Mains power cable installed 6V DC power cable installed Dummy load installed on the transmitter
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Leather belt with lock
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metal buckle with leather belt
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Crystal fitted in the crystal socket
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Internal morse key with adjustments
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AC Mains power cable installed
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6V DC power cable installed
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Dummy load installed on the transmitter

Versions
  • ZO-47
    This is the original version that was developed by Philips in 1947. It is built around Philips Rimlock 41 valves (tubes) and has the text on its front panels printed in orange. All plugs (including the AC and DC power connectors) have their cable entry at the top. It came with a small brown 8-page illustrated booklet that has the model number Z.O. 47 imprinted at the front cover [A]. The radio sets that were found in a forgotten cache in Utrecht in 1992, and there were destroyed before the investigation was concluded, were of this type [2].

  • ZO-47/01
    This is an improved version of the above one, built with Rimlock 42 valves. It was released several years later and has some mechanical and electrical improvements. Furthermore the text on the front panels is printed in white and a much more elaborate manual with full circuit diagrams was supplied. The set featured on this page, is of this type. More...
Version ZO-47   ZO-47/00
The initial version of the radio set was known as ZO-47 or ZO-47/00 and was developed in 1947. The first batch of 100 units was delivered early in 1948. It is believed that there are no surviving examples of this version, although two complete sets were found in a forgotten cache in 1992. 1

The image on the right shows the condition of the set when it was discovered in 1992 [4]. As it had been hidden under the floor of a basement for several years, it was badly corroded and not operational. Nevertheless, many of its features and electronic circuits were still recognizable.

The set is built with Philips EAF41 and ECH41 Rimlock valves, with exception of the EBL21 valve in the PA of the transmitter and an AZ41 dual rectifier valve in the PSU. Many of the passive components in the transmitter and the receiver are from standard (civil) production [2].
  
Click to see more

The LT and HT voltages are delivered by the PSU via a short fixed cables with a straight 3-pin Philips connector at the end. These connectors were also used on professional audio equipment of the era. The set was supplied with a pair of American R-30-U military headphones that had been manufactured by Woolcot in 1945, along with a collection of other accessories. Surprisingly, no external morse key was found with the radio set, suggesting that the internal key was used by default. The set came with a simple 8-page instruction booklet with funny illustrations [A].

 More about the forgotten cache

  1. Both sets were destroyed in 1992, after the investigations had hit a dead end, along with a large number of other surplus radio sets that had been used by the clandestine network.

Version ZO-47/01
Several years after the first 100 radio sets of the above type had been delivered to the secret organisation, another batch was ordered from Philips. As newer valves and even semiconductors had meanwhile become available, it was decided to issue some improvements to the design.

Instead of the EAF41/ECH41 valves, the newer EAF42/ECH42 were used. Furthermore, the AZ41 rectifier valve inside the PSU was replaced by a Selenium variant that was manufactured by AEG. The vibrator unit and the transmitter's EBL21 PA tube had survived from the original design. The basic circuit design had remained unaltered.

Other improvements of the internals are the use of professional components instead of civil parts and better mounting of the neon lamp inside the transmitter. Although small, such improvements generally enhance the equipment's reliablilty.
  
ZO-47/01 spy radio set

The text on the front panels of this variant was screen printed in white, whereas it had been orange on the initial version. Another change to the exterior was the use of right angle plugs for the power distribution, as a result of which the overall height of the set was reduced when it was in full operation. This would have been more convient when using the set from inside a suitcase.

The ZO-47/01 came with a well-written 63-page manual with full operating instructions, circuit descriptions, alignment procedures and full circuit diagrams [B]. It is currently unknown when this variant was released, but it is likely that this was several years after the introduction of the initial version. Judging from the manufacturing codes this might have been between 1951 and 1954.


History
During WWII, The Netherlands had two independent intelligence agencies, both of which operated from England. The first one was Bureau Inlichtingen or BI (Intelligence Bureau). It was controlled by the Dutch Government in exile (London) and was similar in nature to the British MI-6. The BI was tasked with gathering military, political and economic intelligence, mainly on Dutch territory.

The other special agency was Bureau Bijzondere Opdrachten or BBO (Special Operations Bureau). It operated directly under (and was largely controlled by) the British Special Operations Executive (SOE). The main tasks of the BBO were to setup a resistance organisation, and to carry out sabotage actions in occupied Netherlands.

