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UHU   BN-48
BND AM/CW backup receiver

The BN-48 was a small fully-transistorized receiver, developed in the mid-1950s by Wandel & Goltermann in Reutlingen (Germany) for the German Intelligence Service, the Bundesnachrichten­dienst (BND). It was introduced in 1958 and was commonly supplied as an additional (backup) receiver for the SP-15 spy radio set. The official designator is BN-48 but the receiver is also known as UHU. Many thanks to Jim Meyer [1] for supplying additional information and images.
 
The unit measures approx. 145 x 95 x 40 mm and is housed in a metal hammerite-painted enclosure. The case is water resistant and rubber gaskets are used to protect the circuitry from moisture. The receiver is primarily built around the AF125; one of the earliest Germanium HF transtors that were also used in its 'big brother' the FE-8 receiver (BN-58). Circuit diagram here.

The image on the right show a typical UHU receiver [1]. On the top surface are the frequency scales. All connections are at the rear, whilst the controls are on the side panel at the right.
  
BN-48 (UHU)

The receiver covers a frequency range from 2 to 9 MHz, divided over two frequency bands (2 - 5.1 MHz and 5.1 - 9 MHz). The band selector is a three-position switch which also acts as the power switch. In the center position, the receiver is switched OFF. The unit is powered by a single 9V battery (at the left) and has no external power connection. To save power, the dial light is operated by a momentary push-button. A pair of headphones are connected to the rear panel.
 
Antenna and ground wires are connected to the 2.5 mm banana-type sockets at the rear panel. A couple of meters each will be sufficient for good reception. The free running Variable Frequency Oscillator (VFO) is not as stable as the later synthesizer-based alternatives, but it is good enough for the reception of the AM and CW signals (the famous number stations) that were operated by the German intelligence service BND (Bundesnachrichtendienst) during the Cold War.

Despite its simple circuit, the receiver has a remarkable sensitivity and an accurate scale.
  
BN-48 with earphone and antenna

For the 2nd IF stage, a 455 kHz ceramic filter is used. For the reception of CW signals (morse), the built-in Beat Frequency Oscillator (BFO) can be switched ON by pulling the volume knob out­wards. Approximately 600 UHU/BN-48 receivers were built for the BND by Wandel & Goltermann [3]. The receiver would still be useful today, for example as part of an amateur QRP station!
 
BN-48 (UHU) BN-48 (UHU) The BN-48 straight up. The ideal position for operation. BN-48 (UHU) top view BN-48 (UHU) controls Adjusting the volume Switching ON the BFO
Two earphones connected simultaneously BN-48 with earphone and antenna BFO switch and volume adjustment Power switch and band selector Power switch and band selector. The green band (5.1-9 MHz) is selected here. Unlocking the switch Tuning the BN-48 Switching on the scale light
BN-48 seen from the rear Connections at the rear panel BN-48 controls Battery compartment Battery compartment open Battery compartment with standard 9V battery installed Earphone connected to the BN-48 Antenna and ground connected to the BN-48

 
SP-15 spy radio set
The UHU receiver was sometimes supplied as an extra receiver with the SP-15 spy radio set. The image below shows the suitcase version of the SP-15 in which the UHU is strapped-in in the right half of the case (at the front left), next to its 'big brother' the FE-8 (BN-58) receiver.

Briefcase version of the SP-15 with the UHU in the right half of the case

Also in the right half is a power supply unit (PSU) and an RT-3 burst encoder. The latter was used to send messages in morse code at very high speed in order to minimize the risk of interception and detection. The left half of the case contains the usual components of the SP-15 set: the mains transformer, a battery adapter, the transmitter and the headphones and cables.

 More about the SP-15

 
Interior
The BN-48 (UHU) is housed inside a purpose-built die-cast aluminium enclosure that is painted grey hammerite. The interior can easily be accessed by removing the top and bottom panels, each of which is held in place by 6 small screws. Attention: Be careful when removing the top panel.
 
The power switch/band selector at the bottom right, has a spring-loaded tip that might be ejected when the top panel is removed. Remove the tip and the spring and store them safely.

Looking at the interior from the top, most of the space is taken by the two frequency scales. A small light bulb at the centre illuminates the scales when the momentary switch at the side is pressed. The rest of the space is taken by the controls and the electronic components that are mounted on two PCBs at the bottom side. The battery is located outside the main frame.
  
HF amplifier and tuning capacitor

Turning the unit over, reveals the bottom side of the receiver. At the centre is the large tuning capacitor that is operated by the tuning dial at the left. Just below the tuning capacitor are four adjustments, accessible through four holes in a metal cover. These are for calibrating the upper and lower ends of each band. At the bottom left is the power switch/band selector. Note that high-quality epoxy PCB material is used, which is quite remarkable given its age (1958).
 
BN-48 interior, top and bottom panels taken off BN-48 interior - top BN-48 interior - top, seen from the rear BN-48 interior - bottom view BN-48 interior - bottom view Interior - top view Interior - bottom view Circuit diagram inside the bottom panel
Spring-loaded lock on the power switch lever HF amplifier and tuning capacitor Volume potentiometer and BFO switch Headphones socket (top right) Close-up of the audio amplifier PCB bottom view Ceramic filter (blue/gold) Scale light

 
Circuit diagram
The image below shows the circuit diagram of the BN-48 which is printed inside the bottom cover of the case. The circuit is powered by just 9V and only two different types of early Germanium transistors are used: the AF125 (PNP) for the HF sections and the BCZ10 (NPN) for the LF stages.


From the antenna at the top left, the signal is fed through a bandpass filter onto the HF amplifier (T1). The amplified signal is then mixed (T3) with the signal from the local oscillator (T2). The IF signal is first amplified (T4) and then passed to (T5) via a 455 kHz ceramic filter. The optional signal from the BFO (T6) is inserted at the base of T5. The output from T5 is first detected (OA95) and then amplified to earphone level (T8 and T9). The output from the detector is also used to drive the Automatic Gain Control (AGC, T7). Audio volume adjustement is at the bottom left and affects the AGC feedback line.

Half of the tuning capacitor is at the bottom of the local oscillator. It works in tandem with the band filter capacitor at the top left. This allows the band filter to be as narrow as possible, making the receiver much less prone to interference from strong signals on adjacent channels.
 
Accessories
The BN-48 came with the following accessories:
 
  • Rechargeable 9V NiCd battery
  • Mains battery charger
  • Reel with antenna and counterpoise wires
  • Earphone
Frequency ranges
  • 2 - 5.1 MHz (yellow)
  • 5.1 - 9 MHz (green)
References
  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Detailed technical description of the FE-8 (BN-58) in German and English.
    Website QRZ.COM. Personal correspondence. Retrieved December 2013.

  2. UHU circuit diagram
    Original circuit diagram mounted inside case lid [1].

  3. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN: 0952063-36-0, September 2004.

Further information

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Crypto Museum. Created: Monday 12 January 2015. Last changed: Sunday, 02 October 2016 - 09:29 CET.
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