Feld-Hell model TBs-24b

[Introduction]
[Electronics box and circuitry]
[Keyboard and character-drum]
[Printer module]
[Motor-generator]
[Carrying-case]
[Another "S 8 K" base]
[References]

Last page update: 31 January 2017

INTRODUCTION

Until June of 2011, I thought that the ubiquitous model 24a-32 (Siemens-Halske designator T.typ.58) was the only Feld-Hell model. Then I was made aware by Pierre Destexhe of the existence of model TBS 24b-209. Unlike model 24a, the 24b was actually built by the Hell company in Berlin-Dahlem, rather than by Siemens-Halske. The available schematic diagram dates from 1935, the same year that Siemens-Halske started to manufacture model 24a. The machine documented here, was built in 1936, and has serial number 641.

Fig. 1: label on the front of the electronics box of Feld-Hellschreiber type "TBS / 24b-209"

"TBS" stands for "Typenbildschreiber", i.e., Hellschreiber. The Wehrmacht equipment category ("Stoffgliederungsgruppe") "24" refers to "Nachrichtengerät". That is: communications equipment. The "a" in "24a" is the equipment sub-category ( = "Stoffgliederungsuntergruppe"), and implies "Drahtnachrichtengerät". That is: equipment for wired telecommunication. The "b" in "24b" is the sub-category "Funkgerät". That is: radio equipment. The items in each equipment sub-category were numbered consecutively with a "Gerätnummer" (not to be confused with the serial number "Gerätenummer"): "32" in "24a-32" and "209" in "24b-209".

The photo below clearly shows that Tbs 24b has very much in common with Tbs 24a:

Fig. 2: Feld Hellschreiber "Tbs 24b" (left) and the "Tbs 24a"

(unedited originals of all TBS 24b photos on this page are courtesy Pierre Destexhe; used with permission)

ELECTRONICS BOX AND CIRCUITRY OF THE "TBS 24B-209"

Fig. 3: front of the electronics box of Feld-Hellschreiber model "Tbs 24b"

There are some significant differences between 24a and 24b. First of all, the electronics box:

Fig. 4: the primary differences between the electronics box of the "Tbs 24a" and "Tbs 24b"

There is a 6-position rotary switch at the center of the front of the box. It is actuated with a removable key. When not used, the key is clipped to the inside of the Tornister carrying case. The selected configuration is indicated in a window just above the switch.

Fig. 5: the mode switch and associated indicator - for all six possible configurations

The switch configures the output interface (direct on-off keying with the character-drum or tone pulses), the 900 Hz tone filter (on/off), and the tone generator (900 Hz / 2500 Hz). The selectable configurations are as listed in the table below:

Fig. 6: the six mode-configurations, selectable on the front of the electronics box

Note that the tone generator can be selected to 900 Hz and 2500 Hz. It is unclear what the purpose is of transmitting with 2500 Hz tones. In principle, it could support two simultaneous Hellschreiber conversations on the same phone line: one with 900 Hz, the other with 2500 Hz, or simultaneous operation with a voice/phone connection. However, there is no 2500 Hz tone filter in the receiver circuitry. Also, the 2500 Hz transmit option is only used in one of the "Funk" ( = radio transmission) configurations, not during operation over phone lines....

As the simplified schematic of the "24b" shown below shows: there is no rectifier/tone-detector: the amplified tone pulse are fed straight to the printer solenoid. This solenoid is actually transformer-coupled to the output of the driver tubes:

Fig. 7: Feld-Hell "Tbs 24b" has AC-coupling, all the way to the AC-driven printer solenoid

Compare that to the DC solenoid-driver configuration of the classical Feld-Hell model 24a:

Fig. 8: Feld-Hell "Tbs 24a" has AC-coupling up to the detector and DC-driven printer solenoid

This clearly implies that an AC-relay was used, rather than the DC-relay of Feld Hell type 24a (and most other Hellschreibers). Rudolf Hell himself actually describes such a relay for use in high-speed Morse telegraphy printers and early model Hell-printers (ref. 1). AC-type relays were made for specific frequencies, up to about 1000 Hz (ref. 2). The printer relays described by Rudolf Hell, have two separate solenoids in parallel; half of the dual-solenoid relay is resonant at the selected frequency.

AC-relays used in the first Hell-printer models actually required 2 watt energization power, with an associated 5 watt dissipation in the driver tube (§6 in ref. 1). RV 12 P 400 tubes an anode dissipation rating of 1.5 W. In model 24a, the driver tube already operates at its limit: 4 kΩ solenoid resistance, 180 volt DC anode voltage. So, two such tubes were used in parallel in model 24b. An other option would have been to use a different type of tube, with higher anode ratings (but same gain). I am not sure what type of Wehrmachtsröhre could have been used, but it would have violated the "single tube type" principle.

