The electronic circuitry of the Hell-Feldfernschreiber comprises four stages, each with an RV12P4000 vacuum tube (or "valve"):

	Tonsummer – AF-oscillator stage (here: 900 Hz tone)
	Vorstufe – (pre)amplifier stage (here: 900 Hz bandpass filter and amplifier)
	Endstufe – final stage (here: tone detector and amplifier/driver for electro-magnet solenoid control)
	Reglerstufe – regulator stage (here: motor speed regulator)

These stages are shown left-to-right in the schematic below. The Feldfernschreiber is powered by 12 Vdc. The power input is passed to the 3-position main switch via a line filter (not shown in the schematic below) and a fuse ("Schmelzsicherung", S). A 5 Ω series-resistor can be switched in series with the fuse, for operation with an early model of a Hellschreiber-specific transformer-rectifier power supply unit. The heater filament of each tube is powered by 12 Vdc (hence the "12" in the tube designator RV12P4000), as is the motor-generator. The generator provides an anode voltage of 150-180 Vdc (165 V nominal) to the tubes.

Simplified schematic of the Hell Feldfernschreiber
 

The 900 Hz tone is generated with an LC-oscillator. The "L" part of the "LC" is provided by the inductance of the oscillator's output coupling transformer T1.
 

Schematic of the 900 Hz tone oscillator
 

It is a variation on one of the two standard Hartley oscillator configurations (named after its inventor Ralph Hartley, who was awarded the patent in 1920). It has two series-connected (and coupled) inductors and a single capacitor (here: C18). The two inductors are formed by the tapped windings on one side of coupling/isolation transformer SÜ ("Summer-Übertrager"). So this transformer cleverly fulfills two functions.

 

Hartley oscillator configurations
(note that the series-fed circuit arrangement (a) is less suitable for radio applications)

 

When a key is selected via the keyboard, the continuous 900 Hz tone is keyed by the associated track of the character drum ("Geberwalze"). See the "raster can" section of the "how it works" page. The tone can also be keyed with the "Morse" telegraphy key of the keyboard.

 

The keyed tone is output via transformer T2 to the telephone line I/O port ("Leitung" connectors La and Lb/E), and the telephone jack that is connected in parallel to this port. The "Leitung" port has an impedance of 800 ohm (at 900 Hz). This may at first appear to be incompatible with what many people consider to be the (fixed) impedance of standard POTS telephone land-lines: 600 ohm. However, keep in mind that phone lines only have 600 ohm impedance at one single frequency: about 1300-1400 Hz. For 900 Hz (the Hellschreiber tone), the impedance of a standard phone line is actually 800 ohm! At the high end of the phone-voice bandwidth (3400 Hz) the line impedance drops to about 350 ohm. See ref. 1. The impedance of the connected line is not at all critical, as long as the end-to-end attenuation of the 900 Hz tone is less than 43 dB (5 Neper). The "Leitung" port is bi-directional: keyed tones are output to the phone line, and tones from an other Hellschreiber are received from this phone line. The latter are, therefore, also transformer coupled. The output level of the keyed 900 Hz tone is at least 2.2 V at the La-Lb/E port, and at least 6.1 V at the "Empfänger" port (ref. 13).

 

The same transformer T2 provides coupling of the keyed tones to the bandpass filter / pre-amplifier stage; this path allows printing of the text transmitted by the own Hellschreiber. Some Feldfernschreibers are equipped with an optional 12-pin round connector on the front panel. If the corresponding plug is inserted, the keying contacts of the character drum are diverted to this connector, and the keyed tone is no longer coupled to the pre-amplifier stage or the line output.

 

All transformers have a static shield that is connected to ground. Additionally, T1 is placed in a shielding box that is also connected to chassis ground. Shielded wiring is used where it is prone to picking up noise or hum.

