DMG 5k




Page initiated: on 29 February 2024

Current status: 9 June 2024

Part 2   (since 8 March 2024)

Part 3   (since 16 March 2024)

Part 4   (since 23 March 2024)

Part 5   (since 30 March 2024)

Part 6   (since 6 April 2024)

Part 7   (since 14 April 2024)

Part 8   (since 20 April 2024)

Part 9   (since 28 April 2024)

Part 10 (since 7 May 2024)

Part 11 (since 10 May 2024)

Part 12 (since 19 May 2024)

Part 13 (since 26 May 2024)

Part 14 (since 3 June 2024)

Part 15 (since 9 June 2024)

Part 16 (since 14 June 2024)

Part 17 (since 23 June 2024)

Part 18


This photo originates from the end of January 2023, when we just received it on a long-term loan



We have moved with the assistance of Hans Goulooze the DMG5k to a place (facility) where it should be possible to undertake an elaborate restoration



The new DMG5k restoration facility is just adjacent our current Nachtfee repair endeavour


 I would still like to let run our opened Nachtfee Console in the repair mode; as to become really sure that the annoyingly repeating deflection faults being since really cured.


The most low plug-in cabinet is the mains controlling module containing on its far right-hand side the two 'Automatic' fuses (Selbst-Schalter)

The grey circle is actually the main-on/off switch, of which the knob is failing.



On this trolley-table we should be able to repair or accomplish our necessary measurements

What, however, we need then first is at least a pair of extension-cables, which Dick Zijlmans promised me to bring along next Sunday (3 March)



Luckily we found a knob which, with some adaptation does fit to the wanted purpose

The on-off switch is in the schematic belong designated with '3' (inside the circle).



Schematic of the module dealt with below

The 220 V mains is to be fed onto the connections 4 and 5



It is evident, that this module has been build quite condensed



Considering it from a slightly different perspective



Please compare the 'pot-core' numbers with the schematic (6a.) above Though:

The painted pot-core numbers do not match with the schematic (6a) of above, but doubtless it concerns just these 6 coils (forming a chain of low-pass filters).



Viewing the 'pot-cores' just from their sides



The finger of the thermo-relay are just visible in the vertical centre



It proved to be quite difficult to access the neon indicator lamp-holder, which 'bayonet-neon indicator bulb ', you might recognise down in the centre

Which is fit with two base-contacts

The lamp is still glowing yellow/red, but it is inside showing a black coloured deposit on inside of the glass-bulb.



(2)   (since 8 March 2024)

Below on the right-hand side we notice that the revised power-switching module had been inserted in the DMG5k frame again

It proved that the manuals we possess does not imply a sound cabling plan as well as is lacking the detailed schematic of the power-supply module.

It becomes necessary to find out these aspects ourselves. The Hans Goulooze and myself have started a new survey.

What and where do we trace the mains connections.

Firstly, we start with checking by means of a simple ohm-meter what the according pin-connections.



Dick Zijlmans necessitate more time than he expected, we still are lacking the according extension-cables. Instead, we are using first an extension cable used in conjunction with the FuG 10 system. Inadequate, but we need only to extend two cable pins.



The extension cable substitute is linking the 220 V connection onto the soundly constructed DMG 5k Michael power-supply


Now we should focus first on the core of the entire system: the power-supply.


After having removed the various module covers, we get some idea of the various shapes of the separate sections




After connecting mains onto this module and no response could be notices; my attention was drawn onto suspected electrolytic capacitors.


We have now noticed the rear of the power-supply from three sides


Soundly build it certainly is!


We found three electrolytic capacitors, but these were mounted at a non-accessible place


We first tried to access them by removing the bottom-plate, though in vain.

Apparently there must exist a different way.

In such cases: do always consider that it concerns: a German type of equipment, and smart mechanical engineering is almost likely involved!

And, indeed, it is!

All proved to be mounted modularly!



AOB: my first approach, is always try to removed first what is easily to accomplish!

The German engineers of the Luftwaffe employ easily to understand systems.

The fixing screws are always being marked by engraved red circles on top.

Only two screws had to be turned anti-clockwise a few turns. These will remain in the module - due to their thread free section.

Please look closely at the upside turned module, and notice on the far right-hand side that the fixing screw is still hold in the base-plate!

Connectors being lavishly employed without causing mall-functioning. 



The forgoing module now viewing it more in detail on its side

You can also now see the two module fixing screws due to their still dead-threat(??) Re-mounting the module becomes very easy!

The tubular capacitors are of the expensive "hermetically ceramic sealed-off" type; these, even after 80 years, will function like these were newly accepted in, say, 1943!



The second module removed is also a sound master-piece!

The two black devices around its centre might constitute  a voltage- or current regulator. The circular device on the right-hand side nay be a current sensitive coil-system; the black square device may constitute a variable current device.

Though, because we still do not yet have access to an integral circuit drawing, quite much is using the experience of my 'grey cells'. 



The number '0' always indicate ground-level



After some experiments, Hans drew the conclusion that these hexagon-extensions had to be removed as well



Voilą also this barrier had been accomplished


It is so logically and soundly constructed!


The certainly defect electrolytic capacitors are visible through the lower holes in the bottom-plate



AQs to get really fully access to the three electrolytic capacitors, it is necessary to remove also the heavy transformer

Which is quite easily maintained as the cable-tree-wires being fixed 



The three electrolytic capacitors have to be replaced

We tend to access the aluminium cylinders and insert modern types

  The problem might be, that the specification tells us 550 V average 450 V; whereas most modern types of the 450 V types.



Please bear in mind, we have to cope with the circumstance, that we do not possess a detailed electrical schematic of our power-supply

However, what we, Deo volente, may bring-in is more than half-a-century experience, of tracing faults.





(3) (since 16 March 2024)

Ultimately Dick Zijlmans, very kindly, managed to supply us with a bunch of extension-cables he made

Now we can approach our DMG5k survey more seriously.



  Visible is the way these type of cables are matching together; it is even possible to put 2 extension-cable in series as to widen the space between the unit under test and the system-frame



Looking at a new schematic someone so kindly did copy



The component numbers differ from those noticed on the schematic, though, the wire numbers still do match, and we can trace where various components being drawn on the schematics




We notice above the three electrolytic capacitors, which was to be replaced

As to keep matters as it had been before, we have to remove the content of the inside the Al cylinder.

After all not a too great problem.



Dismantling was unexpected simple

Afterwards we cleaned the inside with hot water, as the electrolytic residues have to be removed.  



Viewing the application of an extension-cable differently



When someone has to access plug-in units under operational conditions, extension-cables are essential



Considering it from a different perspective



Slightly complicating is: that nowadays most HT electrolytic capacitors being made for 450 V max.; whereas our DMG5k, particularly in the start-up stage is passing 500 V by far (max. voltage rate printed at the Al cylinder 550 V

We therefore have decided to implement two electrolytic capacitors in series. As to counter production difference, we will implement low power resistors as to secure that the two electrolytic capacitors being fed with about equal dc voltages.