Why does the solenoid-driver stage of the 24b have two RV 12 P 4000 vacuum tubes in parallel? Note that the anode of this tube can sink about 10 mA - but only when configured as a DC-on/off switch, as in the Feld-Hell "24a". In an AC-amplifier configuration, the nominal anode current is less than half of that.

As to be expected, resonant relays have some inherent selectivity around the resonance frequency - here 900 Hz. Hence, the printer will not work with tone frequencies that differ significantly from 900 Hz. This is why in the "Empfang 200-3000 Hz" configuration, there actually is a detector/rectifier placed before the solenoid-driver tubes!

Fig. 9: half-wave tone-detector and summing with constant 900 Hz tone for operation with 200-3000 Hz tone pulses

But how can this work: the rectifier generates DC-pulses and the printer-solenoid needs to be driven with 900 Hz pulses!? The DC-pulses are superimposed onto the grid-bias voltage of the driver tubes. They turn those tubes on and off. The standard grid-bias resistor is implemented as two series-resistors. One of these is used as a coupling-resistor: the constant 900 Hz tone from the built-in oscillator is applied across it. As a result, 900 Hz AC pulses appear at the output of the driver tubes - independent of the tone frequency of the received pulses (200-3000 Hz)! Quite clever.

The simplified schematic diagram of the 24b (Fig. 9 above) shows only a single detector diode ( = half-wave rectifier) and no smoothing capacitor at the output of the detector. But neither do the some of the simplified diagrams of the Feld-Hell type 24a (e.g., in ref. 1). However, the latter does have a full-wave rectifier (2-diode bridge) and an RC smoothing filter. So it is reasonable to assume that the 24b had a similar arrangement.

Fig. 10: the simplified schematic of Feld-Hell model Tbs 24b (12 July 1935)

(click here to get high-resolution image [8 MB])

The electronics box has a label that refers to it as being model "Sv" ("Schreibverstärker" = printer-amplfier). A similar label on the front of the gear-box (between the motor-generator and the ink roller lever) refers to the base-module of the machine as being of type "S8K" (a second S8K module is shown further below). The electronics box, motor-generator, and gear-box of this particular machine from 1936 all have the same serial number: 641. This suggest that at least 641 units were built (which is unlikely, given this appears to be the only surviving machine), or that this is serial-number 41 of series number 6.

Fig. 11: label on the electronics box of Feld-Hellschreiber model24b (left) and label on the gear-box

Behind the aforementioned cover plate marked "Anschlüsse für Prüfgerät und Ferneinschaltung", there is a bridged resistor connection, three pairs of banana plug jacks, and a 7-prong connector. The latter is the same as the one used on Lorenz (and other) fax machines ("Bildschreiber") of the era.

Fig. 12: the test & remote control connectors, and a matching "octa" plug

As with Tbs 24a, the cables of motor-generator and the keyboard/drum unit connect to the left-hand side of the electronics box, with two 6-pin connectors. Here, the pins are arranged in-line, instead of a vertical zigzag; the top two pins have slightly different spacing, to avoid inadvertent swapping. The connectors of Tbs 24b do not have guide pins, and they are marked with a blue and red dot, instead of green and red. Here, the rectangular connectors have a ring-hook, to facilitate pulling hem out.

Fig. 13: the right-hand side of the electronics box with connectors for the motor-generator and one of the two connectors

Fig. 14: top-view of the electronics box

Fig. 15: rear-view of the electronic box

The large block-capacitor in the lower right-hand corner of Fig. 15 above, was manufactured by "Hydra AG" in Berlin. In Greek mythology, Hydra was a many-headed water serpent, hence the company logo with the three snake heads. The Hydra company was founded in 1899. After being taken over by AEG in 1912, its name was changed to "AEG Kondensatoren und Wandler GmbH" (AEG KUW GmbH). It was sold off to the British company Elexis in 1998.

Fig. 16A: capacitor made by the Hydra company

Fig. 16B: listing in the "Kondensatoren" (capacitors) section of the 1929 Berlin telephone & address books

(source: Berliner Adreßbuch 1799-1943 - 1929 (Part II, p. 418)

KEYBOARD AND CHARACTER-DRUM OF THE "TBS 24B-209"

The keys of the keyboard look like regular typewriter keys of the era. This keyboard has fewer keys than model 24a: there is no key for zero, question mark, minus sign, or the pause-character. The "Morse" key has been retained, but is now completely red, rather than having a green dot. The keys are round, as in model 24a, but are of the "metal ring" type. The keyboard has a small lever on the right. It is marked "Fest - Frei"" ( = fixed - released); it may have served to immobilize the slip-contacts during transportation ( = shocks and vibration). The common-track of the charactre-drum is on the far right. The associated carbon-brush is installed at the far left of the angle bracket to from which the springs are suspended. Note: in the Tbs 24a, this carbon-brush is installed on the rear of the unit.