The BI was led by Colonel J. Somer who – before the war – had already started the formation of a secret section of the Dutch General Staff under the name GS III C. During a war, the main tasks of GS III C would be active intelligence gathering (espionage) and stay-behind. By 1939, Somer had succeeded in setting up GS III C, but it was too late to be of much use during the war.

Once the war was over, Somer rejuventated his GS III C section, albeit in a less ambitious form than he had anticipated. His GS-III-C section was actually the post-war variant of the wartime BI. For the stay-behind activities of his section, Somer needed communication equipment. After using a variety of Army Surplus (mainly from Belgian stores), he managed to secure a budget of NLG 20,000 (EUR 9100) for the development of the first and only Dutch spy radio set: ZO-47.
  
Colonel Jan Marginus Somer

The order for the development was given by Somer in 1946. Development by Philips/NSF took most of 1947 and by the end of that year the radio was ready for its first field test. In early 1948, the first batch of 100 units was delivered. By that time however, Somer had already left GS-III-C to become head of intelligence in the Dutch Indies. He was succeeded by J.J.L. baron van Lynden.

 History of the Dutch stay-behind organisation O&I
 More about Jan Marginus Somer


Forgotten cache 1
The explosive case of Parkstraat 3

In the first week of August 1992, the police in Utrecht (Netherlands) got a phone call from the owner of the house at Parkstraat 3. He intended to sell the house, but wanted the police to make an investigation first, as there was a persistent rumour that the were explosives hidden inside it.

The police took the matter seriously and, using a metal detector, discovered some metal packages hidden under the floor of the basement. As the packages appeared to be badly corroded, they called in the national bomb squad (EOD), 2 who took over the job of digging out the packages.

The EOD found six 9 mm FN-Browning pistols, a Welrod suppressed pistol, a box with a number of hand granades, ammunition, detonators and finally two complete radio sets with spares and accessories, spread over several metal boxes. It were thought to be leftovers from World War II.
  
The recovered radio sets on the floor of the police's workshop

It was rumoured that the house had been used by the German Sicherheitsdienst (SD) during the war, and that they had probably hidden the weapons there. But when the EOD closely examined the weapons, they came to the conclusion that they were British-made and not German. From that moment on, the police assumed that the weapons were used by the resistance and had been confiscated by the SD during the war. But when the investigation turned out that the SD had used the house at number 2 — opposite the current one — this theory had to be abandonned as well.

In the meantime, the arrival of the bomb squad had attracted the attention of the public and the Dutch national press. As a result, the discovery appeared in the headlines of several newspapers the next day — 6 August 1992 — and even in the 8 o'clock news on national TV (NOS Journaal).

On 11 August, the news was picked up by Wim Kramer, a freelance author for the Dutch Radio Amateur Magazine (RAM), who immediately contacted the Utrecht Police with the request to be allowed access to the discovered equipment. And against his expectation, he was admitted.
  
A complete ZO-47 radio set after some cleaning

The equipment had been stored temporarily in the workshop of the Utrecht Police, where their weapons expert had already succeeded in partly restoring some of the guns. Resistance experts were still discussing the wartime role of the resistance with members of the investigation team.

When Kramer arrived, the radio sets and their accessories were scattered all over the floor of the workshop as shown in the first photograph of this section. Kramer had expected to find war time British spy radio sets, such as the large B2 suitcase radio and the smaller A3, but to his great surprise he had never seen them before.

After recognising the typical 3-pin Philips power connectors and checking the numbers on the spare valves, he arrived at the conclusion that this was not a wartime British radio set, but far more likely a post-war device built by Philips.
  
Wim Kramer at work in the workshop of the Utrecht Police in August 1992 [4]

Kramer was allowed to further investigate the equipment. After cleaning the units as good as possible, he wrote an excellent 'educated guess' of how the equipment might have worked, and was allowed to publish a detailed description in RAM of October 1992 [2] in the hope that a member of the public would recognise the equipment and come forward with an explanation.

In response to the article, the author received about 20 phone calls, most of which hinted to a secret hitherto unknown organisation nicknamed GLADIO, 3 but neither of them had recognised the set and could not provide additional details.