The bigger two capacitors possessed 16 µF and we employ two 45 µF capacitors in series, making 22 µF and a max. voltage of 900 V.


(4)   (since 23 March 2024)


Hans' aim today is to build-in the former electrolytic capacitor housing

We only possess printed-board electrolytic capacitors of 45 µF and max. 450 V

However, the genuine electrolytic capacitor was 16 µF, but designed for a max. voltage of 550 V; and preliminary experiments showed us that the unloaded voltage rises up 500 V.

Therefore we have to use a staged arrangement with two 47 µF devices in series.

As to keep the voltage division within limits we decided to arrange two resistors of 2.2 MΩ in series, as to force the system to be loaded more or less equally.



Hans is relying upon silicon based insulation sheet (red-colour)

please notice that two capacitors being stagged.



After Hans sealed the Al tube (more or less) we accomplished load measurement

We were astonished with the actual behaviour of the two electrolytic capacitors in series.

Very erratic current was flowing; albeit with reduced currents.

But irregular it remains.

As to look whether this special capacitor type was causing the nuisance, we compared a heavier type though encountered more or less similar phenomena.

The only thing we might have noticed was that after changing to another electrolytic capacitor and returning later to the foregoing sample - it might be that we encountered a bit less erratic current-flowing; but we aren't yet sure that all will later operate correctly.

This unexpected nuisance might have been caused by the so-called 'formateren' (Dutch language) effect. As electrolytic capacitors being based upon an oxide-skin-layer deposited at an aluminium electrode (negative contact) and a positive counter-electrode.

We measured sometimes in the range of 10 - 30 µA at a 365 V dc loading.

We dedicated quite some time in trying to understand the many phenomena we encountered; we certainly have to continue these measurements as to get an 'feeling' of what actually occurs.

Might this being caused by the fact, that the modern type electrolytic capacitors differ in their behaviour compared with the old type densely filled with electrolytic liquid?




With some brief skills - the second electrolytic capacitor was more soundly closed than was the first one



(5)   (since 30 March 2024)


On 28 March 2024

We continued our DMG5K endeavour


Testing now the last electrolytic-capacitor replacement

Our aim now was to view what its current leakage at 365 V dc is.

After a while we measured ca 90 µA ; but please remember that we additionally integrated 2 x 2.2 MΩ is series, as to keep the centre of the two 22 µF electrolytic capacitors being kept at about half of the supplied dc voltage across the two connection wires (black and red). (Roughly 87 µA already is flowing through the 2 resistors in series, which equals about what is shows on the current scale of our test setup.   

Its minus (pole) isn't grounded directly and is wired by means of wire number 31 connected onto an outgoing pin connection (please notice the following schematic accordingly).



Viewing a bit more accurately



Hans placed the "Netzteil" (power-supply) at its fixed position, on its carrying frame again



The mounting frame for the additional 3 electrolytic smoothing capacitor demanded careful considerations as to be certain that all being rewired correctly



Please notice the three electrolytic capacitors numbered 68 and 70 being nominally 16 µF (in our new case each one  ca. 23 µF) and the one left of the foregoing 66 being nominally 8 µF (since replaced internally ca. 11 µF)

We discovered:  that a modification accomplished according production stamps between July 1950 and 1952; the 8 µF capacitor once had been connected incorrectly.

Before demounting them we cut-off the wires but left just some as to notice their according colours.

Wire number 94 doubtless is of red colour and wire number 31 accordingly black coloured; whereas the red capacitor wire was soldered on it! After we did remove the cable cover-insulation we encountered a 'black' cable-marking!



Ü 5 = Übertrager (transformer) number 5; constituting the loaded 300 V dc; as well as 12.6 V for some of the filament circuits


Considering the wire-connections a bit more in details; though not yet fully insulated

The yellow-coloured wires are fire- or heat resistant of which the yellow shining originating from heavy wounded silk; wound in two counter-rotating spinning. Very nice flexible cables, provided in various strengths. Also the red coloured wires being of comparable favourite qualities. 



The wires being now insulated effectively (using heat-shrink tubing); also the transformer Ü 5 being re-installed again



Everything being refit appropriately and we suppose next to connect 220 V ac on the system

Of course, nowadays we  count with 230 .. 240 V ac mains, but we consider we should operate the system by means of a 'variac' provision adjusted at 220 V ac.

Such a provision allows us to start measurements with rather low voltages.

For security, I prefer to start up by means of a series light-bulb which - when all performs smoothly, can be safely short-circuited.




(6)    (since 6 April 2024)


 On 4 April 2024 we continued our DMG5K survey


Having started, it  became soon evident that - due to the complexity of this technical subject, and the encountered nuisances, that it does make sense to start with my photo series first, and thereafter, I will go into the various technical implications and uncertainties.

Mostly being caused by the very fact, that the main frame wiring schematic is, in some respect, using two different numbering systems. Which I have, designated the old-  and the new numbering.

Why I don't know - but I suppose that for whatever reasons, the DMG5K frames once had been modified. And, consequently, the genuine techniques which remained, maintaining using their old wiring number designations. Whereas the successor system employed a new wiring system, and their according modified wiring numbers.


Our aim today had been: to investigate the behaviour of the 300 V stabilized power supply



As to setup a test of the plug-in number V, constituting the main power supply (some being integrated separately in plug-in module (Roman) VI) - down in the main-frame with the yellow glowing indicator lamp and typical German automatic fuses)



Considering the schematic of this power-supply it was evident that some crucial electronics were not yet understood; which will be dealt with separately, later




We we experimentally interconnected some connections (the wire numbers '30' - '31' and '0'). This erroneous estimation caused quite some implications, which had later to be solved. It proved to be necessary to go quite deep into the nomenclature of a different plug-in units including (Roman) VI

One of the implications can be seen on the next schematic: 



Please notice yourself why we in first instance had to interconnect the lines (wires) designated '30' - '31' and '0'

As we otherwise would not have had any HT at all, as the minus contact of the rectifier-bridge would be open and there could not be supplied HT voltage!

That this proved to be an in-valid estimation, we will later learn more about.

We discovered, however, also that we encountered number series, which did not match together.


But, the kind of surveys (Entdeckungsreisen) we generally accomplish, are also causing a forced learning process; which is really necessary to grasp the essentials of circuitries. As 'misinterpretations'  are necessary to learn about their implications.

Your will virtually remember such matters for the rest of your life.  


The 'variac' transformer has been set at about 130 V only, but the 300 V stabilisers were already becoming overloaded!

And, the filament supply which should be 12.6 V ac did  not reach about 7 V ac!

Thus, there must be something fundamentally wrong in the high-tension (HT) circuitry!

But, let us first follow the line of photographs I took that day.




When you look closely down the 'STV' (Stabilovolt an enterprise owned by Lorenz a substitute of the American ITT) symbol

This was indicating that the HT had reached already ca. 300 V at a system supply of ca. 130 V ac!

It is evident, that the stabilising current flowing through the stabilizers are reaching their limits!