 

Tones can also be received from an other Hellschreiber via a radio receiver. The associated "Empfänger" (receiver) input port passes through a line filter (not shown in the schematic above), and is tied across the secondary side of transformer T2. This is sort of a "transformer-coupled wired-OR". This port has an impedance of 4000 Ω (at 900 Hz).

 

At this point, a potentiometer provides audio input volume control. The potmeter output is passed through an LC "Tonsieb". I.e., a bandpass filter. As for the tone oscillator, the "L" part of the "LC" is formed by a transformer. In this case, the T3 coupling to the pre-amplifier tube.
 

Schematic of the 900 Hz filter

Per ref. 2, this filter has a bandwidth of 150 Hz; per the 1941 manual (ref. 3 and 4), the bandwidth is 100 Hz. I measured 130 Hz for my machine (see bandpass curve on the maintenance page). The filter is turned off by manually switching-in a "spoiler" resistor between the audio gain potentiometer and the filter.

The pre-amp has an impressive gain - as to be expected: the RV12P4000 tube has a μ of 4000. At the monitoring port ("Mithören" - "to listen in"), the 900 Hz tone begins to clip at 15-20 Vp (36 Vpp !). A signal of 55 mV at the line-input produces at least 1.8 V at the "Mithören" output (ref. 13).

The output of the pre-amplifier is coupled to the tone-detector via transformer T4. A separate secondary winding of this transformer is output to the "Mithören" port. This enables monitoring of the received radio signals that were passed through the 900 Hz bandpass filter and pre-amp, and tune the receiver to change the pitch of the tone with respect to the filter's center frequency.

Note that all interfaces are transformer-coupled, so no isolation transformers are required when interfacing a Feld-Hell machine to a PC soundcard or solid-state transceiver.

The detector is a full-wave rectifier, comprising two "Kupferoxydul-Gleichrichter": cuprous-oxide-on-copper diodes. Note that in those days, the word "diode" was reserved for vacuum tubes. RC-filtering reduces the ripple on the rectified signal and ensures that only pulses with a certain minimum duration are passed to the printer's solenoid driver-  simple form of noise suppression. Such diodes often came with one or two pairs in the same package (ref. 5, 11, 12).

 
A two-diode Kupferoxydul-Gleichrichter                          The two Sirutor diodes of my Hellschreiber
(a.k.a. "Maikäfer", maybug beetle, for its buggy look)                                                                                               

 

This type of  metal-oxide diode was invented around 1922 by Lars Grondahl and Paul Geiger (their 1927 U.S. Patent 1640335 only lists the name of the prior). They are made by heavily oxidizing copper plate on one side, in order to form black cupric oxide (CuO, called tenorite in mineral form). Subsequent heat treatment (annealing) causes the formation of red cuprous oxide (Cu₂O, called cuprite in mineral form) between the copper and the cupric oxide. The latter is etched off, leaving a semiconductor PN-junction of Cu₂O and Cu. Ref. 30-47 provide an exhaustive scientific treatise of these early semiconductors. These diodes are quite suitable for detectors: they have a forward voltage (a.k.a. "knee" or "turn-on" voltage) of only 0.2-0.3 V, and a relatively linear forward I-V curve. This is similar to germanium diodes (as compared to 0.5-0.7 V for silicon diodes). With larger dimensions, they were also used for rectifiers in radio power supplies (ref. 28, 34), and high power applications.

 

Note that asymmetrical conductivity was already discovered in lead-sulfide some 50 years earlier, by Ferdinand Braun (1874). From the early 1900s on, this was used for "cat's whisker" signal detectors in crystal set radios. Braun also invented the cathode ray tube. Walter Schottky (1886-1976) was head of the Siemens-Halske semiconductor research labs around 1930 (ref. 48). He was the founder of barrier-layer semiconductor theory (ca. 1938), discoverer of shot noise and the Schottky effect, and inventor of the screen-grid tube, the tetrode, and the superheterodyne receiver (Schottky filed the associated patent mid-1918, some six months before Edwin Armstrong in the USA, who developed it independently and to whom it is usually - but incorrectly - attributed. Ref. 14).