The involvement of a secret organisation could explain the presence of the eight codebooks that had been found with the radios: four blue ones and four with a red linen cover. The blue books were labelled DECODEREN (decoding) whilst the reds books were titled CODEREN (coding). Each of the books contained a set of numbered pages. The books had been damaged considerably by moist, but fortunately they were still readable.
  
The eight codebooks that were found with the equipment

Each page contained 100 five-letter groups that were separated by pre-perforated lines, much like a sheet with stamps. The 'stamps' were organised as 10 rows by 10 columns, and each one could be torn from the page individually. It was clear that these books were one time pads.

Despite the best efforts of the investigation team, Wim Kramer, members of the public and the press, the results of the investigation were inconclusive and the spooky case hit a dead end.

As the equipment was about 45 years old, it was assumed that the people responsible for it had long since died. And to make matters worse: the district attourney had decided 4 that all weapons and equipment that had been discovered, were to be destroyed. Despite several requests from museums and private collectors, that is exactly what happened: all evidence was destroyed...
  
Single page from a codebook

Or was it? When Wim Kramer visited the Dutch Signal Corps Museum five years later, in 1997, he stumbled upon a ZO-47 set in near-mint condition in one of the display cases. Apparently a few years earlier the officer responsible for the destruction of the equipment, had exhibited a great sense of history. On his way to the blast furnices in IJmuiden (Netherlands), where the sets were to be incinerated, he dropped off several unique radio sets that had once belonged to O&I.

  1. Story by Wim Kramer, published in Radio Amateur Magazine (RAM) of October 1992 (Dutch) [2]. Translated and partly reproduced here by kind permission and cooperation from the author.
  2. EOD = Explosieven Opruimingsdienst (Dutch national bomb squad).
  3. In the preceeding years, several (inter)national incidents had raised public awareness of a secret stay-behind organisation in The Netherlands, which was confirmed in 1990 by (then) Prime Minister Ruud Lubbers. Shortly after the Parkstraat discovery in 1992, Lubbers officially dismantled the organisation.
  4. It is still a mystery why the district attourney (Dutch: Officier van Justitie) wanted the equipment to be destroyed, but it is possible, if not likely, that the DA had received instructions from someone 'higher up'.

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Using the ZO-47
AC or DC mains
In most situations it was recommended to power the ZO-47 directly from the mains. As the radio was intended for use by the stay-behind organisation (SBO), this means that it had to be suitable for all mains networks in The Netherlands [7]. This was not straigthforward however, as the country had a wild variety of mains voltages (127V and 220V) and even a mixture of AC and DC.

Today all mains power lines in the world are AC, but at the time the ZO-47 was issued (1948), some parts of The Netherlands still had a DC network. 1 As the power supply unit of the ZO-47 is not suitable for DC mains networks, it had to be used on a 6V DC car battery in these areas [A]:

  1. Ameland (Nes and Hollum),
  2. Arnhem (partly)
  3. Boxtel
  4. Groningen (city, partly)
  5. Rotterdam (partly),
  6. Schiermonnikoog
  7. Utrecht (city, partly)
  1. AC = Alternating Current, DC = Direct current.

Battery operation
In places with a DC mains voltage, but also in areas without a mains network (fields, woods, etc.), the ZO-47 could be powered from a 6V DC source, such as the battery of a car. In some cases, a suitable battery was supplied with the set, so that it could be used in virtually any situation.

When connecting a battery to the ZO-47, the AC mains cable has to be removed as it otherwise interferes with the 6V rail. The polarity of the 6V DC connection is not crytical and may be reversed, as the internal vibrator pack converts it to an AC voltage anyway.




Parts
Power Supply Unit (PSU)
PSU
Transmitter (TX)
TX
Receiver (RX)
RX
Metal container with accessories and spare parts AC mains power cable (220V)
AC
DC battery power cable (6V)
DC
Wire antenna Ground wire with clamp
External morse key Headphones Crystals Dummy load (artificial antenna) Spare parts Toolkits
Power Supply Unit   PSU
The Power Supply Unit (PSU) is the heaviest of the three modules. It can be powered either by the AC mains or by a 6V DC source, such as the battery of a car, and converts it into 6V AC for the filaments and 300V DC for the transmitter, which in turn converts it into 250V for the receiver.