Because, as I already noticed, that we encountered schematics with not matching cable (wire) numbers, we decided to measure each of the wires ('30' - '31' and '0') from point to point separately.

Hans down approaching the mains power section whereas I did read-off the Fluke meter.



Luckily, we possess Dick Zijlmans made extension cables, so I could easily handle the matter by simple means




We really did our best to understand the circuitry, but in first instance, we missed some essentials!


We leave the state of affairs, and remounted the two, removed modules


This is a quite easy job as all fits exactly together and being fixed by two or three market screws together



Screening off the power supply firstly

But, we removed it again after we considered that we should finding out, what the problems actually were causing.



One consideration was: might it be that the problems of too much overloading of the stabilizers was being caused by the lack of current consumption in the system circuitry?

It proved, however, that this wasn't the case.




Viewing it from a different perspective, we notice now that the units on the table being interconnected with the main-frame of the system.




Viewing it differently



After I inserted the RV12P2000 valves in the receiver plug-in module (Roman) II, I started measuring the various anode and screen-grid voltages

These responded as I expected, and therefore left it for the time being.


Thereafter I pulled-out one of the STV stabilisers so that these could not be overloaded and adjusted the filament voltage at about, say, 12.6 V ac.

The HT now rose beyond > 500 V dc!

Then we disconnected the lines marked '30'- '31'and '0'

Now, we had no longer any HT available. We disconnected the line '30' from the connectors

and instead,

we used our universal laboratory HT power-supply.

This was already the state of affairs when I measured the values of the anode and screen-grid voltages of the receiver module (Roman) II.


I would like now to approach the nuisance we encounter with the far too high HT output.

Let us match the three essential schematics together; of which one is the:

First is the power-supply schematic - the second one is the schematic of the mainframe wiring, and the third one is the small mains switching circuitry.

Not yet explaining but to get some feeling of what the problems are about.




Abb 31 (from the wartime ↓ manual) Schaltbild des Netzeingangsteiles


We have already noticed the line (wire) '30'; please jump to first schematic (85a.) and look at the upper bridge-rectifier designated '63' and notice its minus contact line (wire) '30'; please follow the line until it leaves the power module.    Thereafter consider the next schematic within the red circle marked 'b'.  And notice a bit down at the line contact number '12'. Please look now a bit carefully and notice the designation: 'V 30' (it looks a bit like a 'Y' but it should be 'V' as the is pointing at plug-in unit (Roman) V which's schematic we have already noticed firstly.  Though, frame cable (wire) number '30' is now becoming line (wire) number '12'.    I suppose that this might once have been caused due to some modified systems, where the mains input module remained in service whereas the rest had been redesigned. The clue or name it solution is provided by a kind of conversion table in the bigger module (Roman) VI. Thus, on the left you notice the numbers maintained in the old type module, and on the right we notice the main-frame lines (wires) including their (Roman) plug-in numbers (I ... VI).

Please, let us now consider the third module, mounted in the lowest right-hand corner of the main system frame.

It is designated Netzeingansteiles (87a)

Briefly translated: Mains (switching) interface

Let us consider '12' which actually is equal to the line (wire) '30' originating from the negative connection of the HT-rectifier-bridge designated '63'.    But we know (from the wartime manual) that within the mains-interface component numbered '16' constitutes a thermo relay. Such a device normally responds upon heating-up due to current flowing through a coil which heats-up a bi-metallic switch.    But there cannot flow a current as long as the thermo-relay coil isn't loaded with a certain (sufficient) amount of current.     Let us now first consider wire (line) numbered '2'. when you follow the circuitry we notice that the latter line is also flowing, via the thermo-relay coils towards '0' and '0' commonly is the symbol for ground.    Though, let us neglect the other components first; that when the thermo-relay switch being activated the line '30' via wire '12' can flow towards wire (line) '11'. Please return now again towards the red circle 'b" and let us view where line (wire) '11' being wired onto.


Contact '11' (left-hand side) is connected onto VI 80; here we encounter or an printing or drawing error: VI is indicating that we remain in the frame bottom section and is being connected onto power resistor (50 W) designated '14'. I interpret that is printed 10 but it should be number '80'! However, the current flows through the resistor reaching point '81' which is a logical consequence of the foregoing number '80'.    However this also being fed onto another 50 W power-resistor (designated '15') is in the schematic on its right-hand side connected onto '0' thus ground. Now line (wire) '30' being finally connected onto ground. Consequently, when current is flowing that there will be a voltage drop across the two power resistors designated '14' and '15'. Whether this will be sufficient we cannot yet determine.


Please, notice the two high-power resistors (green porcelain) at the rear side of main-frame (on the left-hand side in front); both constituting the resistors '14' and '15' in the schematic (86a) Leitungsplan des Gesamtgerätes (within the red circle marked 'a')


The mains-interface module is mounted inside the black box down on the left-hand side.


Let us now take a closer look of what it is about:


The two thermo-relay (vertically mounted) switching contacts-fingers are visible left of the two ceramic automatic fuse/switches

The wire-wound resistor is in the schematic of Netzeingangsteil (87a) designated '17' and, in my perception determining the switching parameter of the thermo-relay.



Finally what is actually causing the thermo-relay to switch on?


Please return again to the second schematic and consider the red circle marked 'b'.

Please, consider in the box Netzeingangsteil VI contact 2 and read off on the outside we notice: (Roman) IV '21' and V '21'.    V '21' is pointing at plug-in unit (frame) of the power-supply (plug-in unit Roman V).    Please consider here fore the first schematic (85a). And line '21' is supplying - 12 V dc, originating from bridge rectifier '76'.    Hence, the thermo-relay is simply being supplied from the main plug-in unit (Roman) V. When the mains switch is being switched to 'On' the - 12 V is heating up the thermo-relay coil. And after some time - it will click in the on position. I suppose, at least delaying the heating-up period of the valves.     When the thermo-relay is being activated the circuitry is keeping it 'on' status.


Whatever someone may think about our today's survey, I hope that you, at least, enjoyed it.   


(7)   (since 14 April 2024)

On Friday 12 April 2024

Our aim today being - to test firstly the 'thermo-relay' in the mains-entrance module.

I would like to repeat the circuitry of the mains entrance module.


Our objective to day was first to look whether the 'thermo-relay' was correctly operating

My screwdriver is pointing at the main 'thermo-relay' contact fingers.


The main fingers is connected onto ground and being wound-around by some resistor-wire

We have measured roughly that the delay time differs between 8 to 10 seconds.


When the bi-metal finger reaches its critical temperature is bents and is causing the contact wires '2' and '46' to inter-connect.

this is causing that relay '18' to switch on and herewith it is causing a current-flow between the wires '12' and '11'; thereafter the circuit keeps itself being operational.


My screwdriver it pointing at the relay and its switching contacts

The next YouTube film is providing a better understanding of what it is about.