 

The diodes in the Hellschreiber are marked "Sirutor". This is short for "Siemens Rundfunk Detektor", a "Kupferoxydul-Pillengleichrichter". Siemens-Halske started to produce these small-signal detector diodes around 1930. The 1940 list-price was 3.75 Reichsmark a piece (ref. 11); this is roughly equivalent to an estimated 1.95 euros (±2.80 US$ in 2009). This diode comprises a small plastic tube in which a number of cuprous-oxide-on-copper "pills" are stacked (2 mm diameter). Diodes were made with 1 to 10 or even 15 pills. The standard diode has 5 pills, and the tube is marked with a "5" (or "5b") accordingly. The Hellschreiber has two of these. Small copper disks are added to both ends of the stack as fillers, as necessary. One end of the stack has a contact-spring, the other a small filler rod. The ends of the plastic tube are threaded; threaded end-caps are fitted to these ends. The Sirutors in my Feld-Hell have a conical cap on both ends; others one conical and one straight cap (see below).

  The parts of a Sirutor "5" diode


source: ref. 29

 

Each diode-pill has a relatively small reverse-voltage: about 6 volt. So a number 5 Sirutor has a reverse-voltage of about 30 volt (at 0.1 mA). See ref 7, 11, 12. This is why the Hellschreiber diodes are marked "30V" in some original schematics. The diodes are marked with a "+" at one end, but in the early days, polarity markings were not yet fully standardized. As the 1931 warning below shows, even in those days, this caused confusion.

 

       

        Funkschau, Vol. 4, Nr. 2,                             Polarity marking in Telefunken 1948
            11 January 1931, p. 12                                 Presse-Hellempfänger schematic

 

Siemens-Halske also screened Sirutor diodes to higher voltages. They are marked with a yellow band ("Kennstreifen"). They are not used in the Feld-Hell. As Sirutors are more robust, much lighter and smaller than tube diodes, they were also used in the three ring-modulators of the guidance and control control computer of the A4 ("Aggregat 4") missile. (ref. 15, 26). The A4 was renamed "Vergeltungswaffe 2" (V2) upon its deployment in the fall of 1944.

 

The rectified and filtered signal goes to the grid of the final amplifier tube, and controls the tube's anode current. This tube is the solenoid driver. As soon, and as long, as a tone is received, the anode current energizes the solenoid, and the armature pushes the printers paper tape up against the turning spindle of the printer mechanism.

 

A separate tube stage tightly regulates the speed of the 12 Vdc electric motor. The anode current of the tube passes through the motor's governor-field windings. This current is switched on/off by a centrifugal switch on the motor shaft. See details in the "motor control" section of the "how it works" page.

 

Practically all resistors and capacitors have the Siemens-Halske (S-H) logo on it. Resistors are typically carbon type. No color coding is used: component values are printed on the component. The "Telegraphen-Bauanstalt von Siemens & Halske“ ("The Telegraph Construction Company of Siemens & Halke") was founded in 1847 in Berlin. Since 1966 it is simply called Siemens A.G. A brief overview of the company's Hellschreiber related history is here.

 

But there are also components from other manufacturer's...

 

Logos of the companies involved with the design and manufacturing of the Feld Hell

Several capacitors and resistors are marked HOGES, which means that they came from the Hochohm Gesellschaft mbH of Berlin-Schöneberg and Berlin-Adlershof. HOGES manufactured tubes and passive components, especially resistors (as the name suggests). They also marketed components from other manufacturers under their own label. The HOGES resistors are typically dark green, as opposed to S-H grey. There are also some HOGES capacitors in the motor-generator. E.g., the yellow filter capacitors C70 and C71 (in the schematic further below; 200 pF, 10%, 1500 V). Hochohm's earliest patent dates back to 1927 (Reichspatentamt, Nr. 565176).