When powered from the AC mains, the correct input voltage (126V or 230V) has to be selected, by placing the mains fuse in the correct fuse holder under the rengtangular 'bulge' at the front edge. Spare fuses are also stored there.

The output of the PSU is available from a short fixed cable with a right angle 3-pole female plug at the end. On the initial version of the ZO-47, a straight plug was used for this purpose, but this increased the total height of the radio set when all cables were connected, and was probably a cause for intermittent (cable) contact problems.
  
ZO-47 power supply unit

Inside the PSU is a conventional transformer with two primary windings (to allow for 110V and 20V AC mains voltages) and a so-called synchronous vibrator pack, that was used to convert the 6V DC (battery) voltage into an AC voltage that could be 'stepped-up' with the transformer.

Transmitter   TX
The transmitter is suitable for the 2.5 to 9 MHz frequency range and is quarz crystal operated. It accepts the usual crystal sizes, with a pinning distance/diameter of 12.5/2.5 mm and 19/3 mm respectively, that should be inserted into the socket at the bottom left, close to the power cable.

The crystal can be used at its fundamental frequency (i.e. the frequency that is printed on its body) or at the 2nd harmonic. In the latter case there will be a power output penalty, as the frequency doubler causes some loss. It is there­fore advised to use the fundamental frequency.

At the top left is the band selector. It selects the correct tap on the transmitter's tuning coil and should be set as close to the desired frequency as possible. With the correct crystal installed, the antenna tuning knob at the bottom right should tuned for maximum output of the PA stage.
  
ZO-47 transmitter

This is done with help of the tuning indicator (lamp) at the top right. The knobs should be adjusted for maximum light. Once the optimum is found, the ring of the indicator lamp can be turned to reduce the amount of emitted light. The transmitter can be used in always-on mode (Z), or in break-in mode (BI). The latter requires separate RX and TX antennas to be installed.

The transmitter is only suitable for the sending CW-signals (morse). This can be done either with the built in morse key, or with an external one that is connected to the socket marked SLEUTEL (key) to the right of the crystal socket.

An external morse key was not supplied with the set, so we have to assume that the operator was required to bring his own preferred key, or that the built-in was used by default. Despite its rather high position, the quality of the internal key is actually quite good. It can be adjusted with the two screws at the radio's front edge.
  
Operating the built-in morse key

One of the major issues when sending in CW, is the appearance of so-called key clicks, a clicking sound that can be heard in regular broadcast receivers in the vicinity of the transmitter, when the user presses down the morse key. Such clicks are unwanted as they might reveal the presence of the clandestine radio station. The transmitter of the ZO-47 contains a filter to avoids key clicks.

Note that when using an external morse key, the pins of its banana plugs should be no longer than 10 mm. When longer (i.e. regular) banana plugs are used, they will engage an integrated switch that permanently enables the oscillator. According to the manual [B] this was reserved for special applications, which probably means that it was provided for connection of a burst keyer. 1 This feature was only present on the ZO-47/01 and was omitted from on the initial 1948 ZO-47.

The transmitter is powered by the PSU by connecting the short fixed cable of the PSU to the 3-pin socket of the transmitter. It provides the transmitter with 6V AC (LT) for the filaments, and 300V DC (HT) for the anodes. The transmitter converts the anode voltage to 250V, which is available on the fixed cable at the bottom left. This cable should be connected to the receiver's power socket.

  1. A burst keyer is a device that allows a pre-recorded message to be sent in morse code at high speed, in order to evade interception and radio direction finding. Also known as burst transmitter or burst encoder. When sending at high speed it is generally necessary to turn the transmitter's oscillator ON permamently.

Receiver   RX
The receiver is the final unit in the power chain. It receives its 6V AC (LT) and 250V DC (HT) power lines from the fixed cable of the transmitter. A wire antenna of approx 6 metres long (shorter than the 20 m TX antenna) and a similar counterpoise, should be sufficient for a good reception.

The antenna and counterpoise are connected at the top left. A high-impedant or low-impedant pair of headphones should be connected to the banana terminals at the bottom left. A suitable pair of headphones was supplied with the radio.