Film 000164:    Please notice first the relay and its according contacts in front. The finger-pair, being activated by the relay designated '18'. Please notice the left-hand side taller switching-fingers. The most left-hand side one is wire-wound by some resistance wire; which is heating up the bi-metallic left-hand side finger. In front you see the two sets of switching relay contacts. The bi-metallic fingers are moving, when a certain temperature has reached, to the right-hand side. The adjustable resistor in front is to adjust the switching-on time-delay.    Rö 1 designated black tube is the housing of the 220 V mains supply neon-indicator. Please view carefully the slowly responding bi-metallic finger towards the right-hand side. The sound of a buzzer is indicating the instant the circuitry being activated.  

I would like to advice you, to view this film more than a single time. Because you notice a progressing process without really noticing that something is moving.


The mains entrance unit has been re-mounted

After we noticed that it operates soundly.


It would first like to recall a foregoing wiring plan


The voltage across the two high-power series resistors '14' and '15' as being shown with circle a

On the left-hand side of resistor designated '14' we notice line (wire) '80' (however you might read '10' but when you view on the right-hand side of resistor '14' the we notice '81'


The two resistors in series are constituting the actual resistor in series with the minus connection of the bridge rectifier designate '63' and particularly wire (line) '30'; on the next following schematic


We encountered a far too high HT level of (measured with one STV 150 pulled out) of ca. 500 V dc


The curious remedy was implementing a series resistor arrangements of component '14' and '15' towards ground.

Let us now notice what is measured at the terminal (wire) 20 of the HT -  as well as at line (wire) '30' connected onto resistor '14' point '80' (inside the red circle designated 'a')




This meter is measuring ca. 230 V dc across point '80' and ground '0' (86a red circle 86a circle 'a')



We are measuring ca. 205 volt across the HT line '20' in the foregoing schematic (85a) against ground '0'

This is ca. 100 V too low, as it should be about 300 V.

Though, as to simulate a kind of operational condition I have loaded the HT line ('20') with a 30 W light bulb. (actually two 60 W bulbs in series as we do not possess a better HT load).


This bulb is only operating at ca 205 : 2 is say ca. 100 V


After we trusted that all is functioning correctly we attached the two demounted modules again

The two silver-shinning glass envelopes are constituting the two two STV 150 V stabiliser devices.

Please compare schematic designated '72'and '73' loading line (wire) '20' (schematic 85a)


Again noticing the mains entrance unit, with a glowing neon indicating the mains being switched on


Again the system is operating quite smoothly now


It is quite astonishing that the system operates after say, 80 years so soundly!

Let use closure today the survey with a second YouTube film


Film 000171:    Again we start with no mains (220 V) being supplied (using a variac transformer) which is adjusted at about 220 V ac supply. (Please be aware that I notice "within the red circle 'b' whereas it should be, within circle 'a', instead!  The 'thermo-relay' delays about 8 ą 10 seconds.   The two meters being not operated at similar sensitivities. The left one being operated via the 1:10 voltage divider (x10)!


(8)   (since 20 April 2024)


Friday 19 April 2024

Today, we (Hans Goulooze and myself) approached the DMG5k power-supply (Netzteil) again.


Our first aim today was to take a look at the relay module

Secondly, to solve the load deficit of the 300 V HT

We have already noticed before: that the plug-in modules III and IV are inside totally ruined! Therefore the valve and related circuitries were loading the 300 V stabilised circuitry; causing that the two

STV 150 Volt (LK 199) in series being forced to consume the missing HT load. We therefore decided to do dome experiment as to find out the actual current load that is missing.

We finally decided to load the 300 V circuitry with a 7.5 kΩ additionally.

As U = I · R     → I =U / R  →  300 /7.5 = 40 (when we count voltage in Volt and R in kΩ than the results being in mA; thus we create an additional load of 40 mA.

This apparently is sufficient to prevent overloading of the two STV 150 V stabiliser valves in series.



The two silver-shining glass envelopes constituting connected in series as a 300 V dc stabiliser (2 x 150 V)

The 3 relay are visible on the right-hand side of stabiliser valves.



The relays module is quite well recognisable (on the right-hand side of the two 'STV LK 199' stabiliser valves (each one handling 150 V)

As to turn this plug-in unit this far, we extended the existing extension cable with an additional one.



I consider this photo being self-explaining



Show the two 15 kΩ power-resistors connected in parallel, hence providing 7.5 kΩ additional HT current load of 40 mA

Why do we mount these resistors here?

Putting them inside the real power-supply is hardly possible, and we therefore used it at the 300 V HT terminals ('20' and '0') of the receiver plug-in module (Roman II)

This module is to be investigated soon anyway - as it is already demounted and placed adjacent to the the power supply on the same moveable table.



The current loaded 'STV LK 199' stabiliser-valves are together operating at 300 V HT, in a reasonable manner, with not too much neon-gas (ions) glowing



Please notice the square window (?212-D), it is visible that the scale illumination functions, thus the 12 V filaments of the: various RV12P2000 and LD1 and the LG1



We are viewing at the noise signal available at the valve (RV12P2000) anode of the 6th IF stage

This at least is indicating that the IF strip does operate.

Whether the oscillator operates correctly I do not yet know, though at least the entire IF strip operates. 



Marc Simons ( donated last year a 25 V power-supply; as our formerly operated for more than a decade, is showing some forms of oscillations

I told him, that we essentially need an indication of the d.c. current consumption.

So Marc provided a complete package and Hans manufactured the Al frame.

As we can see, Hans did a great job to create a mounting for it.



This final photo today is showing our substitute aircraft display and additional gear


(9)   (since 28 April 2024)

On 26 April 2024

we continued with our DMG5K survey


Current status: 28 April 2024


Hans brought along his HP signal generator, as our R&S stops at 520 MHz; whereas Hans' generator goes up to about 900 MHz and even beyond.


Hans is pre-adjusting his HP signal generator


Our first concerned the limiter stage before the two discriminator diodes


Hans' signal generator being adjusted at 525 MHz

as this is about the centre of the operational frequency spectrum of 505 to 550 MHz

FM deviation 20 kHz

output adjusted at 50 mV

At the moment of when our experiments started we did not have access to any circuit description.

WE even did not know what the actual IF frequency was.

After returning home this first thing that bothered me was to read the DMG5K manual (

And the circuitry had been quite well explained.

The discriminator possessed of two coils of which differed + and - 80 kHz from the centre frequency of 650 kHz.

In the next principle schematic designated by me as 'a' and 'b'


AOB: Why do the use such a broad-band discriminator band-width?

Left us do not forget, that beside voice communication that two full telex-channels were incorporated within the DMG5K system.

And, telex , teletype or Fernschreiber as the Germans designated it, were using nearly square wave signals, and as to main them the bandwidth should accordingly be prepared for it.


Our first incorrect assumption was that the signal had not been picked up from the centre between the two diodes; actually being two RV12P2000 pentodes, where all electrodes were interconnected versus the cathode. But that the audio signal was derived from the upper end) end of resistor '165'.

For better understanding I added two 'silicon diode symbols'.

Our first measurements concentrated at measuring the signals available at the two coils. The arrows 'a' and 'b' were used to viewing the various behaviours at these points.

