LORENZ FuG10 EL and EK

I recently updated my collection with two German aircraft receivers, the Lorenz FuG10 EL and EK (FunkGerät 10 Emfänger Langwelle, Empfänger Kurzwelle). They were bought on eBay in Germany and they both are in very good condition externally and internally, they are both working. I am now restoring, checking and aligning them. I will describe the FuG10 radio system, my receiver modules and the measures I had to take to get them in top condition. I will use the receivers to listen to utility stations on the longwave band and broadcast stations on the tropical bands.

1. HISTORY

During the years 1936-37 Lorenz AG developed the aircraft radio system FuG10 for Luftwaffe. From 1939 it was the preferred system in all large aircraft with several crew members like Heinkel He 111, Messerschmidt ME 110, Junkers Ju 88 and Dornier DO 217.





The bomber and reconnaissance aircraft Heinkel He111, the most common German bomber during the war, was using the FuG10 radio system, consisting of the long- and shortwave transmitting modules SL and SK and the corresponding receiving modules
EL and EK complementad by aerial switching and tuning units, power converters and fuse units.

When the war ended Lorenz AG and many licensees had manufactured close to 300.000 units!

2. FuG10 EL

Data
The receiver is a single superhet with one RF- and two IF stages without AVC. It is designed for telegraphy (CW) only and covers the frequency range 300 - 600 kHz long wave. It uses the IF frequency 140 kHz and has a narrow bandwidth, damping 1000 at +- 3 kHz. The receiver uses 8 valves of type RV12P2000 and is very sturdily constructed in three cast metal chassis modules. The designation EL means Empfänger Langwelle, the receiver measures 21 x 20 x 20 cm and weighs around 7 kg.

My Receiver
The receiver has the following text on the type plate (riveted metal plate):
E 10 L
Lieferer: C. Lorenz A.G.
Gerät-Nr    124-109 B-1
Werk-Nr    103766-41
Anforder Z    Ln 26593
Bauart Lorenz     Herst Lorenz

Number 124-109 stands for FuG10 EL and B-1 means the receiver is temperature compensated and has a built in dial light. Werk-Nr is a serial number that was given by the checking authority at the time of delivery checkout and it implies the receiver was manufactured in 1941. A stamp on the back of the RF/IF chassis shows the number 68963-42, so this seems to be a spare unit. Ln means Luftnachrichtentruppe and 26593 designates FuG10 EL. The receiver was designed by Lorenz but in this case they have also manufactured it.

The receiver is in a very good untouched condition inside, complete with the dial lamp assembly and the bolts for securing it in the aircraft, details that are often missing! The front cover and the case had some dents and some of the knobs were scratched. Two flags in the fixed frequency system did not work properly. Otherwise all controls, tuning, volume, beat oscillator and fine tuning worked well.

Restoration
The dents in the front cover and case were straightened out. The frequency flag unit was exchanged for one from a spare receiver. The resistance at the heater and high tension inputs was measured:
Heater +BB to MBB to -BB showed 7.0 and 8.7 ohm. +BB to -BB showed 14.1 ohm.
Anode +Ae to Ebf showed 33.4 kohm. All values seemed good.

Applied heater voltage 2 x 13.5 V gave the current 0.32 A.
Anode voltage 200 V was applied, the anode current was 14 mA. The dial light worked, no strange phenomenons were observed. Thus the "smoke test" was successful! Head phones (4000 ohm) were connected at the front connector and an antenna at the back. The receiver worked fine, the beacons Saab OL and T as well as Malmen LCF were heard!

The dial showed + 10 kHz in both ends and in the middle, indicating that the tuning capacitor was not properly aligned to the dial and had to be realigned in the gearbox. The frame with the dial was loosened, Saab OL at 421 kHz (approximately midscale) was tuned in, the dial was set to the same frequency and the frame was fixed again without disturbing the tuning capacitor.

The oscillator was aligned with the coil at 320 kHz. The alignment at the upper end was done by adding a 3.3 pF NP0 capacitor (neutral temperature coefficient) over trimmer C21 because this was stuck and I did not want to risk destroying it! The added capacitor can be seen in white below the trimmer top left on the photo. The photo shows the oscillator stage to the left, the mixer stage in the middle and the RF stage to the right. Below is the AF module. The dial was after alignment exact at 580 and 420 kHz, and shows 3 kHz high (+ 0.8%) at 320 kHz, which is a very good result!

With the beat oscillator valve removed the top of the IF curve was measured at 141.8 kHz, with the output 0 V (not visible) at 143.5 and 140.0 kHz, which is + 1.7 and - 1.8 kHz. High selectivity and even side bands! No appreciable frequency drift was noted on the beacon Saab OL 421 kHz which was monitored for 1 hour from cold condition! Now the RF stage and the mixer were aligned, both coil slugs and trimmers were movable. The sensitivity increased considerably and many NDB:s could now be heard. The sensitivity will be measured later.

When tapping the chassis I heard loud "crashes". The capacitor C11 in the RF stages was found to be too close to the chassis shorting the anode voltage. After bending the leads the fault disappeared. The sound in the head phones was intermittent, found to be caused by oxide in the AF valve socket. All valve sockets were sprayed with Oscillin and the valves were removed and reinserted a couple of times to clean the sockets.

After bettering the paint on the cover and case the receiver was ready for the final test and photography. The receiver is now in top close to original condition!