  A HOGES filter capacitor from the motor-generator

 
HOGES resistor catalog (ref. 17) and capacitor catalog (ref. 18) from the 1930s

The capacitors are marked with both operating voltage ("Betriebsspannung", Bsp. or Betr.-Sp.) and test voltage ("Prüfspannung", Psp.) or max/peak voltage ("Spitzenspannung", Spitz.-Spg). Typically 250/750, 500/1500, or 750/2250 volt. Some electrolytics are 25/30. Typical operating temperature limit is 70 ºC (160 ºF). Resistors have 5% tolerance.

The electrolytic capacitors in the motor-generator are made by Richard Jahre GmbH of Berlin (hence: JAHRELYT). These capacitors are actually packaged inside a small cardboard tube that measures 40½ mm x 12 mm (length x diameter; ± 1⅝" x ½").

 JAHRE electrolytic capacitor from the Feld-Hellschreiber

  
 JAHRE catalog from 1938 (ref. 19) and Jahre advert

JAHRE advert from Telegraphen-, Fernsprech- und Funk-Technik, Nr. 8, 1936, p. 235


JAHRE advert from Schneiders Bauhefte Nr. 6 of 1942

Richard Jahre GmbH Kondensatoren & Induktivitäten (in Wilhelmshafen), is still a capacitor manufacturer today, still specialized in mica ("Glimmer") capacitors. Mr. Jahre founded the "Apparate- und Modellbau" company in November of 1919, at the age of 24, originally to develop and manufacture demonstration devices for university physics courses. During the early 1920s, as radio became increasingly popular, he recognized the market for series production of components (e.g., capacitors, inductors, detectors) and test equipment (decades, precision reference components, wavelength meters, etc.); the name changes to "Radioapparatebau". Jahre's first patent dates back to 1931 (Reichspatentamt , nr. 587098). In 1965, Jahre sells the company to the Roederstein group (ROE). In 1977, the company moves to Wilhelmshafen. Richard Jahre died in 1994 at the age of 99.  Ernst Roederstein Spezialfabrik für Kondensatoren GmbH started out in Berlin, and moved to Landshut (Lower Bavaria) in 1958. In 1993, Roederstein was acquired by Vishay (a leading global manufacturer of discrete semiconductor components and passive components).

 
Listings in the "Kondensatoren" (capacitors) section of the Berlin "yellow pages"
Berliner Adreßbuch 1929 (Part II, p. 418) and  1943 (Part II, p. 371)        
(source: Berliner Adressbücher 1799-1943, telephone & address books of Berlin)

As an interesting side note:  in 1979, the Jahre company acquired the mica capacitor manufacturer Scherb & Schwer Glimmertechnik (mica) of Berlin-Weißensee. In the early 1940s, this company was called "Scherb & Schwer Electro- Glimmer- und Preßwerke". In 1941, Scherb & Schwer had acquired Jaroslaw's Erste Glimmer-Waren Fabrik (Jaroslaw's First Mica Products Factory), also of Berlin-Weißensee.

 
Listings in the "Kondensatoren" (capacitors) section of the Berlin "yellow pages"
Berliner Adreßbuch 1940 (Part II, p. 300) and  1941 (Part II, p. 309)        
(source: Berliner Adressbücher 1799-1943, telephone & address books of Berlin)

 

The latter company had been confiscated by the German government in 1940, for ethnic reasons. In the 1930s, Jaroslaw's had been one of first manufacturers of "Hartgewebe": a cotton-reinforced laminate of a particular thermosetting polymeric resin (phenol-formaldehyde, PF). They used the trade name "Turbax".

