German World War II Communications Receivers
Technical Perfection From A Nearby Past
Part III of this elongated series concerns itself with the
Kw.E.a superheterodyne receiver which was a basic Telefunken design.
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1. avsnitt: un.....ld. Unleselig ord, jeg tÝr ikke tippe(!)
Kw.E.a front view (LA8AK)
KwEa seen from above (LA8AK)
BY DICK ROLLEMA, PA0SE
Part III now takes up with a 1938 design for a superheterodyne receiver. The Kw E a as you will see from the photos is a marvel of mechanical engineering. While PA0SE regales us with the theoretical aspects of the design, one can only imagine how that was transformed into the mechanical beauty we see un.........ld. Still, the thought of lugging 77 pounds of hardware around leaves a lot to be desired.
SHORT WAVE SUPERHET Kw E a
The first of the two superheterodyne receivers we are going to discuss is the Kw E a (Kw = "Kurzwellen" = short wave, E = "Empfanger" = receiver, a is serial indicator). This radio was designed by Telefunken and it became operational in 1938.
Photograph 8 gives you a first impression of what it looks like. Photograph 9 shows the receiver out of its cabinet and photograph 10 a rear view.
The dimensions of the radio are 69.2 cm wide, 27.4 cm high and 34.6 cm deep. It weighs 38 kg (77 lbs). Not directly a set to take on holidays.
The total frequency span of 1 to 10 MHz is covered in five ranges, as follows: 980-1610 kHz; 1560-2550 kHz; 2470-4060 kHz; 3940-6395 kHz and 6205-10000 kHz. Again the subbands are kept relatively narrow which improves the ease of tuning and the frequency read-out
Looking at photograph 8 you notice several by now familiar features. Left of the dial the band-selector control above ("Grob" = coarse) and main tuning control with crank ("Fein" = fine) below. The dial features the rotating mask we already met on the previous receiver with a slot that displays only the frequency scale in use. The mask also shows the limits of the selected range and the frequency increment that corresponds to one dial division, 10 kHz on range III, as seen in the photograph. The receiver uses the same filament type tube in all 11 stages. It is the type RV 2 P 800 pentode we also encountered in the Torn E b. The radio consumes 1.6 a. at 2 v. for the filaments and 15-20 ma. at 90 v anode current. The bottoms of the tubes with extraction knob can clearly be seen in photographs 9 and 10.
The simplified diagram in the instruction manual is too complicated to reproduce here. Therefore we present a block schematic diagram as fig. 8.
The mixer is preceded by two r.f. amplifier stages. There are five tuned r.f. circuits that are ganged to the oscillator tuning circuit. The user has the option to use a single or a double tuned circuit between the antenna and the first r.f. tube. Normally one circuit is used. But the manual says that when interference is experienced from a very strong near-by transmitter the second circuit between antenna and first r.f. tube should be brought into operation. The switch for this is the top one of the two controls at the right on the front panel.
The lower one of these controls is an antenna attenuator. Not a resistive one but a variable series capacitor between antenna and input circuit. To avoid detuning of this circuit a second section of the capacitor adds just as much capacitance in parallel with the tuned circuit as the series capacitor detracts. In other words the input attenuator is a differential capacitor. It is called "Ankopplung" (= coupling) on the front panel. There is also a series trimmer in the antenna that compensates for different antenna capacitances. It is set once and for all for a particular antenna and therefore has screwdriver adjustment (top right just to the left of the antenna connector in photograph 8).
The six sets of coils that have to be switched (or the five frequency ranges are mounted in a coil turret. This one is of particular beauty Photograph 11 shows the turret, taken out of the receiver, which is a simple operation. The turret is moved from one position to the next by means of a Maltese cross mechanism that can just be seen at the extreme left of the turret. But before the turret starts to rotate the contact fingers are lifted from their partners on the turret by means of a camshaft that can be observed in photograph 11 in front of the turret. When the turret has come to rest in the new position, the contact fingers are lowered onto the turret again. The fingers make a slight wiping movement when pushed onto the ring shaped contacts on the turret, thereby removing possible dirt deposits.
The receiver uses the relatively low intermediate frequency of 250.9 kHz. But because five tuned circuits are used ahead of the mixer the image response is sufficiently suppressed (on the order of 80 dB).
The oscillator is of the tuned anode circuit variety. A coil in the grid circuit is inductively coupled to it. A second coil, coupled to the anode circuit of the oscillator is in series with the coil that forms the r.f tuned circuit connected to the grid of the mixer tube. In this way the oscillator signal is injected into the mixer.
There are three i.f. amplifier tubes. They are preceded by double tuned i.f. transformers on 250.9 kHz.