3. FuG10 EK

Data
The receiver is a single super with one RF and three IF stages with AVC. It is intended for both telephony and telegrahy, and it covers the frequency range 3 - 6 MHz shortwave. It has the IF frequency 1460 kHz, damping 1000 times at +- 18 kHz. The sensitivity is 4 uV for 10 V out in 4000 ohm. The receiver uses 11 valves of type RV12P2000 and is very sturdily constructed in three cast metal chassis modules. The designation EK means Empfänger Kurzwelle, the receiver measures 21 x 20 x 20 cm and weighs around 7 kg.

My receiver
The receiver has the following text on the type sign (printed paper):
E 10aK
Gerät-Nr    124-108 C
Werk-Nr
Anforder Z      Ln 26594-1
Bauart Lorenz     Herst

Type number 124-108 C means FuG10 EaK, the a means the receiver is volume regulated (AVC) and has built in dial light.

Werk-Nr is a serial number which was given by the checking authority at the time of delivery checkout and it is missing here. Ln means Luftnachrichtentruppe and 26594-1 designates FuG10 EaK. The receiver was designed by Lorenz but no manufacturer is given. Stamped on the back of the RF/IF chassies and on the front module is the serial number 20602.

Towards the end of the war metal and manpower were scarce, so simplifications were made, for example in this case the type sign is printed paper. The Germans were also more careful to reveal how many units were built and where. Thus the Werk-nr and manufacturer are missing on the type sign and no year is given!

The receiver is in a very good untouched condition inside complete with the dial lamp assembly and the bolts for securing it in the aircraft, details that are often missing! The front cover and the case have some dents and some of the knobs are scratched. All controls, tuning, volume, beat oscillator and fine tuning work well.

Restoration
The dents in the front cover and case were straightened out. The resistance at the heater and high tension inputs was measured: Heater +BB to MBB to -BB showed 7 and 8 ohm. +BB to -BB showed 14 ohm. Anode +Ae to Ebf showed 35 kohm. All values seemed good.

Applied heater voltage 2 x 13.5 V gave the current 0.47 A. Anode voltage 200 V was applied, the anode current was 17 mA.
The dial light worked, no strange phenomenons were observed. Thus the "smoke test" was successful! Head phones (4000 ohm) were connected at the front connector and an antenna at the back. The receiver worked fine, for example Radio Nederland at 5955 kHz was heard! The photo below shows the receiver during the voltage/current checks.

The slugs in the RF, mixer and oscillator stages were all bottomed, indicating that the tuning capacitor was not properly aligned to the dial and had to be realigned in the gearbox. Slugs and trimmers were put in a middle position, the frame with the dial was loosened, and the signal generator was tuned to 4500 kHz (approximately midscale). The dial was set to the same frequency and the frame was fixed again without disturbing the tuning capacitor.

The oscillator was aligned with the coil at 3200 kHz. The alignment at the upper end was done at 5800 kHz with the trimmer C21. After repeating this a couple of times the dial alignment was very exact.

Now the RF stage and the mixer were aligned in the same way. All maxima were within the tunable range of slugs and trimmers. The sensitivity increased considerably and many broadcast stations could now be heard. The sensitivity will be measured later.

With the beat oscillator off (mode A3) the top of the IF curve was measured at 1460 kHz, with the output 0 V (not visible) at 1449 and 1471 kHz, which is +11 and -11 kHz. High selectivity and even side bands! No appreciable frequency drift was noted on Radio Nederland at 5955 kHz which was monitored for 1 hour from cold condition!

After bettering the paint on the cover and case the receiver was ready for the final test and photography. The receiver is now in top condition, very close to the original!

4. PHOTO GALLERY

FuG10 EL


Left: FuG10 EL restored

Right: The front module without cover, top right is the dial light assembly (brown) and the fixed frequency flags




Left: The shield covering the oscillator, mixer and RF stages (it is often missing in these receivers)

Right: The back with the serial number stamped on the RF/IF unit, below the AF unit with the power connector




Left: Top view. On the right from back to front are the RF, mixer and oscillator valves. In the middle from the front the first IF filter, the second IF filter and the antenna coil. To the left from the front back are the first IF valve, the second IF valve and the detector valve, all in screened compartments!

Right: The right side with from the left the oscillator stage, the mixer stage and the RF stage, below is the AF module



Above: The left side with from the right the first IF stage, the second IF stage and the detector stage
Below them is the beat oscillator with the valve removed



Right: The front module with tuning mechanism and dial. Under the dial
the cam discs that can be locked to any position of the dial to give four
fixed frequencies

Left: RF/IF unit seen from below with the tuning capacitor screen removed





Left: The AF module with the AF stage (right) and the beat oscillator (left)

Right: The double gears for the tuning capacitor


FuG10 EK


Left: FuG10 EK restored

Right: The front module, top right the holder for the dial light (brown) and the flags for the fixed frequencies



Above: The back with the serial number stamped on the RF/IF unit, at the bottom the AF unit with the power connector




Left: Top view. Bottom from right to left the RF, mixer and oscillator valves, in the middle the IF filters

Right: The right side with from left to right the oscillator, mixer and RF stages, at the bottom the AF module with two AF valves in parallell



Above: The left side with from left to right the first, second and third IF stages, below them the detector stage
At the bottom are the beat oscillator and AVC stages

Right: The bottom side with the screen for the beat oscillator

Links

Museum for historical military technology with a complete FuG10 installation
Helge Fykses radio collection with the FuG10
Peter Gierlach DF3KT user and maintenance manuals
Norsk Radiohistorisk Förening NRHF



More on WWII receivers

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Updated November 30th 2007