 

 

The Hell Feldfernschreiber contains a small block of Hartgewebe in the centrifugal speed regulator, and several gears are made of it, as are the tracks at the bottom of the unit. Turbax is still used today for gears (e.g., in the automobile and motorcycle industry): it is very durable, lightweight, quiet-running, and can be machined like wood and metal, and can also be molded. Phenol is a by-product of coking coal, and formaldehyde is obtained by converting aldehyde, e.g., by oxidizing vaporized methyl alcohol (methanol -highly toxic, unlike ethanol). The latter is also known as "wood alcohol", as it is a by-product of the distillation of wood. Both coal and wood were in plenty supply.

 

There are no manufacturer's markings on the Hartgewebe parts of the Feld-Hellschreiber. However, the official 1941 manual (ref. 4, 5) mentions "turbax". As Jaroslaw was located in Berlin (like Siemens-Halske), and manufactured gears, it is probable that Jaroslaw supplied the turbax parts.

 

 
 Jaroslaw brochure from 1938 (ref. 20) and a 1935 product overview (ref. 21)

 

An equivalent of Turbax is HAREX, a trademark of  Hermann Römmler GmbH. Römmler was founded in 1867 in Spremberg, some 80 km (50 mi) southeast of Berlin. In 1921 they obtained license-free access to the bakelite patents. Römmler had a background in the processing of textiles (e.g., cotton flocks for shellac gramophone records), which facilitated the step to cotton-reinforced PF. One of their molding materials had the trademark HARES (registered ca. 1908), derived from the phonetic pronunciation of the company logo "HRS" (the initials HR of the company founder and S for Spremberg; ref. 23, 24). I have contacted the company early 2010, to find out of there is a similar explanation for "HAREX", but they had no info on this. In 1938, the company became a wholly-owned subsidiary of Brown, Boveri & Cie (BBC).  BBC was established in 1891 in Baden/Switzerland, as a major player in the field of large electrical motors, electrical power generation and distribution; it merged with the Swedish ASEA company in 1988 to form ABB. Based on the success of their Resopal Schichtpreßstoff (a laminate, dating back to the early 1930s), the Römmler company name was expanded to Resopal Werk H. Römmler GmbH in 1971.

 

 

Numerous other manufacturers produced similar Hartgewebe materials with (registered) brand names such as Novotext, Tenazit, Thesit, Taumalit, Esconit, Bernit, Resinol, Resitex, Tenatext, Trolitan, Tufnol, Celeron, and Micarta. Later on, an other polymer replaced PF for many applications: polyepoxide (a.k.a. "epoxy"); it was patented in Germany in 1939 by the giant chemical conglomerate Interessen-Gemeinschaft Farbenindustrie AG ("I.G. Farben" for short), a company that became infamous for other chemicals during WWII. I.G. Farben originally comprised other well-know companies such as Bayer, BASF, Hoechst, and AGFA.
 

The wire-wound volume control potentiometer of the Feld-Hell was made by "Kabi": Karl Biermann, a small manufacturer of electro-mechanical components in Berlin-Johannisthal during the latter half of the 1920s, through WWII.

 

            
"Kabi" potentiometer of the Feld-Hell and "Kabi" advertising
(advert source: "Die Sendung - Das Rundfunkwesen", Vol. 8, Nr. 26 June 1931, p. 511;  Nr. 32 / XII, 4 Dec. 1931)
 

Phonebook entries for Karl Biermann (top to bottom: 1925, 1926, 1932, 1939)
(source: Berliner Adressbücher 1799-1943, telephone & address books of Berlin)

 

