PA0SE (E52 part one)

pg 12a German WW2 Radio equipment
[Scanned by LA5FH]

Technical articles under preparation (received from PA0SE Dick Rollema):
This is the third of a series of three articles. The first one, German World War II Radio Equipment, appeared in RB65 and described the German technology with aircraft radio equipment FuG 10 as an example.
The second article was titled Lorenz. Short Wave Receiver Lo6K39a - The Ultimate Tuned Radio Frequency Set and was featured in RB72. We will end this trilogy with a description of a remarkable superheterodyne receiver, made by Telefunken. The photo below shows the radio. The name Köln (Cologne) was chosen as the project designation for what eventually became a short-wave communications receiver that may be considered as being the finest ever developed and built during the Second World War. The receiver was designed by Telefunken at the beginning of the war for the German military forces and represents a true example of the state of the art at the end of WWII.

10a Introduction to German World War II Radio Equipment (FuG10)
12a. German World War II Radio Equipment - Köln E52 receiver - part 1 (PA0SE Dick Rolema)
12b. German World War II Radio Equipment - Köln E52 receiver - part 2 (PA0SE Dick Rolema)
12c. Lorenz Shortwave Receiver Lo6K39a (Lo6L39), The ultimate TRF set [PA0SE]
12d. Telefunken World War II Superheterodyne Receiver Kw.E.a (Lw.E.a) [PA0SE]
12e. Telefunken World War II Universal TRF receiver Torn.E.b [PA0SE]

Planned page, some possible faults to be corrected when I have the manus returned

Tyrkleif de LA5FH (to be checked when LA6NCA returns the papers):

Under tittelen STRATEGIC...: 8x10-6
Under tittelen SPCIFICATION...: 30x10-6 (Den er plassert bakerst i del 1)
Omega rettet til ohm

Telefunken World War II
Superheterodyne Receiver E52 'Köln'

In the years just before the outbreak of WWII German industry, as well as their military principals, were convinced that at least in Germany the art of building communications receivers for frequencies up to 30MHz had reached a point where no further technical development of importance was to be expected in the foreseeable future. Therefore, instead of allowing a further proliferation of receiver types, which could bring little more than minor improvements in size, weight or control convenience, the German Air-Transport Ministry decided in 1939 to introduce a new receiver for the Luftwaffe (Air Force) terrestrial communications that should become the standard communications receiver.
The exacting specifications for the radio set called for a truly advanced product that, besides showing supreme electrical qualities capable of passing the test of many years to come, also had to be of sound and uncomplicated mechanical design. Particular emphasis was placed on production by an industry that possibly could be forced to strategically spread over different geographical locations.
Thereupon, Telefunken, the most important German manufacturer of radio equipment at the time, and being one of the tenderers for the ambitious project, developed the prototypes of four receivers. They were all similar looking in outward appearance and code-named after the German towns of Leipzig (40-1600kHz), Köln (1.5-25MHz), Ulm (2468MHz), and Kulm (60-150MHz). Early in 1941, entering their second year of the war, the Luftwaffe accepted the Köln, whilst the receiver was also recommended to be adopted by the Army, the Wehrmacht. Consequently, the Köln went into production, later followed by the Ulm. The Leipzig (a long-wave double superhet with 2MHz and 130kHz IF), and the Kulm, never made it to the production line, as far as known. Thus, with their Köln, soon registered under the Luftwaffe nomenclature of 'E52'. Telefunken introduced 'The Ultimate Communications Receiver' as it was intended to remain for many future years. And it certainly did, as turned out later.
Taking stock of electronic technology as it existed immediately after the war in 1945, there could be little doubt that the E52 was still one of the most advanced radio receivers of its time. Even during many years following WWII, nowhere in the world, so it seemed, was there a series-produced article to be found that could match this receiver. Today, of the approximately 2500 Köln (E52) short-wave receivers produced, only a rather limited number are still in existence, although in most cases perfectly serviceable and cautiously guarded by some of the more fortunate connoisseurs of German radio equipment. These relatively few precious sets miraculously survived the war, as well as the succeeding mania of senseless destruction of 'former-enemy radio equipment' as ordered by some of the Allied military commanders after the German capitulation. However, what does appear to have been definitely lost, unfortunately, are many details of an impressive research that stood at the beginning of the highly advanced technology that made it all possible.

