My investigation on how the 'Power Meter' (Leistungsmesser) according Ln20978 might work

State of affairs: 10 February 2010 (in progress)


First, I have changed J2 for J1

Instrument J2  is only used for checking whether diode valve Rö 2 has sufficient 'emission', which is the only function of this meter! The moving coil instrument fitting into this circuit is J1*; as, according this simplified schematic, its function is: to watch (controlling) the balance between the radar power, thus its provided HF voltage across dummy load resistor W1, against the adjustable voltage indicated by meter J3. Which meter scale is directly calibrated in kW. When the voltage across C1 is equal to the one across J3 there cannot flow an equalizing current either way!        *For this purpose normally an instrument having a central null (0) is used, which indicate in which direction the actual current flows. However, we will see later that it actually is (was) used differently. Our instrument J1 did not function in the beginning.  After opening its housing it proved that the meter pointer was touching the glass window, after careful bending it a bit, it responds normally again

It is also clear, that the moving coil instrument J1 (the one with central null) is also used for indicating the relative EM power inside the wave meter cavity resonator (control switch set at 'Welle'). This measurement reading is only of relative value, as it should indicate at which point of the (brass)scale resonance (frequency) is at its maximum. It will therefore deflect into a single direction (in our case from central null to the left).


The uncorrected detailed schematic 2

What might have meant switch section 's' which is being activated when the control selector is set at 'emission measurement', but without having any wire connections? (marked with ?)


The second correction is disconnecting the incorrect wiring (red) and replacing it by the most likely connection (green)


Selector switch U2 set for "Leistung" (power measuring)

What is not easy to understand is, that line '11' is connected with meter J3 tap having a + symbol. In our case potential 12 and 22 is having a negative potential against line 11! The compensating voltage is thus having a negative value. Which has to be compared with the negative voltage resulting from (after detection of) diode Rö2. As line number 12 is connected onto the + section of the moving coil meter, potentiometer W6 is within the minus circuit and its potential is set (adjusted) by means of W6


To simplify the understanding of the control setting for test function "emission"

When we follow the yellow and red lines (12 and 13), it is clear that the actual voltage across the connecting points 11 and 13 of meter J3 is adjusted by means of potentiometer W5. As we will notice below, this pre setting is essential (but not yet responding in our apparatus).



Front panel of the control module


According the instruction chapter at paragraph 4 of manual section D017-4:

Schalter U2 auf "Emission" schalten (consider the true table above). (Der Zeiger des Instruments J2  muß innerhalb des gelben Skalensektors stehen. Der Zeiger des Instrumentes J3 ist mit dem Drehwiderstand W5 auf die rote Strichmarke einzustellen.)


Connecting now the power meter by means of a coaxial cable onto the Würzburg transmitter output


Chapter C. Leistungsprüfung (measuring output power)

1.    Drehknopf "Komp grob" entgegen dem Uhrzeigersinn (anti-clockwise) bis zum Anschlag drehen.

2.    Schalter U2 auf "Leistung" schalten. Zeiger des Instruments J1 schlägt aus.

3.    Zeigerausschlag des Instruments J1 auf Null einstellen (Kompensieren).

        a)    Drehknopf "Komp. grob" so weit nach rechts drehen, bis Zeigerausschlag an J1 nazu verschwindet. (meter pointer just at central null)

        b)    Bei gedrückter Taste "Komp" den Drehknopf Komp. grob" so weit drehen, daß Zeigerausschlag an J1 genau Null wird.

                Beim Weiterdrehen von "Komp. grob" schlägt der Zeiger des Instrumentes J1 nicht nach der anderen Seite aus. Der am Instrument J3 abgelesenen Wert der Leistung ist nur dann richtig, wenn der Zeiger gerade den Nullpunkt erreicht.

4.    Zeigerausschlag am Instrument J3 ablesen. Zeiger muß sich innerhalb der Skalenteile befinden, die auf dem Schild über den Instrument angegeben sind. (in our situation we don't have any further indication)


What kind of information is provided?

The moving coil instrument J1 has a central null scale, which is commonly used in compensating (bridge) systems. However, according this description, the instrument is only able to deflect into a single direction (to the left). Why have they adopted a more expensive (sensitive) moving coil instrument type? I believe, after considering all pro and cons, that it was done as to guarantee the most sensitive reading-off at the null (zero) meter deflection. Hence, although we may speak of compensation, the actual point of full compensation is indicated only at the exact centre of the meter scale, thus its pointer is just not responding at all. With a regular meter this would be most likely far less accurate ('null' at the far left corner).


We still face the fact, that although following the instructions of paragraph 4, we are yet unable to set the pointer of meter J3 at the red hairline marker (at about 20 degrees from the left). It is also not yet clear what the electrical circuit inside meter J3 is. Is it, maybe, a cross-coil instrument? We might have to open it.  !! It just appeared in my mind - that the integration of a second (crossing) moving coil might result in a more exponential scale reading. As its initial current bias might hamper (countering) the linear deflection of the voltage read-off by means of W6. We know that we measure a voltage across resistor W1 (70 ohm dummy load) and we know also that V2/R = P. It is likely that this is why the fixed biased second moving coil is implemented in the system.    *We may expect that the actual maximum pulse voltage across the dummy load is about 750 Veff (7502 / 70 = 8035 Watt, which is the maximum HF output of Würzburg. However, Mainz and Mannheim radars generated about 900 Veff across this dummy load) What is against my thoughts is, that according instruction 4. b, both controls (W5 and W6) has to be re-adjusted, which would interfere with my earlier guess that a cross moving coil is involved! (why adjusting previously W5 at the red marker of J3;  is it for controlling the voltage at line 11 and thus the voltage reference for the emission measurement ??)


Investigating why meter J3 doesn't work correctly and finding out if it have crossed moving coils

10 February

First, J3 is not having crossing coils


After opening the control box we found the following situation:


Meter J3 is the top one (please notice the additionally built in fuse holder)

After disconnecting the three wires, I measured first the resistance between the + marked pole and -15V. It showed, that this circuit was open. My next step was to remove the instrument and finding out what might have caused it.


The brown bobbin in the front was hanging loose in the air and the wire was cut-off from its contact-tag (was this sabotage?). I forgot to take a picture of the loose hanging situation. Only the loose resistor wire is just visible (it is pointing differently from the wire at the solder tag)


Although, not very good visible, the resistor wire is soldered again

I measured both series resistors, the -15 V one is having 7.62 kΩ and the 150 V is having ≈ 78 kΩ.


The red marker on the meter scale is good visible


Before mounting the power supply unit inside the control box again, we notice its sound construction. One might wonder, that I have used modern elco's. Our practice is to leave all original components inside, but disconnecting (de-soldering) the positive tag and replacing it electrically by means of modern smaller sized elco's. This precaution is taken because - when a component fails one never know what might be damaged as well. Nevertheless, the original state of affairs can very easily be restored 


This photo shows that the pointer of meter J3 is now being set at the red marker. Which was done by means the procedure described in the manual (in mode 'Emission' adjusting with potentiometer W5). Given the circumstances when this photo was taken, it proved to be impossible to get rid off all the interfering outside light reflections (although, Photoshop CS 3 suppressed it to the shown level)


I also investigated whether switch section 's' is really without wiring. As far as I could see, both contacts are interconnected and are being used as a multi-wire soldering point. Without destroying the original wiring, it is not possible to find out where these wires are exactly connected onto.



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