The Feld-Hell has a 2 watt light bulb for its signal lamp (Ba9s bayonet-base). It was made by OSRAM. OSRAM is the contraction of Osmium-Wolfram. Wolfram is also know as tungsten, from the Swedish for "heavy stone". OSRAM is the alloy used for the helical filaments of incandescent light bulbs. An other alloy used for filaments at that time was Wotan: Wolfram-Tantalum. Early 1906, OSRAM was registered with the imperial patent office in Berlin as a trademark for "Elektrische Glüh- und Bogenlichtlampen von der Auer-Gesellschaft" (electrical incandescent & arc lights of the Auer Co.). The "Auer Gesellschaft" (Auer von Welsbach) was also referred to as the "Deutsche Gasglühlicht-Anstalt" (the German Gaslight Establishment). In 1919, the Auer company, Siemens-Halske, and AEG, founded the OSRAM G.m.b.H. & KG (Kommanditgesellschaft = limited partnership), to combine their lightbulb manufacturing activities. They adopted OSRAM as their brand name. The first OSRAM logo (see below) is from the same year. In 1929, during the Great Depression, the International General Electric Company (subsidiary of GE USA), acquired an interest in OSRAM.  During WWII, a number of production sites were moved from the Berlin area to the eastern part of Germany.  Some remained in east-Berlin. After the war, the sites located in the Soviet occupied zone were dispossessed, and OSRAM lost its foreign subsidiaries and trademark rights. The OSRAM G.m.b.H. KG was converted to OSRAM G.m.b.H Berlin/München in 1956, with the same shareholders as the original KG. During 1976-1978 Siemens AG became sole shareholder. Ref. 25.
 

   

First OSRAM logo (1929), one of my old light bulbs, and early OSRAM advertising
 

TO BE EXPANDED:

• H. Mende & Co. The electronics box ("Verstärker- und Anschlußsatz", "Amplifier & Interconnect Unit") of some Feld Hell machines was manufactured by Mende in Dresden.

• Chemische Fabrik Griesheim-Elektron. The electronics box, the keyboard chassis, and the lower unit of the Hellschreiber (i.e., gearbox, paper tape compartment) are die-cast alloy. Practically all die-casting was done by Griesheim-Elektron. Though I have found no markings on or inside the castings that confirm this.

The label on a Feld-Hell electronics box -  triangular Mende logo at top right hand corner
(photo courtesy Chris, ON4DFR)

The detailed circuit diagram for Feld-Hell model 24a-32 (a1, a2) is here (8 MB high-resolution jpg file), and here (1.4 MB pdf file); a low-resolution version is shown below. Note: the diagrams may not look good in your web browser, but should be fine when the files are saved to your PC and opened with your favorite image viewer. My document "Schematic, Component-Layout and Connector Pin-Outs of the Hell Feldfernschreiber" contains all the information that the title suggests. It is available here.

The components inside the Amplifier and Interconnect Unit all have a small round sticker on them, with a number that corresponds to the component number in the schematic (W.. for "Widerstand" (resistor), C.. for "Condensator" (capacitor), D.. for "Drossel" (choke coil / inductor), GL.. for "Gleichrichter" (rectifier diode) etc.). Very helpful during troubleshooting and repair! Also, all the solder lugs on the pertinax circuit card, the interconnect blocks, and the tube sockets, have a number printed on them or next to them; it corresponds to a numbered point in the schematic. Note that numbering and naming is not necessarily consistent between all available schematics, and there is variation between models.

        Rear-view of the Amplifier & Interconnect Unit of the Feld-Hellschreiber - cover removed         
(the components and solder lugs are numbered per the schematic)


 Rear-view of the Amplifier & Interconnect Unit of another Feld-Hellschreiber
(this unit was manufactured by "Radio H. Mende & Co. GmbH" in Dresden, rather than by Siemens-Halske; note that in this unit, the component number stickers were placed at the edge of the components, so as not to cover the component value markings)
 

As can be confirmed by tracing the circuit diagram below, when the Hellschreiber’s main switch is in the “Bereit” (ready/standby) position, the red signal lamp extinguishes when one or both of the motor’s carbon brushes is not in contact with the commutator, or there is an open connection in that circuit.

Schematic of the Hell Feldfernschreiber  (drawing nr. 24a-32 (a1/a2))
- a very high-res version of this diagram is here (9 MB) -