a survey


Puck was the code-name of the test generator operated in conjunction to Naxos

It radiates a spark signal which is designed such, that one may expect that its radiated spectrum is preferring the 10 cm wave band (S-Band). Although, actually it does it over quite a broad spectrum.


I was never able to obtain such a device, or be it far too expensive. I therefore decided recently to build a similar unit myself, from scrap materials available.

Finding first out is it feasible?


Survey status: 28 May 2013



My test set design is made from memory, as I have seen it somewhere decades ago for the last time


In the scrap box I found two rectangular lead tubes (Pb), which remained once from a roof renovation.

Although, lead is a very soft material, it is, nevertheless, easy to handle and I therefore choose to use it first. The Al reflecting mirror was easily curved by means of a solid tube. I had to work from memory and I did not possess much details. Only after my first reconstruction, I phoned Dieter Beikirch asking him for photos of his genuine apparatus. 



Please compare the previous photo by this one made by Dieter Beikirch a few days ago from his Puck transmitter

The main difference is that Beikirch's 'spark unit' is mounted more to the front and the two pin connections being just down the mirror surface, whereas mine is only having the spark interrupter including the antenna strip kept within the mirror space - the rest is behind its surface. The difference being only, say, two cm, whether this will make much difference can only be determined after a second replica can be operated. We luckily have two spark units.



Viewing one side of the experimental Puck imitation. On the right hand-side the rear opening with a 'flat battery' well known for old continental torches. The filtering is implemented as I encountered high levels of spark interference, when the spark device being fed from a separate power supply; likely owing to radiation from the feeding cables


The next photo may be looking a bit chaotic as well


Dieter Beikirch's Puck is not looking much better

Beikirch suggested, that the dimensions of the wartime batteries might have been slightly smaller, as his one does not fit using a modern 4.5 V battery pack.



Viewing the pluggable sparking signal source

The flat strip being the radiator (antenna) which's length is more or less resonating at say 10 cm (actually a ¼ λ)



The sparking interrupter is adjusted by means of the tuning-screw, which is to be set such that it operate- and is re-starting soundly


Viewing the experimental Puck signal


I was, however, not very pleased with the signal to noise ratio adjustable at the Naxos display FuG 350Zc


Two decades ago it performed much better!


I then measured the filament voltage of the EF14s, as these being fed from the internal transformer; which itself is powered from a rotary converter (Umformer) U10E. This device delivers, of course, regular HT as well as an ac voltage of 333 Hz. The filament supply was maximally 5.7 V; too low for sound operation in my perception. What can be done about it, as the system is being powered by two heavy 12 V batteries in series?




The schematic of the Naxos FuG 350Zc passive receiver and display unit

Please notice the power transformer on the right-hand side which is being fed from the U10E Umformer

We are actually looking at the version made for the Navy, which differs slightly from ours. The difference is that the HT in the GAF version is derived from the 24 V rotary convertor which provides also the 333 Hz ac for feeding the transformer. The dual phase rectifier (EZ11) is omitted in the GAF version.

I first doubted whether C 2 is defect, as the part list provides only the information that C2 is according DIN xxx and is paper-roll capacitor, not providing its capacity. I located the component and it proved that it is a superb Sikatrop type, which is a hermetically sealed-off type. These normally perform as they did 70 years before!


A new approach is valid! 


Before we moved towards the new premises, we operated an external power supply unit; which once was used to test Nato aircraft 'pay loads' (pods).


Avionics Power Supply type SP 32-50

It was stored in a shelf and weights far more than a kg per Amp! As it can deliver 50 amps you can imagine how heavy this device is!  Getting older, moving it is demanding quite some muscle power; I had to go up to my very limits!


However, it was eventually connected such that a single pole of the 24 V battery arrangement was disconnected and the system being fed experimentally from this mains driven power supply instead.

By this means, the Naxos filaments were being adjusted at 6.76 Volt. The deflection voltage generated from the ZM290M 1/44 antenna motor had to be readjusted, as the CRT HT was logically increased as well (CRT deflection sensitivity is decreasing).


Viewing the Puck signal, whereas the system being fed from an increased supply voltage of about 27/28 volt, we formerly operated at 24 - 24.6 V. The weaker signal some degrees east of 180° may be due to an antenna side lobe



Watching the General Radio generator signal (3210 MHz)

The GR generator using internal square wave modulation of 1 kHz (duty cycle 50%)


It is always a compromise finding the correct balance between trace brightness and  system sensitivity versus the surrounding light.

In a nightfighter aircraft brightness was operated at a very low level.  For this reason, we use a screening tube, which often is extended as to keep too much daylight from the CRT screen, when screen shots being taken.



We are viewing both at south the Puck sparking signal and at say 45° the General Radio signal source

It is evident, that the Puck is generating a stronger signal, likely also owing to the sharpness of the Puck pulses. We also have to take into account, that the sparking source is being fed from a 4.5 V battery in series with the magnetic coil; generating visible sparks. Which might, I guess, reaching at least fifty volt pulses. When this estimation is true, the effectiveness of the ¼ λ radiator attached to the one end of the sparking arrangement is rather poor. Maybe, a better made Puck replica can improve the performance, but my estimation is it will do it only slightly.



On 28 May 2013


I tested first whether couple C 2 needs an increased value. The most simple way was linking C 3 parallel onto it and watching whether it does make sense. It did not so, the only phenomenon occurring was the the brightness versus signal amplitude changed a tiny bit.


The Naxos receiver - display being opened is a good opportunity taking some extra photos.



Viewing the FuG 350Zc display when the cover have been removed. On the right-hand side we see the 'Umformer' U10E

Quite curious, the set is normally mounted upside down (the CRT on top).



Viewing the Naxos set from the left-hand side

In my perception, this set is build rather neatly! Although, built likely in 1944 they used still high standard blocking capacitors; all of the so-called hermetically sealed-off type



Looking now a bit more from below

The English text to the four potentiometers on the left-hand side proof that this set was once being operated in the US, as this is the country where I obtained it from in the 1980s.

The 'four contact connector' (femail) is meant for a headphone when signal search is also done acoustically. According the manual, this is quite heavily loading the circuit resulting in a reduced video signal level.

What was new for me, is the fact that in Fritz Trenkle's personal papers, which he gave me in the 1980s, is mentioned that this type was designed by RPF (Reichspostforschungsanstalt) which had an equal role as once British GPO.



Viewing the set from the right-hand side, U10E being removed

This mounting was originally meant for a dual function in conjunction with the PT 10; where at choice a mains driven power unit can be operated or one from a 24 V battery.  

The valve parallel to the DG2-7 CRT is the HT rectifier RPG5 which provides the high tension for the CRT.



Viewing the FuG 350Zc transformer in details

Please notice the previous schematic.



SG 350 Zc, Ln 29448, Werk-Nr. 129027

Werk-Nr. may be a kind of cryptic serial number, like production item 27 or, for instance, 127. Ln 29448 was the GAF stock number. SG stood for 'Sichtgerät' (display apparatus)



To be continued in due course


By Arthur O. Bauer




Please go back to, or proceed with: Naxos