The receiver can be tuned freely over the entire frequency range between 3.2 and 7.2 MHz in a single band. It has a frequency tuning window with two scales: one that has been calibrated in MHz, and a linear scale from 0 to 100. The latter can be used to accurately mark the position of a station, so that it can be found back more easily.
  
ZO-47 receiver

The receiver is suitable for the reception of Amplitude Modulated (AM, or A3) signals, as well as CW (A1, or morse). In the latter case, the BFO has to be turned ON by turning the BFO knob clock­wise. The knob can then be used to adjust the signal ± 2 KHz, for optimum tone quality.

Accessories and spares
The radio set came with a variety of accessories and spare parts, such as power cables, antenna wires, crystals, spare fuses and spare valves, that were neatly packed inside a metal container.

The container is simular to the other units, and has a removable lid that is held in place by a leather strap. Inside the lid is a storage bay for up to 20 crystals, plus an artificial antenna. Spare light bulbs and fuses are packed in a small carton box that is also stowed in the container.
  
Aceesories and spare parts

Mains power cable
The cable shown in the image on the right was supplied for connecting the PSU to the 110V or 220V AC mains. The cable has a 5-pin female plug that mates with the mains power socket at the top right of the PSU's front panel.

This type of connector is extremely rare and was only briefly manufactured by Philips subsidary Nederlandse Seintoestellen Fabriek (NSF).

 AC connector pinout

  
AC Mains power cable

Battery power cable
The ZO-47 could also be powered by a 6V DC source, such as the battery of a car, 1 in which case the AC power cable had to be removed and the DC power cable shown in the image on the right had to be installed in the 7-pin DC socket.

Like the AC connector, this 7-pin plug was briefly manufactured by Nederlandse Sein­toe­stellen Fabriek (NSF) and is extremely rare. 2

 DC connector pinout
  
6V DC power cable

  1. At the time of the ZO-47, most civil cars had a 6V battery.
  2. This connector was missing from the ZO-47 set in our colection, but could be reproduced from donated spare parts of an old NSF-device in the collection of Arthur Bauer.

Antenna
A metal spool with 20 metres of isolated copper wire, was supplied as the transmit antenna. The wire has a banana plug at the end, that mates with the transmitter's antenna socket.

A separate 6-metre piece of isolated copper wire was used as the receive antenna. It should be connected to the antenna socket on the receiver.
  
Antenna wire on spool

Ground wire
For a proper operation of the transmitter, the ground socket should be connected to ground (earth). This is especially the case when using a ¼λ antenna, instead of the proposed ½λ one.

A suitable ground wire is supplied with the set. It consists of a short 3-metre wire with a banana plug at one end, and a clamp at the other one. The clamp should be fitted to an appropriate ground connection, such as the water supply.
  
Ground cable

Morse key
No external key was found with the set featured on this page, but it is unlikely that the internal key was used at all times, as its position is rather unpractical. It is possible that the operator was given the option to bring his or her own key.

In most cases, this would have been a British, American or German leftover from WWII, for example the German Ta.P. Wehrmacht key that is shown in the image on the right. It's connector directly fits the SLEUTEL socket to the right of the crystal. Note however that the pins of the connector should not be longer than 10 mm.

  
Ta.P. Wehrmacht morse key

Headphones
The ZO-47 is suitable for both high-impedant (2000 Ω × 2) and low-impedant (150 Ω) head­phones, by connecting them to the appropriate termials at the bottom left of the front panel.

The standard headset (shown here) consists of two R-30-F earpieces, held together by a canvas head strap (missing here), allowing it to be worn under a helmet. Alternatively, the R-30-U in-ear variant with metal bracket could be used.
  
Low-impedance headphones

Crystals
The crystal socket on the transmitter accepts two types of crystals, both of which were common at the time. The socket accepts crystals with a pin distance of 12.5 mm and a thickness of 2.5 mm, but also the larger variant with 3 mm thick pins and a pin-distance of 19 mm.

Although it is possible to double the crystal frequency, it is recommended to use crystals of 2.5 - 9 MHz at their fundamental frequency. Up to 20 crystals can be stowed in the metal lid of the accessory container.

  
Examples of Crystals that were used with the ZO-47

Dummy load
Tuning the transmitter for maximum output whilst the wire antenna is connected, raises the chance of detection and discovery by means of Radio Direction Finding (RDF), leading to the operator's arrest and endangering the mission.