In the first article of this series (RB65) it was shown in what aspects German radio technology was different from what was more or less standard in other countries. They can all be found in the Telefunken E52. We repeat them here:
1. Instead of a metal chassis, a die-cast frame of a special aluminium alloy was used. Into this frame, modules housing different parts of the electronic circuitry could be bolted or clicked together. The modules could be manufactured at the most suitable production site, even being tested and calibrated there.
2. Coils with iron dust cores or of silver windings burned onto a ceramic former.
3. Ceramic capacitors with controlled temperature coefficient.
4. Limited number of valves.
The first article also described why the Germans favoured tuning by means of variable oscillators over crystal control. This aspect is so important that we pay some attention to it again.
In September, 1936, Hitler launched his '4 Year's Plan', thus making the German economy subordinate to a general preparation for war. It was Goering who received the virtually unlimited power for realising this plan. Also for the electronic production this led to important consequences, like the reminder to the German industry of its dependence on foreign basic materials.
One of these was quartz. Brazil, being the sole world quartz producer of importance, was geographically too far away from Germany for a reliable supply to be secured in times of war. Consequently, the industry took up the challenge of finding new solutions, avoiding the use of quartz where possible, nevertheless aiming at meeting the strict frequency stability specifications as laid down by their military customers. What eventually emerged was a development that did not lead to what one could easily have expected to become an ersatz solution. Instead it turned into a victory of electronic hardware technology. Temperature-compensated variable oscillators of exceptional frequency stability were the result. Though primarily developed to replace quartz crystals in transmitters, the technology was also applied to variable oscillators used in superheterodyne receivers like the E52.

The antenna (Figure 1) is followed by a double-tuned band-pass filter that drives the first RE amplifier. Next comes another double-tuned band-pass filter, feeding the second RE amplifier. Then follows a single tuned circuit and the mixer that converts the incoming signal to the 1MHz IF. The mixer also receives the signal from the local oscillator. The five preselector circuits and the local oscillator are tuned by the sections of a six-gang variable capacitor.
Directly following the mixer is a fixed tuned IF filter with no fewer than six circuits. Next comes the first IF amplifier, a crystal filter with continuously variable bandwidth, the second IF amplifier, again a crystal filter, the third IF amplifier and a single tuned circuit that feeds the detector circuit for audio and AGC voltage. When used for reception of telegraphy the signal from the beat frequency oscillator (BFO) is also fed to the detector. The oscillator frequency can be adjusted from the front panel; but it can also be controlled by a quartz crystal on 1000.9kHz. When crystal controlled, the oscillator also provides calibration signals. Finally, an audio amplifier brings the signal up to headphone level. We will now take a closer look at the details of the E52 receiver.

