This year I have worked with a very interesting project, the restoration of a previously modified E52 Köln to get it as close to
its original state as practically possible. I want to share my experience from this project to inspire others to restore and keep this
and other examples of the great engineering skills that emerged during the second world war. The restoration is following a logical
pattern that can be used also for other restoration projects.
History
The receiver Empfänger 52 was specified by the Reichsluftfahrtministerium (RLM) in 1939 as one in a series of four similar
receivers covering the entire frequency spectrum from LF to UHF, intended for use in the Luftwaffe ground stations, but also for other
services and authorities. A request for quotation went out to a few German electronics manufacturers and Telefunken developed
and built a prototype of the short wave receiver Köln in the beginning of 1941. Telefunken was chosen to manufacture the series
of receivers according to their specification ”Luftboden-Empf.-Programm 2 – 7500 m für die Bodenausrüstung der deutschen Luftwaffe”,
Telefunken Berlin-Zehlendorf 5.5.1941. I succeeded to download a copy of this very interesting detailed document from ref. 1.
Out of four specified receivers E51 Leipzig (LF), E52 Köln (HF), E53 Ulm (VHF) and E54 Kulm (UHF) only Köln was built in large series.
In total around 2500 receivers were built from July 1942 to March 1945. The E52 was made in 10 models, of which the E52a-1
(simplified with motorized tuning) and E52b-2 (simplified with manual tuning and unified scale) were the most common. Of all the E52s
built there are probably 300 - 500 left, of these perhaps 50 in there original state (ref. 2 and 3).
The receiver is modularised, the modules are mounted in a moulded aluminium chassis and all connections between the modules are
through connectors in a backplane! Both the construction and the performance were far ahead of its time. Only one type of valve, the
RV12P2000, was used in the receiver except for the power rectifiers. The intention was that all modules and valves should be
interchangable in the field without the need for realignment. The first test series of around 20 receivers was made in Sachsenwerk
in Radeberg from July 1942 of the simplified type E52a-1 and possibly also the type E52b-1. The manufacture was not up to speed
until the beginning of 1943. The assembly and alignment was made in Sachsenwerk Niedersedlitz while the modules were made in
several different electronics industries (ref. 2).
Variants
The receiver is an advanced single superheterodyne with two RF stages, mixer/local oscillator, three IF stages, detector/BFO/AF
amplifier and an output stage for headphones, see block diagram (link below). As a total there are 6 tuned circuits in the
RF/mixer/oscillator part and the receiver has a very high sensitivity and selectivity. The IF stages have a 6-circuit filter plus 4 tunable
circuits in a variable crystal filter, the bandwith is variable from +/- 5 kHz down to +/- 200 Hz @-3dB with very steep slopes, damping
100 times at +/- 10 kHz in wide. The tuning has a coarse scale and a fine scale projected from a micro photo disc with very high
resolution and accuracy. The oscillator has very accurate temperature compensation and thus high stability. Frequency coverage
is 1.5 to 25 Mhz in five bands, the sensitivity for AM 3 - 5 uV, CW wide 1.0 uV and CW narrow 0.3 uV for 5 V output. Mirror frequency
damping is at least 50000 and the IF damping is better than 100000 at 1.5 MHz (the IF is 1 MHz). The stability is better than 30x 10E-6/degree C,
that is 30 Hz/MHz/degree C for a freerunning oscillator! Even today this is competitive against many modern receivers comparing
sensitivity, selectivity and stability.
Remarks: Ln = Luftnachrichtentruppe (RLMs catalogue number”)
21000 = receiver number, E52
x-n resp n = model
It should be pointed out that there is no absolute right or wrong when it comes to the original state of the different models. The receiver
was continually changed and the borders between the models are floating. The modules were modified and exchanged between the
receivers for repair, there can be a-modules in b-, c- and d-models and vice versa. You can find components in the schematic diagram
for E52-b2 for which there is a remark in the parts list ”erscheint nicht”, does not exist!
My receiver
My receiver is marked 21000-4, corresponding to the model E 52a-1, simplified with motorized tuning. It was sold after the war as
surplus to hams and I got it around 1960. It was then painted in green wrinkle paint, probably when demilitarised. When I restored it
I found that the original paint was the standard Luftwaffe black grey. The number of the scale unit is 774, a rather early unit, perhaps
the end of 1943, but the modules have high numbers and could be exchanged units. It is also possible that all modules had numbers
with four figures.
As I was young and foolish I modernised the receiver with new miniature valves in the RF-, mixer and AF-stages and added an S-meter
and a crystal calibrator. The real madness was the removal of the mechanics of the motorised tuning to reduce weight! I dissembled
the receiver and painted it in grey hammer paint. When assembled it had lost its alignment and did not seem very sensitive, probably
due to misalignment of the tuning condensers. The receiver then was stowed away because of lack of time (read family!). The first photos
in the photo gallery show the receiver before restoration, rather neat but not in its original state and with bad performance.