Audio had been derived from the designated point 'c'

The dc level designated at point 'd' is varying in dc level as a function of the correct tuning centre

This will be described later.     


We first measured at the anode of the limiter valve in this schematic designated '139'

It has to be noticed though, that the component numbers differ from what is actually encountered in the current DMG5K

However, in contrast, with a few exception in the mains module all wire numbers being in full accordance to the schematic.

As to find out matters we use the wire (cable) numbers instead. The signal at anode of the limiter valve is clearly cut-off at the upper part. However


However, at the coil end (blue) 'a' and 'b' we still measure quite some noise

Deviation had in this case been reduced to 7.5 kHz at the HP signal generator. The two


The two RV 12P2000 valve designated: 1. Demodulator and 2. Demodulator

are actually constituting the two discriminator valve diodes.


We measure in dual channel 'Alternating' modes; where each channel connected on to arrow (blue) a and b

It is interesting to notice alternating between two signal the channels is occurring in segments. Next we see a second switching sequence.  


I had to take various photos before a caught just the photos succeeding quasi.


Up being channel one

and below representing channel two


This time  we measured:

the two signals at the arrows a and b

But now in between the two maxima

so that we are viewing at two signals of, say, equal signal amplitudes but only having a phase difference

measured again at the arrows designated a and b and roughly possessing a 90° phase difference (At a centre frequency of about 650 kHz.)


We still measure at 525 MHz

When we consider that the covered frequency setting being 505 MHz to 550 MHz that our 525 MHz is roughly in the centre, which well is matching the the scale currently set at about 70

What also became apparent, was that after a while the frequency drift of the font-end local oscillator hardly called for additional receiver adjustment.

Please notice the U-shaped construction:

This is a switching provision as to provide a mechanical centre of the Motor-Nachlauf (centring) tuning motor (equilibrium)  


On top of the main frame we notice a audio monitor

So that we can also distinguish whether the audio suffers too much distortion due over-loading of incorrect tuning.


I suppose you have got now a better understanding of what we have done today


The basic schematic of the automatic motor fine tuning as to keep the main signal in the centre of the discriminator circuitry

The two Neon bulbs being part of a bridge circuitry using the discriminator dc-output as a fine-tuning reference.

Later we will enter this subject more in detail, as some of it is not existing in our set any longer, as we have noticed that the plug-in unit actually looks alike

see (M2155     M2155return)

So we must improvise quite allot.



(10)   (since 5 May 2024)

On 3 May 2024, Hans and myself continued our DMG5K Survey


In the foregoing period between my last contribution of 28 April 2024, I have accomplished some theoretical schematic-analysis.


Please bear in mind the forgoing brief schematic about Such- und Nachstimmvorgang (Motor controlled)


Let us please consider first the correct tuning (Nachstimmung) mechanism

In the upper schematic half we notice a line originating from the discriminator output.

The circuitry by itself is not delivering a defined 0 V level and then going up- or below 0 V when the carrier centre is positive or negative against ground.

I designated quite briefly how the fine-tuning stage is functioning.

Valve '263' is during 0-adjustment being connected onto ground and the tuning meter zero point in the meter at its centre.

The 'Kontroll' push button being released and the discriminator signal now being fed onto valve '263'

We tested it and it started responding, but apparently a spindle-shaft had been broken-off.

So there was no way around then to repair this nuisance.

The polarity being drawn connected onto the 'cold' end of the current meter '268'

This polarity relay is now bringing the 'Nachstimmrelas' (relay) on the far left-hand side down of the combined drawing, out-off its centre position in such a way, that the fine-tuning motor runs in the direction to compensate the frequency deviation. And when this has been reached stops rotating.

The green and blue colour currents are flowing as to counter the origin of a signal deviation (letting rotate the motor clock-wise or counter clock-wise).

It doesn't matter whether the own local oscillator- or the received signal is causing a signal deviation, the "Nachstimm-Motor" will counter it. 

The way this Nachstimmrelais is being placed (mounted) as shown on the next photo.


Please notice the typical S&H relay down on the right-hand side


Though, what about on the next right-hand side brief schematic?

 Let us first take a brief view of what the lower relay circuitry is about

'Ausschalter für Suchvorgang' is the switch  


The lower part of the foregoing schematic compilation, actually is represented, on the rear, with the three + one additional relays (the big brown/black) circular device, is the hold-delay relay-assembly

Though, our Roman III and IV plug-in unites aren't functioning any longer. And therefore all circuitry linked onto these two units have to be omitted entirely.  


Viewing partially at the schematic of the plug-in unit (Roman) II 

Switch '286' constitutes the main signal search on-off facility

For us this is not relevant, because the actual measuring circuitry is no longer facilitated in our current DMG5K system.

Test have today shown that principally the 'Nachstimm' facility does function; albeit that the coupling between 'Mo 1' and the tuning mechanism is broken-off.



Luckily we possess another unit, albeit in quite poor shape, the defect essential being intact

I am pointing in two cases the three screws by which means the entire tuning-mechanism can be dismantled.


The second screw that simply has to be set free; the third one is about - in the middle left of the tuning motor


With some luck the entire assembly can be demounted easily.


Always bear in your mind: German gear is quite often thought over from a serviceable point of view

The repairs we are currently accomplishing, never took place in the field!

Such repairs had to be maintained in special factory controlled repair-facilities.

This unit is only supplying the spindle (shaft)  from the motor on the far down right-hand side towards left-hand side.

The rest we do not consider essential in our current context.


Viewing the spare-part carrier module from what normally never is visible

It looks as if it is ceramic, but actually it consists of a light-metallic die-cast.

The label on the inside of the lid, it informing the user which bearing (ball- or sliding) has regularly to be lubricated.



Text: Beim Auswechseln des Antriebes Druckschrauben nachstellen

Briefly: when replacing this module, please readjust the pressure-screw.


Please consider the spindle-shaft in front running horizontally from left towards the far right-hand side


It proved quite difficult to demount the the essential spindle-shaft

We first tried to accomplish it by means of heating the miniature ball-bearing up; this attempt proved fruitless!


A typical Luftwaffe universal 24 V (also made in a 12 V version) dc motor

Quite versatile and powerful devices.


Finally we succeeded and the spindle-shaft has to replace the broken-off shaft in our nicer module

Please notice the small pressed structure of the Al gear wheel:

This was a typical Siemens way of a deformation of the crystallic-structure; this prevented the (later) deformation of pressed metallic structures.


The spindle which looks fine and is to replace our broken-off device


Please notice down Aus Ein Träger suchen constituting switch '286'


Is '286' (Ausschalter für Suchvorgang) (schematic block up on the right-hand side) is the switch shown down in the centre of the foregoing photo

As our DMG5K system had been adapted after the war, and it served in systems of the West-German Bundespost and it was modified for carrier-telephony (five channels) applications, the two (Roman III and IV) plug-in modules being in a useless to us; we therefore cannot rely on the wartime schematic (in this respect).

The Bundespost made all wartime systems redundant about the introduction of the UHF TV channels (zweites Programm), in the early 1960s.

