Kw.E.a (1943) - part 1

We're currently in the process of restoring the Kw.E.a receiver. This is one of the most complex heterodyne receivers made by Germany during WWII. It uses 11 RV2P800 vacuum tubes in the following stages: 2 r-f amplifiers, one local oscillator, one mixer, 3 intermediate frequency amplifiers, one low frequency amplifier, detector, one AVC, and a signal generating calibration tube.

There was an incorrect electrical configuration from the beginning. The H+ terminus of the 1st RF amplifier tube was directly connected to the +Umf. (Umformer) (3rd) terminus of the power receptacle by a separate wire. So the + filament voltage had to be also applied to the middle prong of the voltage socket. Also, it was quite apparent that someone tried to tune the receiver by turning the screwdriver adjustable iron dust cores on the coil turret drum. This was not a good sign. All of the screws had epoxy paint applied to them during assembly. This ensured that screws did not loosen during vibration or transport.

Once the voltage output was checked on all of the vacuum tube connectors, and was OK, next came the tubes. The RV2P800 vacuum tube is very sensitive to filament voltage increases. Any strong increase in filament voltage will most likely damage the tube, and since these tubes are not being manufactured today, one must be very careful in controlling this voltage.

The tubes were checked with my RPG 3/4, and 8 out of the 11 in the set were found to be significantly below desired capability. I located a set of 10 tubes - There were 7 Telefunken, 2 Valvo, and one TeKaDe. all 10 tested at or above high usability.

The tubes were inserted into the Kw .E.a, and the receiver was switched on. An antenna cable was connected to the radio, and 2000 Ohm headphones were connected to the sound output jack. The signal from the headphones was very quiet and weak. There was a station or two that came in very quietly, but the performance was that of a receiver that did not work properly.

Next, we attached our signal generator to the antenna connector. The AM frequency was 1100 KHz with a frequency modulation setting of 400. There was a faint tone that came through the headphones with an output setting of +20 Dbm. This was a very strong signal, since +0 Dbm should be enough to produce a normal sound. When the modulation setting was changed to 1000, the output tone raised the pitch. No other modulation setting was audible. This all means that there is a problem with one or more of the stages.

We started testing the stages in reverse order. We attached a 200 pF capacitor to the output connector of our signal generator, and started testing the intermediate and low frequency stages of the receiver. Vlad Dvorkin assisted us with this, and it was established that the detector (Audion) was not working. Vladimir tested the components around the detector vacuum tube and discovered that the tube's screen biasing voltage was too high, which can be several things, but in this case it was a bad solder which disconnected a resistor. All of the connections were re-soldered around the detector, and the radio started working.

This Kw. E. a worked on frequency ranges II, III and V. It was non-functional on frequency ranges I and IV. The frequency coil cores screws were tapered with, and needed to be re-tuned. Also, the coil blocks of frequency ranges I and IV need to be opened, the coils need to be tested individually and the trimmers need to be un-frozen. This will be time-consuming and careful work. It takes nothing to rip coils, break the coil iron dust cores or break the ceramic trimmers inside the coil blocks. Fingers will be used initially to tune the coils and trimmers, and a plastic screwdriver if these screws rotate easily. penetrating oil will be used on screws which do not turn easily by hand.


This is what I pulled out of the 1st HF amplifier socket. An RV12 P2000 vacuum tube. This tube consumes 12.5 Volts of filament voltage. The filament (H+) connector was wired to the +Umf. terminus of the power plug, so that 12.5 Volts could be supplied to that tube exclusively. Looks like instead of finding another RV2P800, the previous owners modified the receiver for the wrong tube.



The low frequency amplifier (10), 3 intermediate frequency amplifiers (5,6,7), the detector (Audion - 9), the tone tester (8), and the current control (11) vacuum tubes. The problem was discovered around vacuum tube #9, the Audion detector. This circuit is almost identical to the one found on the Torn. E. b.



These screw connectors were tightened with a screwdriver. Later, I will clean each one individually with a contact cleaning solution.



Here Vlad Dvorkin is testing the detector by sending a signal to the input pin of that vacuum tube. We could not hear anything on the speaker.



Here Vlad Dvorkin is trying to see if a signal is coming through on the oscilloscope.



There is a weak signal.



Troubleshooting of the detector (Audion) circuitry.



Intermediate frequency stage troubleshooting (Vlad Dvorkin's lab).



More IF stage troubleshooting (Vlad Dvorkin's lab).