To minimise the time on-air, the dummy-load, or artificial antenna, shown in the image on the right was used during the tuning stage. Once the transmitter was properly tuned, the real antenna was reconnected. The dummy load is stowed inside the top lid of the accessory container.

  
Dummy load

Spare parts
A small carton box with spare fuses and light bulbs was supplied with the set. It was usually stowed inside the metal accessory container.

Some of the larger lamps are wrapped in paper, whilst other are fitted in the cut-outs of a small wooden block. For an overview of the items in the carton box, please refer to this list.

 List of items

  
Spare fuses and lamps

Toolkit
Each set also came with a selection of tools – such as screwdrivers and pliers – divided over two small canvas tool bags. As both these bags and the tools are missing from the ZO-47 in our collection, we are unable to show a picture.

 List of items

  

Complete ZO-47 radio stations with accessories Power connectors Power sockets and fuse ZO-47 power supply unit Opening the voltage selector Power selector and protective lid Power selector and spare fuses Spare fuses with red dot
Mains voltage selector ZO-47 transmitter Selecting break-in mode (BI) Crystal fitted in the crystal socket Close-up of the built-in morse key and its adjustments Operating the built-in morse key Antenna sockets, Z/BI switch and tuning light Dimming the light
ZO-47 receiver ZO-47 receiver Accessory box with additional parts stowed in lid Aceesories and spare parts Accessories in storage case Crystals and fixating plate Crystal removed from the crystal bank Spares in carton box
Spare fuses and lamps Dummy load Dummy load installed on the transmitter AC Mains power cable AC mains plug 6V DC power cable 6V DC plug Ground cable
Antenna wire on spool Low-impedance headphones Low-impedance headphones ZO-47 headphones in-ear loudspeaker Headphones connector Connecting the headphones to the receiver Y-junction  with clothing-clamp
Examples of Crystals that were used with the ZO-47 Small crystal Large crystal
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Complete ZO-47 radio stations with accessories
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Power connectors
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Power sockets and fuse
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ZO-47 power supply unit
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Opening the voltage selector
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Power selector and protective lid
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Power selector and spare fuses
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Spare fuses with red dot
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Mains voltage selector
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ZO-47 transmitter
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Selecting break-in mode (BI)
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Crystal fitted in the crystal socket
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Close-up of the built-in morse key and its adjustments
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Operating the built-in morse key
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Antenna sockets, Z/BI switch and tuning light
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Dimming the light
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ZO-47 receiver
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ZO-47 receiver
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Accessory box with additional parts stowed in lid
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Aceesories and spare parts
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Accessories in storage case
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Crystals and fixating plate
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Crystal removed from the crystal bank
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Spares in carton box
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25 / 43
Spare fuses and lamps
D
26 / 43
Dummy load
D
27 / 43
Dummy load installed on the transmitter
D
28 / 43
AC Mains power cable
D
29 / 43
AC mains plug
D
30 / 43
6V DC power cable
D
31 / 43
6V DC plug
D
32 / 43
Ground cable
D
33 / 43
Antenna wire on spool
D
34 / 43
Low-impedance headphones
D
35 / 43
Low-impedance headphones
D
36 / 43
ZO-47 headphones
D
37 / 43
in-ear loudspeaker
D
38 / 43
Headphones connector
D
39 / 43
Connecting the headphones to the receiver
D
40 / 43
Y-junction  with clothing-clamp
D
41 / 43
Examples of Crystals that were used with the ZO-47
D
42 / 43
Small crystal
D
43 / 43
Large crystal

Interior
All three modules are housed in a nearly identical enclosure.

Power supply unit
The Power Supply Unit (PSU) can be opened by removing three bolts from the bottom of the case and sliding off the case shell (which can be rather difficult as it is a tight fit).

The image on the right shows the interior of the PSU. About half the space is taken by the large transformer. The two cylinders at the center are the vibrator, which is used when powering the PSU from a 6V source, and a Selenium rectifier. In the ZO-47/01, the Selenium cell replaces the AZ41 rectifier valve of the original ZO-47.
  