However, the rest of the modules are functioning damn well!


Hans Goulooze labelled the wires with their original cable numbers

(these labels may be about 50 years old, but luckily we possess still complete serials between: 1 and, say, 100)


The 'new' spindle (shaft) originating from the other unit being mounted again

An interesting detail, is, that this module being fit with 'sliding bearings' whereas the other module being fit with ball-bearings.

As the speak about gliding-bearings in respect to lubrication, I suppose the ball-bearing concerned a later modification.   


The Germans had a penchant for die-castings

Complicated constructions can easily be produced and adjusted elsewhere.

By the way, the spindle (shaft) in front is the one with a broken-off spindle-end.


(11)   (since 10 May 2024)

Yesterday: Wednesday 8th May 2024

We approached the motor fine tuning stages again and again, but with no success at first.


Hans' first approach was to mount the tuning frame inside the DMG5K receiver plug-in frame (Roman II) again


After this had been accomplished we continued with our survey

But firstly without a break-through.

Then Hans made a remark, which I was not aware of:

This was once a not connected wire and I soldered it at the most near point!

But first we had to pass through other deadlocks.


Please notice the two contacts left of the left-hand meter housing


The swich-finger show in the last photo actually being '264' in his principle schematic

But still without a sound result.


This slightly lifted blue wire was the 'corpus delictum'!

Hans once did solder its open end at the point in between the two orange capacitors!

Actually interconnecting the centre of the two green resistors with the centre of the two orange capacitors, which were already inter-connected!

Then I remembered Hans' remarks and I judged whether the wire-length did fit to the nearby anode connection and it did!


What actually happened indicated by means of the red cross

Eureka! At the cathode I measured ca 60 V, whereas I did not measure 1 V!

Line '210' (about the bottom of this drawing) actually constitutes the filtered discriminator output, containing only dc and all audio information being eliminated.

Wow, the audio output multiplied by more than 500% and the circuitry is operating soundly since.

See YouTube film 00176 for the explanations.



 A static photograph is not useful when a dynamic process has to be demonstrated


Manually it is possible by means of tuning the potentiometer-control '256' in the cathode circuitry does respond a bit, but by no means what someone may expect


YouTube contributions:


Film 00175:    Today we would like to approach our deadlock again. The motor tuning is responding, but controlling the receiver tuning by means of the discriminator does not operate. Manually we can force the tuning motor to rotate in either direction, but by no means is showing a capturing response.


Film 00176:    We are now viewing the section which we consider much cause our nuisance! The two valves on the left-hand side constitute the two discriminator diodes. Hans told me this morning, that when we approached this circuitry he saw an open wire and connected it to what he thought the wire had been broken-off. Then we found that the the interconnected cathodes possessed a dc level of about 60 V, whereas at the loading resistors we hardly measured 1 V. Please notice the foregoing circuitry with the red crossed indicating what connection was failing. Since the circuitry operates soundly.  


Film 00178:    Hans is detuning his HP signal generator and we can see the various responses of the motor-controlled frequency tuning.



(12)    (since 19 May 2024)


On Friday 17 May 2024

we continued our DMG5K Survey


The Receiver plug-in module being remounted in its appropriate cabin, and is functioning


Down on the left-hand side, we notice the transmitter module; the various cover-plates have been removed already

This Roman I plug-in unit is connected by means of two extension cable (once kindly made by Dick Zijlmans)


In front we notice the HT power supply

The flat device on top of the red selenium rectifiers we see a wire-wound resistor which is to be adjusted such, that the current flowing through the two STV (150 V 40 mA), not being overloaded - even when a proper 'load' is failing.


Not well visible is, that the resistor wire is about in the middle broken, which I suppose can de repaired by means of an old trick

The wire thickness is, I guess,  >0.1 mm. 


Hans' screwdriver is pointing at a critical component: an electrolytic capacitor of 8 µF 500 V (max. 550V). Date printed on it tells us: 7.44 (thus produced in July 1944)

Tested it, capacity hardly measurable; which is rather normal after, say, 80 years have passed since!


Please notice what is inside the ellipse designated A

Our main concern today, inside the ellipse, is component '78'

The electrolytic capacitor, today about 80 years old and being most unreliable!

Secondly of some concern: is the variable wire-wound resistor designated '80' which is, according the manual 3 kΩ.

Apparently its very thin resistor-wire (> 0.1 mm diameter) is broken, but this nuisance can be cured, without too much problems; Deo volente, of course.


As usually - the construction is allowing quite easy serviceability


Considering the serviceability a bit differently


Quite striking the thinness of the resistor-wire


Hans firstly had to demount the electrolytic capacitor of 8 µF 500/550 V

We replace it by two modern electrolytic capacitors in series - with a pair or resistors across each 22 µF capacitor; as to divide the, say, 500 V in equal voltages.


Hans took care with demounting the electrolytic capacitor


The left-hand side rectangular capacitor carries two dates: 12 October 1942 and 22 October 1942

I suppose that these hermetically sealed-off types, by the way typical German technique, being in an operational shape still.

Operational voltage 1200 V (dc) and maximally tested at 2500 V; temperature range -30 to +70° C


The internal matters has been disposed off


The vertically mounted circuitry is the modulator stage of the transmitter


The selector is to adjust the audio-level onto the regular system telephone signal


(13)   (since 26 May 2024)

 On Friday 24 May 2024


We continued also with our DMG5K transmitter module


Today we reached a real milestone!


It proved that our flat-wire-wound resistor (> 0.1 mm) was broken at several places and we decided to remove its resistor-wire windings (with a sad mind); and to replaced it by means of substitute power-resistors; as we would like to operate the system by means of a dummy-load

Hampering is, that the free-space on top of it left is quite limited.


That the wire had become so fragile might have been due to the many years this set had been maintained in service by the Deutsch Bundespost (up to, I suppose the end of the 1950s when the "Zweitesprogramm" (UHF channels) became operational


It became soon evident that the genuine STV 150/40 neon stabilisers were defect and operating un-reliably, even mechanically

Luckily, Dieter Beikirch once provided me, very kindly, with a pair of the LK 199 stabilisers; so that all operates well, a minor nuisance is that we cannot watch the ignited status of the neon gas; but measured a outside glass-envelope temperature of, say 42° C. Which at least is informing us - that least the neon gas is conducting.



The meter is indicating is the current flowing through the external resistor arrangement

It finally was decided to keep the two 4.3 kΩ 10 W in parallel thus constituting 2.15 kΩ 20 W.

Current-flowing between 55 and 60 mA (mostly caused by the electronic circuits inside the TX module.


Voltage across the two STV stabilisers in series being: 271.1 V

This is according the data-sheet.


We actually notice: 56 mA current is been provided




Within red the circle B we notice the audio signal amplifier and transmitter modulation module

We encountered problems within the audio module: we at no point could measure any HT voltage of say 270 V.

Failing were: the voltages at line '72 and line '60'

Therefore we removed the audio module and tested all resistors. At the inter-connector sockets all voltage were available.