The receiver offers the selection of seven different bandwidths, of which four are meant for telephone, the fifth, sixth, seventh and eighth are for c.w. only. The bandwidth in positions seven and eight is identical, but in the eighth position the b.f.o. is switched to the other side of the passband. The bandwidth selector control can be seen in photograph 8 directly under the meter.
The principle of the bandwidth variation is indicated in fig. 8 in simplified form. In positions 1-5 the bandwidth of the i.f. amplifier is changed by varying the coupling between the tuned circuits of the i.f. transformers. By going from position 1 to 5 the coupling capacitors between the tuned circuits are made smaller and smaller. This would also shift the center frequency of the passband slightly. But this is compensated for by adding extra capacity in parallel with the tuned circuits as the coupling capacitors become less. Also the damping resistors in parallel with the tuned circuits are increased in value as the bandwidth narrows. In position 5 no extra damping is used. In positions 6-8 of the bandwidth control the i.f. bandwidth remains the same but the a.f. bandwidth is reduced. This is done by a tuned circuit that resonates at 900 Hz between the detector and the a.f. final amplifier. In position 6 it is brought into the circuit but the response is broadened by means of a parallel resistor. In positions 7 and 8 the resistor is removed and the bandwidth is at its narrowest.
The b.f.o. is crystal controlled and works at a fixed frequency of 250 kHz, thereby generating a beat note of 900 Hz with the 250.9 kHz i.f. signal. The b.f.o. can be brought into operation by means of a separate switch. It is directly under the bandwidth selector switch. If in position seven interference is experienced the operator can go to position eight. The b.f.o. is now changed from 250.0 to 251.8 kHz, again generating a beat note of 900 Hz but now using lower sideband reception in stead of higher sideband in positions six and seven. The b.f.o. has a separate crystal at 251.8 kHz for this.
The detector is of the leaky grid type. Both i.f. and b.f.o. signals are introduced on the control grid of the RV 2 P 800 pentode tube that is used through out the receiver. The detector tube is coupled to the final a.f. tube by means of the tuned circuit at 900 kHz that we already mentioned. In positions 1-5 of the bandwidth control it is replaced by a RC-type coupling.
According to the manual the set should normally be used with manual gain control. This varies the screen grid voltage of the first and third i.f. amplifier tubes. But in case of fading automatic gain control can be used. There is a separate i.f. amplifier (or the a.g.c. It receives the same i.f. signal as the leaky grid detector and it feeds two diodes in a voltage doubling rectifying circuit for the a.g.c. voltage. This is fed to the second r.f. amplifier and the second i.f. tube. The manual gain control is made inoperative when the a.g.c. is switched on and replaced by an a f control that is on the same shaft as the manual i.f. gain control. But the i.f. gain can still be controlled manually when using a.g.c. by means of a separate potentiometer that has the same function as the manual gain control for use without a.g.c. but is only operative in the a.g.c. position. The manual says it should only be used in case of very strong interference.
The combined i.f./a.f. gain control can be seen to the left of the left handle that is used to pull the receiver out of its cabinet. The separate i.f. gain control that only works in case of a.g.c. is to the left of the right handle in photograph 8.
The last item of the Kw E a we will discuss is the metering facility. It is like the one on the Lo 6 K 39 a. The meter is at the top left of the front panel and to the left of it you will notice a switch with 13 positions. In the first the filament voltage is read and in the second the anode voltage. The voltages should be within a red or blue sector on the meter face for the two voltages. The remaining 11 switch positions are for metering the anode currents of the 11 tubes in the receiver. A black mark on the meter dial indicates the minimum reading for a serviceable tube.
The frequency dial also carries red markers for frequency checking. The calibration signal for this comes from the 250 kHz crystal in the b.f.o. Harmonics of the b.f.o. signal can be fed to the input of the receiver. For this a pushbutton must be operated that can only be reached when the set is out of its cabinet. In case the dial reading is not ok this can be corrected by rotating green encircled adjustment screws on the coil turret.
It is remarkable that the manual for the set does not contain performance specifications. I have not yet had an opportunity to test the receiver in my own shack. But this is going to happen in the future and I am quite convinced that the radio will come up to my high expectations.
differences between Lw.E.a (72-1525kHz) and Kw.E.a (980-10200kHz) [LA8AK]
Front view differences between Kw.E.a and Lw.E.a
Kw.E.a IF filter. It is three similar IF-stages on 250,9kHz. The circuit is drawn such that the similarities/differences can be seen.
Lw.E.a IF filter. It is two similar IF-stages on 60,9kHz. The circuit is drawn such that the similarities/differences can be seen.