PSU interior

PSU removed from its case PSU interior seen from the front panel PSU interior PSU interior PSU interior Vibrator unit Selenium rectifier made by AEG Top view of rectifier and vibrator
E
×
E
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PSU removed from its case
E
2 / 8
PSU interior seen from the front panel
E
3 / 8
PSU interior
E
4 / 8
PSU interior
E
5 / 8
PSU interior
E
6 / 8
Vibrator unit
E
7 / 8
Selenium rectifier made by AEG
E
8 / 8
Top view of rectifier and vibrator

Transmitter
The design of the transmitter is pretty straight­forward. It consists of an oscillator, built around an EAF42 (or EAF41 in the earlier model), and a power amplifier (PA) with an EBL21.

When sending in morse (CW) using the built-in key or an external one, an internal filtering circuit avoids key-clicks. The anode voltage (HT) of both the oscillator and the PA are switched ON and OFF with the morse key, which limits the maximum speed. When using a burst encoder, the oscillator was turned ON permanently by a switch in the key socket.
  
Transmitter interior

Transmitter removed from its case Transmitter interior seen from the front panel Transmitter interior Transmitter interior Wiring detail Neon tuning lamp Wiring detail Transmitter detail
F
×
F
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Transmitter removed from its case
F
2 / 8
Transmitter interior seen from the front panel
F
3 / 8
Transmitter interior
F
4 / 8
Transmitter interior
F
5 / 8
Wiring detail
F
6 / 8
Neon tuning lamp
F
7 / 8
Wiring detail
F
8 / 8
Transmitter detail

Receiver
The receiver can be opened by removing a single bolt from the bottom, plus two at the corners of the control panel. It is the most complicated of the three units and is built around five valves.

The circuit comprises a single super heterodyne receiver, consisting of an RF amplifier, a variable frequency oscillator (VFO), mixer, IF amplifier, BFO and AF amplifier. All valves are EAF42, with the exception of the mixer, for which an ECH42 is used. The IF frequency is at 475 kHz.
  
Receiver interior

Receiver removed from its case Receiver interior seen from the front Receiver interior Receiver interior Bottom view Tuning capacitor and trimmers Tuning cogwheels behind the front panel Wiring detail
G
×
G
1 / 8
Receiver removed from its case
G
2 / 8
Receiver interior seen from the front
G
3 / 8
Receiver interior
G
4 / 8
Receiver interior
G
5 / 8
Bottom view
G
6 / 8
Tuning capacitor and trimmers
G
7 / 8
Tuning cogwheels behind the front panel
G
8 / 8
Wiring detail


Connections
Receiver power
The diagram below shows the pinout of the 3-pin Philips power input socket on the receiver. It needs four wires: the three pins, plus the shield which acts as the ground (0V). The pinout is shown when looking into the socket from the controls. Note the position of the reference mark.

  1. 6V AC (LT) for filaments
  2. +250V DC (HT)
  3. Sidetone
Transmitter power
The diagram below shows the pinout of the 3-pin Philips power input socket on the transmitter. It needs three wires: two of the pins, plus the shield which acts as the ground (0V). The pinout is shown when looking into the socket from the controls. Note the position of the reference mark.

  1. 6V AC (LT) for filaments
  2. +300V DC (HT)
  3. not connected
Battery power
The diagram below shows the pinout of the battery power socket. Note that the position of the notch on the plug should be at the bottom. Inside the plug there should be three loop wires as shown below. Pin 5 is unused. The 6V battery voltage should be supplied to two thicker pins (1 and 3). The polarity of the 6V DC voltage is not critical and may therefore be reversed.

  1. 0V
  2. Loop with 7
  3. +6V
  4. +6V
  5. not connected
  6. 0V
  7. Loop with 2
Mains power
The diagram below shows the pinout of the mains power socket on the PSU, when looking into the socket. The AC mains is connected to the two thick pins that are the furthest apart. The position of the notch on the plug should be at the top. Note that a loop wire has to be present between pins 2 and 3. It provides the necessary internal connection for the 6V rail.

