Kw.E.a 1943 - part 2
(tuning)

Part I of the restoration ensured that the basic functionality of the receiver was established. After this, our task is to tune each frequency range (Bereich) of the Kw.E.a. The circuitry for each frequency range is somewhat different depending on the specifics of that range. Since there are two HF stages followed by the heterodyne block (mixer and local oscillator), this entire input section must be tuned properly to maximize selectivity and sensitivity.

We will start with the first HF stage. Each of the HF stages has two tank-circuit band pass filters each comprising of a iron dust core inductor and a trimmer capacitor connected in parallel. The iron dust cores move rather freely and allow for effective tuning of the lower portions of the frequency bands. But NONE of the trimmer capacitors can move. This is a problem that will be solved with penetrating oil and TIME. In the meanwhile, we will be tuning the bottom region of frequency range I (980 KHz - 1610 KHz)

Here is the first HF stage of the Kw.E.a with the aluminum shield removed. Note the cylindrical capacitors with porcelain caps. These are known to be some of the best quality types produced by the Germans. The tunable "Drossel" to the right of the bypass capacitor prevents the IF from leaking into the antenna. Why it is tunable is unknown at this point.

 

 
Here is our FET probe attached to the 1st HF stage output pin on the RV2P800.

 

 
This is what the output looks like for the first HF stage on the lower edge of the first frequency band (980 KHz). It is not tuned properly therefore you can see the unbalanced resonance curve on the spectrum analyzer.

 

 
After tuning, the resonance curve looks much better (close to what it should look like). We need to achieve the same result in the upper edge of the frequency band, but for that we need to unfreeze the trimmer capacitors.

 

 
The dual 1st HF stage filters. The larger openings below the ceramic bar give access to the tunable coils (tuned by rotating the iron dust screws). The smaller openings below provide access to the trimmer capacitors. Here the retaining clasps have been unscrewed and removed.

 

 
Close up of the block endpoints. Note the spacers between the tank circuit container and turret drum.

 

 
1st HF stage removed from the receiver.

 

 
Reverse of the container.

 

 
Coil turret drum with 1st HF stage removed.

 

 
Strange mark on the tank circuit cover.

 

 
Cover removed. I need to get the trimmers moving again. For this I use penetrating oil and a thin insulated wire.

 

 
Better photos of the ceramic trimmers.

 

 
Solder connections to the spools.

 

 
Part #17

 

 
Part #31 12?

 

 
The mysterious 4/301 stamp found on the 1st HF Stage

 

 
After unfreezing the trimmers and installing the HF block, I went back to test the 1000 KHz resonance curve, and found that it was unbalanced again.

 

 
The receiver is powered on.

 

 
Tuning takes place through the two oblong openings in the middle of the photo.

 

 
Here the FET probe is attached to the anode pin of the RV2P800 socket.

 

 
Here is the 1000 KHz resonance curve after tuning

 

 
At 1600 KHz, the resonance curve has a different shape. Each horizontal division is 20 KHz, which gives a very wide bandwidth at this end of the range. Tuning the trimmers would not really change the shape of this curve by much.

 

 
In the middle of the range (approx. 1300 KHz), the curve looked good. Still about a 60 KHz bandwidth. It's ok to have such bandwidth in the HF stages.

 

 
FET Probe connected to the spectrum analyzer.

 

 
For comparison: 1000 KHz

 

 
1300 KHz

 

 
1600 KHz

 

 
The 2nd HF stage of frequency range 1 has been removed.

 

 
Markings on the outside of the 2nd HF stage.

 

 

Internal components.

 

 
Spools have been unscrewed and lifted up to provide access to the trimmers.

 

 

The 2nd HFamplifier filter block was not working so replaced it with a different one that works.

Here the block bas been taken apart to show the individual components. One of the trimmers has become defective.

 

 
Here is the defective trimmer - note the gap between the screw and the silver powdered surface on the top of the trimmer. This is a result of age, or perhaps someone trying to tune it without unfreezing it first.

 

 

Here is what a trimmer in good order should look like. As you can see, the slder is firmly connected from the screw to the silver finish.

Do NOT try to solder the screw to the powdered finish - you will evaporate it. Use high-quality silver paste instead.

PART 3

 

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