A Survey

to enhance the performance

of our Würzburg apparatus


Determining why the interaction between the WB SÜ62d Transmit-Receive stage, the Leistungsmesser (power meter) and the Rehbock artificial target is inconsistent.  


It was recently already discovered that the Leistungsmesser, which is mainly a dummy load of 71.3 Ohm, is not showing the correct impedance or resistance. Sometimes > 90 ohms or even higher.

My first thought was, that this goes back to contact problems. So far it is found, that this estimation is correct. This will, however, not say that all problems have yet been solved!


First the front panel has to be demounted.


The tapered module contains inside the dummy load resistor of 71.3 ohm

The typical conical shape of this device is to keep impedance everywhere in the coaxial system in accordance to the local impedance (at its begin Z = 70 ohm and at its final end Z = 0 ohm). This is, by the way, a fundamental way of accomplishing impedance matching, even nowadays when power measurements is to be dealt with!


As to cope with the local space, the central control unit had to be de-mounted


The four screws are easily accessible since. 


After having disconnected the interconnecting cables, the dummy load module with attached wave meter and HF signal level pick up (power measurements) can be accessed freely

Rö 2 SA 102 is the UHF diode especially designed for rather high voltages combined with low capacitances against the other valve electrodes. Rö 1 (SA 101) is the diode for the wave meter. When full power is concerned HF voltages up to 900 Veff  is to be dealt with. Certainly not in our situation, though, doubtless during is genuine wartime applications.

Time and again I checked whether the dummy load resistance changed, what it actually did.



Please notice the traces of oxide in the upper part of the brass conical conductor. This certainly can causing instable contact response. One might think that it once was being soldered, but in such a case it is not easily possible rubbing it away my means of strong litze


Cleaning it brought seemingly a solid contact, though, it soon proved that the dummy load resistance still was not stable. The only means feasible is that at the other end of the resistor, thus where it is touching ground level (impedance = 0 ohm) causes a problem. This end is having a brass plate designated W 1 which is being fixed by means of three screws. I only was able to rotate it a bit and, maybe also important, I cleaned the Zn oxide from the three screws as to assure that the brass plate is sufficiently connected onto ground level.

Seemingly it functions without a fault now though, from my long term experience (50 years), I know that there still is a chance that the failure will return again. Thus, always bear Murphy in mind, don't replace the apparatus front panel before everything is for > 100 % all right.

It was also found, that the housing of the dummy resistor must have been opened more often, why doing this when there is no need for it? We must thus expect, that this failure occurred even in wartime days or shortly thereafter.  


 Viewing the counter contact of the previous line


I cleaned both ends with rubbing it with heavy 'Cu litze' which proved not curing the entire problem.

Maybe not well visible, though, just below this centre contact is a capacitive pick up capacitor which is being spaced about a cm from the centre conductor. Its separating space can be adjusted, and the service manual is explaining how to do this correctly in respect to the power reading (indication) of the apparatus. The slit at about North-East is actually a circular window which gives access to this provision.


To be continued in due course

By Arthur O. Bauer


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