Not unlike many other Wehrmacht receivers at the time, the RF preselection of the E52 is impressive. No fewer than five tuned circuits, divided over two band-pass filters and a single tuned circuit, precede the mixer stage. This extremely high RF selectivity was not unusual in German communications receivers. It results not only in an exceptionally high 'second-channel' or image frequency suppression (thus blocking those unwanted RF signals at a different frequency which happens to produce the same IF), also the receiver shows no trouble with any 'intercept point' which today seems to occupy the mind of so many receiver manufacturers. In fact, it would not be difficult to demonstrate how the RF section of the E52, working on its own, would exhibit sufficient selectivity for use as a high-quality straight Tuned Radio Frequency (TRF) receiver. And that in spite of the slightly unequal bandwidth of the RF band-pass filters, owing to the unavoidable frequency-dependent coupling factor of the tuned circuits.
It is not impossible that this comprehensive RF pre-selection in military receivers stems from a historically grown prejudice. It appears that in pre-war German military circles a certain feeling existed against the principle of the superheterodyne receiver, mainly because of the inherent danger of this second-channel interference. It is known, for example, how in particular the German Kriegsmarine (Navy) objected persistently against the introduction of the superhet, even until as late as around the outbreak of WWII. (Incidentally, this mistrust led to some spectacular designs of TRF receivers, as for instance the Lorenz L06K39a, discussed in RB72). The anxiety could also stem from the fact that on shipboard, several transmitters and receivers can be in operation at the same time which could easily result in spurious responses of a superheterodyne receiver. In a TRF receiver this possibility is almost absent. It is also rumoured that the Kriegsmarine feared that the radiation of the local oscillator in a superheterodyne could be used by the enemy to locate the receiver by means of direction finding. But we have found no confirmation of this in the available literature. If there was a way to prove that second-channel interference in superhets could be reduced to harmless levels, the factory specification for the E52 of better than 94dB at 20MHz (worst case) was convincing indeed. Likewise, the IF suppression was highly effective: at 1.5MHz (worst case) better than l00dB. The ganged tuning capacitors, mounted along the full width of the main frame, show high precision and stability. For the six-ganged variable capacitor, including the section for the local oscillator unit, at any position over the entire range between 0 and 180 degrees, the deviation in capacity between any of the sections does not exceed 0.25 percent. The rotor plates of the capacitors are crimped onto a ceramic shaft, the stator plate units attached to the metal frame by ceramic stand-offs. This permitted a compact construction and, in spite of the consequent smaller spacing between the plates, production tolerances could be reduced and the temperature coefficient improved. The five tuning capacitors for the RF-input circuits are divided into two groups, with the dual 90° mechanical coupling to the main shaft of the receiver tuning unit in between them. The conical gear wheels are, as a matter of course, of the spring-loaded anti-backlash type.
The way the coils are arranged still betrays something of the traditional preferences of the Wehrmacht radio designers for rotating coil turrets rather than using RF bandchange switches with their inherent contact and spuriouscapacity problems. Coil turrets were a German speciality and many examples can be found of Wehrmacht receivers containing such masterpieces of coil-rotating mechanisms. In the E52, however, no turret is used, and it proves that Telefunken could also produce switches of excellent quality. Thus, in the E52 the contacts are not situated around the coils, but the coils are placed around the switch contacts. The shaft of the band switch runs along the full frame width, and has been extended at the left-hand side of the cabinet. This permits a mechanical coupling with switches in a possible receiver attachment like, for example, a direction finder. In a similar manner, the tuning-capacitor shaft can mechanically drive the loop-tuning capacitor in the direction-finder tuning unit.

The die-cast frame of the tuning module contains the variable local oscillator capacitor, two electric motors, a highly accurate mechanical 'memory' for storing receiver frequencies, as well as an unusual frequency read-out device where the frequency scale is displayed on a glass screen situated at the top of the receiver cabinet. The memory, and the method of frequency read-out, must he one of the most sophisticated designs created for a military radio receiver in WWII. See Fig. 2. The tuning knob drives, via a mechanical reduction, a sturdy main shaft which controls directly the critical elements. The crucial point about the design is that any backlash between the local oscillator capacitor and the tuning indicators is simply impossible. The reason for this is that the pointer for the 'coarse' scale (the half-round disc just above the tuning knob), the glass disc containing the 'fine' scale, inside the module, and the rotor plates of the tuning capacitor, are all, directly and immovably, fixed onto the same main tuning shaft.
It is this particular concept of using light as a medium, and avoiding any form of mechanical translation, that permits the highly magnified and yet intrinsically rock-solid and precise 'fine' frequency read-out, which makes the E52 such a unique radio receiver. The frequency scale is projected onto a small rectangular ground-glass screen (illuminated from behind) located above the tuning knob and 'coarse' indicator. The total absence of backlash, combined with the highly effective temperature compensation of the associated tuned circuits, as well as an individual frequency calibration at the factory to remove the last component tolerances, result in an accuracy that, certainly in its time, was unheard of.
The information of the frequency scales of all five bands is stored on the outer 6mm rim of a glass disc of about 100mm diameter. This glass disc is safely mounted inside the sturdy framework of the module. The extremely fine print on the disc was achieved by using microfilm techniques. The information is projected by a narrow light beam passing through the selected portion of the glass rim (depending on the position of the band switch) via a lens. The light originates from a low-voltage projection lamp (located just under the projection screen) and is, before it passes through the glass disc, focused by another lens towards the rear of the module where it is reflected by a mirror. Thus, not only a complete projection system has been realised in a compact space, even the projection lamp, a dispensable item like the valves, can he replaced without opening up or moving the radio set.