But, we could not find any failure in the wiring of the module as well as resistors.

Finally, we replaced the module again and it operated as may be expected; Why? We do not know, what might have caused problems were inter-connector pin problems.

The first two photos of today were about the wire-wound resistor in circle A designated '80'. 


The pre-amp being valve designated '24' and the modulator valve being valve designated '39'.

AOB: Not known to me, but I would not wonder - that the actual audio-response might possess a certain characteristic as to emphasise like is usually in FM- broadcasting techniques, known as: pre- and de-emphasis, where parts of the audio-spectrum being designed that, after detection, the signal-noise ratio is improving. Afterwards, this can be compensated for in a reversed manner; so that the frequency response seemingly becomes linear again. 


But today we entered a new step: is it possible that the transmitter being received in our RX plug-in module?


Indeed it does; and this instant can only be noticed by means of YouTube films.


YouTube Films:

Film 000181:    Today the 24th of May 2024 we are receiving (bridging less than 1 m) our transmitter signal (via a dummy-load at ca. 525 MHz) We achieved to day that the receiver is operating in such a way, that it compensates the temperature related frequency drift.


Film 000183:   Continuing our demonstration in which the receiver module is responding at audio-tone modulation of the transmitter. Fist the audio-generator being set at 1000 Hz, then the pitch being increased to 2000Hz showing, by the way, a higher audio response.



(14(since 3 June 2024)

I have to apologise for some delay, as my sister did pass away and had to travel to Groningen in the North


also privately there were things to arrange first.


On Thursday 30 May 2024


We approached the bolometer system.

Bolometer, mostly being operated for measuring accurately HF energies.

There rather delicate devices being operated in a bridge circuitry, where one bolometer is being loaded some set current and the second bolometer is being loaded with the same amount of current. In this case the two bolometers in series being adjusted rather accurately. Whereas one bolometer additionally is absorbing some low HF current. In the context of DMG5K it is in the UHF spectrum.



 As to increase its system sensitivity, the 'bolometer bridge' has to be aligned by means of wire-wound potentiometer 'P'. This has a resistance of 3.2 Ω the two adjacent resistors '101'and '103' being of 15 Ω (still as if these are new)

We will learn, that the variable potentiometer P has broken resistor wires. Hans thinks that he can repair the device.

What measuring technique is used here?


The bolometer circuitry is entirely being fed from the 12.6 V ac of valve '112' thus supplied with 12 V ac 50 Hz.

Please imagine now: there is no HF energy supplied (being involved). When the bridge is adjusted accurately then there will be no signal available across the bridge - thus across the left-hand wires '124' and '125' towards the transformer windings; thus concerning, theoretically,  50 Hz ac voltage only (as long as the bridge is not in balance). 

But when HF energy is flowing through the transmitter coaxial system the sensitive bolometer is rapidly changing its resistance.

The bridge balance is no longer existing fully and across the primary windings of transformer '111' a 50 Hz signal being supplied onto the amplifier stage.

When now, the HF current being interrupted it takes some response-time before the bridge balance being established again.

This 50 Hz amplifier circuitry output delivers an indication of the available HF energy; in our case it is at, say, 525 MHz into the dummy-load.   


Please consider the two centric Al bodies inside the two circular tubes - visible are just the two glass-envelopes of the bolometers


Consider please the rear side of the two bolometer bodies (envelopes)

Please notice on the upper part of the brown 'pertinax' mounting plate the circular hole; which is, normally holding the wire-wound potentiometer designated: '102' which value is 3.2 Ω


Viewing it from a slightly different perspective


Pointing at one end of the wire-wound potentiometer which is broken


 Pointing at the other end of resistor wire

Both being broken, but apparently already once 'repaired'

Why broken?


More often found in US constructions, but apparently also in German wire-wound resistors

This is due to - not well understood of the behaviour of metal-crystallography!

Sharp bending is destroying the local metallic crystal-structure and is causing, over the time, a disruption.

Often found in 'General Radio' wire-wound controls.

Simply, lacking the sound understanding of what does cause such a phenomenon.

I did suggest to Hans to try to repair it by removing partially the obstructing plastic potentiometer (outside) body.

Replacing isn't simple as the so-called "de-humming" potentiometers at least having 50 and up ohms.


Hans suggested to replace it by means of a regular control of, say, ca. 50 ohms

But let us try to cure the genuine one first.


(15)   (since 9 June 2024)

Friday 7 June 2024

We proceeded our DMG5K  Bolometer subject.


We encountered today again problems with the Bolometer circuitry

We are currently looking at the so-called Michael oscillator module operating between ca. 500 .. 560 MHz.

Isn't it a beauty to look at?

The oscillator valve, not inserted yet is of type LD 1

It is a remarkably stable oscillator, of course, when a certain time warming-up has been considered.

 (down on the far left-hand side: L 3 is designated in the schematic below being: '53'

and C8 being '54' in the schematic

Line '91' is also visible 'running from 'C8' to the anode of valve designated '51' (LD 1)

Our main concern today is Bolometer measurement-system

In particular about cavity '94'

Hans first tried to implement the repaired Bolometer ''bridge" potentiometer, formerly designated 'P'.

Whatever we accomplished: bridge balance was adjusted whereas the transmitter wasn't excited; but the wave-meter cavity was tuned optimally at the transmitting frequency.

Balance was easily accomplished, viewing the 50 Hz at the primary or secondary coil windings of transformer  '111'.

When bridge balance was adjusted the 50Hz sinewave signal viewed at the CRT screen showed at optimal balance a wave shape and not a sine-wave like response.

Whatever we accomplished, there hardly could be noticed a dis-balance of the Bolometer bridge circuitry.



As a kind of "Deux ex machina" I discovered, entirely unexpected, some DMG5k transmitter parts and among it also a beautiful Bolometer arrangement.

Hans, changed the beautifully genuine looking Bolometer circuitry: 'Bolometer Anordnung' entirely.


I decided to change the transmitter module entirely including its attached Wellenmesser designated '94'


I must admit - that I do not think that our problems being solved.


Our newly discovered module

You might not be aware, but notice the lock screw near to the Bakelite telephone arrangement

This one is genuine whereas in our set it once had been replaced unprofessionally

This is easing to recognise which module we are actually viewing at.


You might recognise the tall screw, which compared with the forgoing photo differs quite much

The front-panel has to be removed first; the tuning knobs constituting the main obstacle due to oxidation of the die-cast knobs inside. 


Front-plate being removed

The left-hand scale is for the transmitter tuning, whereas the right-hand scale is the "wave-meter" tuning-scale.


The old transmitter and wave-meter (Wellenmesser) module being mechanically disconnected

Please notice the "tall screw" (now in a horizontal position behind the upper circular scale)

This latter screw is telling us that we are looking at the old unit.


Please notice again the tall screw

Viewing the module from a slightly different position.


Noticing the space where the newly discovered module has to be placed at

Please notice the blank circles around the holes, these guarantee electrically sound grounding-contacts. 