  1. 127 - 220V AC
  2. Loop with 3
  3. Loop with 2
  4. 127 - 220V AC
  5. not connected
Specifications
  • Weight
    10 kg
  • AC power
    127 or 220V, 50 Hz
  • DC power
    6V battery (external)
  • TX frequency
    2.5 - 9 MHz (crystal operated)
  • RF power
    4 to 4.5 Watts
  • RX Frequency
    3.2 - 7.2 MHz
  • IF Frequency
    475 kHz
  • BFO range
    ± 2 kHz
Accessories   ZE-47/01
  • Dummy load
  • Neon bulb for antenna tuning
  • Wire antenna, approx. 30 meter with banana plug (transmitter)
  • Wire antenna, approx. 6 meter with banana plug (receiver)
  • Spool for antenna wire
  • Ground wire with plug and clamp
  • Frame for holding 20 crystals
  • Full circuit diagram (printed on metal)
  • Mains power cord
  • Battery power cord
  • Low-impedant headphones
  • High-impedant headphones
  • Carton box with spares
Spares
  • 3 x fuse 0.5A 1
  • 3 x fuse 0.8A 1
  • 3 x fuse 10A 1
  • 2 x lamp 12V/5W Philips 10/12 A7 - 12914 T, for dummy load 1
  • 4 x lamp for antenna current indicator 1
  • 1 x neon lamp for sidetone 1
  • Vibrator unit
  • Set spare valves (1 x EBL21, 1 x ECH42, 4 x EAF42)
  • Bag with banana plugs (3 x black, 3 x red)
  • Banana interconnector
  1. In small carton box.

Tools
  • Tube puller
  • Small screwdriver 1
  • Small pliers 1
  • Ferret drill 1
  • Side cutter 1
  • Pipe wrench 1
  1. Divided over two tool bags.

Valves   Tubes
 
PSU
Circuit ZO-47 ZO-47/01 Remark
Rectifier AZ41 - Replaced by AEG selenium rectifier
 
Transmitter
Circuit ZO-47 ZO-47/01 Remark
Oscillator EAF41 EAF42  
PA EBL21 EBL21  
 
Receiver
Circuit ZO-47 ZO-47/01 Remark
RF amplifier EAF41 EAF42  
Mixer ECH41 ECH42  
IF amplifier EAF41 EAF42  
BFO EAF41 EAF42  
AF amplifier EAF41 EAF42  

Missing items
Crypto Museum are still looking for the following items:

  • Spare valves: 1 x EBL21, 1 x ECH42, 4 x EAF42
  • Spare vibrator Philips 6V type 7948
  • 4 x white egg-isolator
  • Tools with bags
  • Canvas strap for headphones
Documentation
  1. Bedieningsvoorschrift van de zend-ontvanger type Z.O. 47
    Operating instructions for transceiver ZO-47 (Dutch).
    Philips, 1947. 8 pages, brown cover marked Z.O. 47.

  2. Handleiding Z.O. 47/01
    Manual with photographs and circuit diagrams (Dutch).
    Philips, date unknown but possibly 18 February 1954. 1
  1. Derived from date codes in the circuit diagrams.

References
  1. Herman Schoemaker, Een geheime organisatie in beeld
    De Nederlandse stay-behind-organisatie, geheim, onafhankelijk en zelfstandig?
    The Dutch stay-behind organisation, secret, independent and autonomous? (Dutch).
    Thesis under supervision of Prof. Dr. B.G.J. de Graaff, Utrecht University. 7 June 2013.

  2. Wim Kramer, Mysterie in Utrecht
    RAM Magazine 136, Oktober 1992. pp. 17-19. 1

  3. Wim Kramer, Mysterie in Utrecht na jaren opgelost
    RAM Magazine 191, Oktober 1997. pp. 32-35. 1

  4. Wim Kramer, Photographs of the equipment found at Parkstraat 3
    Personal correspondence. Received April 2015. 1

  5. Historische Collectie Verbindingsdienst, ZO-47/01 radio set
    Signal Corps Historical Collection (Netherlands).
    Retrieved February 2009 and May 2016.

  6. De Telegraaf, Dansen op explosieven in Parkstraat
    Dancing on explosives at Parkstraat (Dutch).
    Newspaper, 6 August 1992. Retrieved April 2015.

  7. Wikipedia (Dutch), Lichtnet
    Retrieved June 2016.
  1. Reproduced here by kind permission of the author Wim Kramer.

Further information
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Crypto Museum. Created: Friday 15 July 2016. Last changed: Tuesday, 17 March 2020 - 09:56 CET.
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