The precision and stability of the frequency scale of the E52 may he considered as being the best that can he achieved in a mass-produced mechanical analogue system. Yet, each of the five projected frequency scales - of which at any time only a section of a few centimetres is displayed - represents a full length of more than 1.8 metres! Of course, the extreme accuracy of this superb opto-mechanical system had its price. At the factory, for each receiver the projection disc had to he individually calibrated and photo-printed. Resulting from this calibration, of each disc four identical copies were produced. One was mounted into the tuning module of the receiver to which it belonged, another copy was safely hidden against the backwall of the receiver cabinet as a spare, a third copy was kept at the factory, while the last one was stored at a central military depot in Germany. In this way, the frequency-scale information on the glass disc, the precious heart of the tuning mechanism, was considered to be best safeguarded against damage or loss. What a highly expensive and elaborate system it must have been, only possible in a situation where an industry, set out to produce the very best possible, irrespective of any reasonable economical consideration, knew itself fully backed by their plutocratic principals.
There were limits, though. With the war progressing, and suffering from the effects of the increasing bombardments reaching ever deeper into Germany, the E52 receiver production also began to feel the stress and had to yield to certain compromises. At a later stage of the war, the system had to be simplified.
It seems that at Telefunken, the aim for the utter technical limit in the production specifications more than once brought the industry into deep trouble. Instances occurred where the specifications turned out to be too highly strung, once the production was on its way. Permitted tolerances which general have a tendency to even out, do sometimes add up, and in these cases there was little or no room left for correction. Solving the consequent problems required tremendous efforts during the manufacturing process and indeed, it would appear that certain supply bottlenecks were not always unwelcome as they provided the justification for the inevitable modifications necessary for keeping the production flowing. Eventually, at the main production lines the luxury of turning out individually calibrated frequency scales for the E52 had to be abandoned, and later models had to be issued with universal scales, based upon a limited number of typical 'average' calibration patterns (these scales can be recognised by their black-on-white print, instead of white-on-black). It speaks for the overall production tolerances that the accuracy of even these simplified receivers nevertheless remained extraordinary high.

It is one thing to produce a frequency read-out with a high resolution, but this is of little use if the calibration is not maintained with changing temperature. In RB65 it was told that the German firm Herinsdorf-Schomburg-IsolatorenGesellschaft, also known as Hescho, in the 1930s managed to produce ceramic capacitors with a controlled temperature coefficient. These made it possible to compensate the variations with temperature of inductance and capacitance of the components in an oscillator circuit. The local oscillator temperature compensation in the E52 is, as customary in most Wehrmacht receivers, effected by a combination of capacitors with a negative temperature coefficient, both in parallel and in series with the tuned circuit of the local oscillator, in order to make the compensation correct at all positions of the tuning capacitor. Each compensating capacitor is composed of several tubular capacitors, connected in parallel, selected for their (colour-coded) temperature coefficients. These capacitor-batteries are physically located at strategically chosen positions in the frame to take into account both frameend ambient temperature variations. The many capacitors together represent a large exposed surface with fast reaction time, while the distribution of their different temperature coefficient figures shape the correct compensation.

As an aside: Has the strategic significance of these industrial developments ever been evaluated by Allied Bomber Command when they were picking their targets? It was mainly owing to the spectacular progress in these technologies, far ahead of anywhere else in the world, that the German Kriegsmarine (Navy) could be equipped with compact transmitters with free-running variable master oscillators (with valves!) that demonstrated overall frequency stability specifications - without the use of quartz - of better than 8 x 10-6 °C. Or, for a better appreciation of this figure: it was this order of accuracy that enabled the German submarines to apply their tactics of 'blind' high-speed communication in foreign waters. This example of precision had to be performed during a surfacing manoeuvre, lasting only seconds, at an exactly pro-determined hour and frequency an operation that excluded any tuning correction as the idiosyncrasy of the German 'Enigma' ('Ultra') coding system could not permit the loss of any single code character during a transmission.

Part Two of this article will continue the in-depth examination of this exceptional receiver.

Specifications for the E52 'Köln'  
Frequency ranges 1480-3020kHz
Modes: A1A, A2A, A3
optional FM, SSB, TV
Power requirement 110-230VAC 50-60Hz, or 12VDC
Dimensions 245mm x 446mm x 350mm
Weight 40,8kg

AM (wide) 3,5µV
CW (wide) 1µV
CW (narrow) 0,3µV
Image rejection Better than 94dB @20MHz
IF rejection Better than 100dB
Antenna 60/150 ohm
IF 1000kHz
BFO: 1000,9kHz
continuously variable between
10kHz (-3dB), 26kHz (-60dB)
  and 200kHz (-3dB), 4kHz (-60dB)
Frequency stability better than 30 x 10 E-6 °C

Some possible additional pictures to be used, but I haven't the actual figures to use available at the moment:

Fig 1-2. Principle of frequency read-out system

fig 2-6 IF filter circuit