The genuinely locked special screw is telling us that this unit concerns our replacement

The module on the left-hand side with designation Ü 3 concerns the Bolometer amplifier which's purpose it is to amplify the Bolometer bridge signal.

Whether it will do now? I have my doubts, but we have to go through it anyway.


The Germans used telephone-like "tree-wiring"

Which makes reconstruction rather easy.


We mounted our Roman I plug-in unit on the table and should, Deo volente, test the rig again


Next week we would like to repeat our forgoing tests again

The strange tube in the background constitutes our miniature "dummy load"

On top we notice the BNC terminator-resistant of 70 ohms.


Maybe you get a better understanding of what it is about


Viewing the Dummy load from a different perspective


The coaxial inside is also quite impressive

The reason, as to minimise the cable loss between the DMG5k and the antenna-array on a remote mast.


(16)   (since 14 June 2024)

On Wednesday 12 June 2024

We continued our regular DMG5K survey, but also initiated two new web-pages:

One on the very kind donation of an AEG K4 Magnetophon tape recorder (year of its introduction was 1938) incorporated in its genuine suitcase.

Though, lacking its amplifier and recording module.

Secondly, the kind donator of the K4 Magnetophon apparatus, would like to know what the content on some tapes bobbins is.

I warned him already that our Tonschreiber-b1 machine had not been operated for decades and once had been re-lubricated about 1986, and that it might not operate instantly.

It was true: It did not operate well, actually everything had to be reconsidered firstly.


   Hans' concern today being - getting the 'bolometer' circuitry functioning properly.

Last week represented by our previous contribution, we did change the entire module incorporating the transmitter and attached wave-meter module '91'

The bolometer module (Anordnung) is represented by the schematic on the left-hand side.

Let me try to explain its principle again:

The two bolometers designated ''98' and '99' are two accurately matched resistors-like components, though being rather sensitive to (low) HF currents flowing trough it.

Let us first consider: that there is no HF (in our case UHF) signal across bolometer '98'.

The matched-pair bolometers '98' and '99' are of equal resistance value. Through the flow of current, which being exactly equal in both the bolometers: '98' and '99' devices.

The controlling variable resistor (designated '102') is now adjusted such that the 'bridge' circuit is exactly in balance and only a residue signal being supplied between the lines '120 (at the arm of the potentiometer) and line '125'. (AOB: this might originate from some wiring asymmetry)

When now loop '92' is picking-up some HF energy because the cavity resonator is becoming excited about the transmitter signal-frequency (ca. 525 MHz) bolometer designated '98' is coming exposed by some additional energy and is changing its current resistance value.

This is causing a dis-balance of the bridge circuitry and the according 50 Hz signal being fed onto a especially at 50 Hz 'tuned' amplifier.



It is is evident, that the signal lines designated '120' and '125' will feed the 50 Hz signal amplitude onto the so-called: Bolometerverstärker circuitry


The HF probe was here used with a far too 'long' wires and Hans later used the probe-head directly attached

The two circular Al bodies do carry each a bolometer.


Our fist concern was, to find the exact transmitter frequency and the (accordingly) matching wave-meter scale.

The transmitter scale is realised by means of a number scale; whereas the wave-meter cavity being calibrated in fixed 'click' numbers.


This was accomplished by measuring with a diode-probe the HF voltage across the lines '121'and '123' (thus across bolometer '98')


Significant here is only the maximum meter deflection, but not its actual value!

As we necessitate in this brief experiment to assure that the transmitter signal that is fed onto bolometer designated '98'


Quite a nuisance is the 20 ms trace projection on the crt-screen versus the exposure time of our digital Leica camera

I had to take time and again a series of photos and select later which one is suiting best.

Shown here is the remaining 50 Hz residue signal measured at the grid of valve (between line '128' and '1')



Now we connect the HT onto the transmitter again, and the bolometer circuitry is now being brought in a state of dis-balance.


Viewing (at the grid of a RV 12P2000 valve designated '112'), here wired as to create a triode) now the output of the bolometer circuitry caused by the HF energy fed onto bolometer '98', causing an additional dis-balance of the bridge-circuitry

Where the value of dis-balance is also indicating the amount of HF energy fed onto the wave-meter thus also bolometer' designated '98'.  


   The meter response connected onto the lines '133' and '134' across resistor 105

The diode bridge is, say, 80 years old might this be the reason of the 'low' meter deflection.



The meter is combined with the control panel and is designated '121'

It is normally always connected, with the exception of pushing a test button at choice.


Hans tested a range of diodes wired in a regular bridge circuit

All with comparable results thus still a rather minor meter-pointer deflection.

An aspect which one should always bear in mind, is, that a bolometer arrangement is responding with some delay, as the HF energy has to supplement onto the existing bridge arm currents; and this necessitates some (temperature) response-time.

Hans even disconnected the diodes wired in series as to prevent negative deflection, without any results visible.  

Meter designated '121' is visible about the centre of the unit.


(17)   (since 23 June 2024)

On 21 June 2024

Hans continued with his Bolometer survey; albeit that I followed Hans' doings only quite briefly.



It wasn't easy today.

Hans suspected transformer designated '111' its secondary winding resistance was fluctuating

He therefore demounted the one of our spare unit.

The results, however, were marginal better.


The left-hand side meter shows a current of ca. 55 ą 60 µA

Which is for a, we guess, 100 µA (0.1 mA) meter about mid-scale meter pointer deflection. Please consider the next photograph


Please view left of the meter-pointer the very difficult visible black line; it is evident that the meter pointer is only slightly out of its zero position

It is, in our perception hardly possible to decide whether the transmitter is functioning appropriately.


What could be causing the insensitivity of the moving-coil meter is the remaining magnetic flux of the internal magnet.

The nuisance is, that we do not know what the actual sensitivity of this moving-coil instrument.




The bolometer amplifier stage is providing sufficient 50 Hz signal


(18)   (since 29 June 2024)

On 26 June 2024

We proceeded before the summer period.

Our endeavour getting the DMG5K operational again has reached a state of functioning, albeit, there are still matters to accomplish. But, now more in the field of placing the outside modules at their according places in the frame again. This implies also placing the various shielding covers.

But let us first show the final bolometer reading from the replace genuine meter, taken from a spare module.


 It is evident that the meter deflection is within the yellow marked region



Viewing the ultimate bolometer-output signal (measured at the anode of valve designated '112' foregoing schematic


YouTube demonstrating the Bolometer full functioning YouTube


Film 000189:    Today we have reached a stage of affairs, where we may consider that we have reached nearly all our goals. The DMG5K is operating as it should once have been. Albeit, that we only consider: sending an audio signal tone which is being received. Bridging a distance of, say, a single meter as the transmitting energy being absorbed in a dummy-load.  Two modules (Roman III and IV) are useless, as these were deliberately destructed and the other one had been changed for carrier-telephony in the early days of the German Bundespost. We start showing it functioning. Then we switch the system off and restart it again. The main power-supply HT being delayed about 12 to 15 seconds due to a delay system.



To be continued in due course


By Arthur O. Bauer