Fig. 17: keyboard and character-drum of the "Tbs 24b"

The character-drum is also different from that in Tbs 24a. Not only because there are fewer keys: the font (pixel pattern) of the characters is completely different! I have combined several photos of the drum, and reconstructed the font. The font of Tbs 24a has 7 columns of 14 pixels (or 7 x 7 with "half" pixels). However, the font of 24b has 10 columns of 11 pixels. This particular drum is not very accurate: the pixel-ring of various characters is misaligned by as much as half a pixel. However, this is not at all critical in Hellschreibers.

Fig. 18: circumference of the character-drum of the "Tbs 24b"

Fig. 19: the font of the Tbs 24b - based on Fig. 18

Note that some characters look strange or incorrect (e.g., "Y"), but the depicted font is that of the actual drum shown in Fig. 18. As the number of columns is even, characters such as T and I are not centered on the pixel mosaic.

Fig. 20: the slip-contact arrangement of "Tbs 24b" - character drum removed

Though the slip contact arrangement is the same in Tbs 24a and 24b, the choice of metals is not. See photo above and below.

Fig. 21: the slip-contact arrangement of Feld-Hell "24a"

This machine no longer has the hinged cover for the paper trays. The frame of the keyboard does not have a release button for such a cover - there is a simple retainer clip, just above the trays. The paper trays have a release button on the right-hand side of the base, instead of the front.

Fig. 22: paper trays of the "Tbs 24b"

PRINTER MODULE OF THE "TBS 24B-209"

The printer unit appears to be the standard Hellschreiber one. Compared to Tbs 24a, the levers for the paper-transport rollers and the ink-roller are simple wrought metal, instead of cast metal.

Fig. 23: the printer unit of the "Tbs 24b"

The printer spindle of all Hellschreiber models turns in clockwise direction (looking towards the Hellschreiber, along the spindle shaft). In the Tbs 24b, the spindle thread is "right handed" (a screw with a right-handed thread is tightened by clockwise rotation). However, the Tbs 24a has a "left handed" thread. As a result, the spindle of the Tbs 24b prints character columns in a top-to-bottom sense, whereas the Tbs 24a prints the columns bottom-to-top. Of course, both are consistent with the way the columns are laid out on their respective character drum.

Fig. 24: spindle of the "Tbs 24b"" (left) and "24a""

MOTOR-GENERATOR OF THE "TBS 24B-209"

The 12 volt DC motor-generator turns at 3600 rpm, like model 24a. However, the generator of model 24b outputs a nominal voltage of 250 volt DC (50 mA), instead of 165 volt DC (25 mA). The higher generator current is consistent with the higher drive current of the printer solenoid. The speed-control regulator-cap of the motor-generator is not made of black plastic, but of metal.

Fig. 25: motor-generator of "Tbs 24b" and a close-up of its label

THE "TORNISTER" CARRYING-CASE OF THE "TBS 24B-209"

The Tornister carrying case of this particular machine has 3-color camouflage painting. This is typical for pre-WWII. It is actually a slightly modified case of a Tbs 24a: a holder for two felt ink-rollers has been added on the inside of the case, as well as holder for the configuration-selection key.

Fig. 26: the "Tornister" carrying case of the "Tbs 24b"

The 3-color "Buntfarbenanstrich" (lit. colorful painting) pattern is sprayed on. Prior to the 1930s, paint was brushed on. Between 1935 and 1937, a standard combination comprised the colors RAL 27, RAL 22, and RAL 18g. "RAL" refers to the "Reichsausschuß für Lieferbedingungen", the German State Commission for Specifications. The RAL was founded in 1925 as an initiative of the German private industry and the German government. The purpose was to standardize specifications of, e.g., colors, and hence promote industrial rationalization. In 1927, the RAL defined a table of 40 colors. It was successively revised and expanded to the current list of 210 colors. After several years, the name was expanded to "Reichsausschuß für Lieferbedingungen und Gütesicherung", to address quality control aspects. Since 1980, the RAL activities are managed by the "RAL Deutsches Institut für Gütesicherung und Kennzeichnung e.V." Note that the some of the original color numbers have been retained, though the associated colors have changed.

ANOTHER "S 8 K" BASE-MODULE WITH KEYBOARD

During May of 2012, the existence of yet another Feldfernschreiber base of model "S 8 K" was brought to my attention. Unfortunately, the amplifier box is missing. So it is unknown if that box was the same as the one of the Tbs 24b described above.

Fig. 27: front of a Feldhellschreiber with base-module model "S 8 K"

(original unedited photos: eBay 2012)

Fig. 28: equipment label of this "S 8 K" - located on the gearbox, above the printer

The serial number "1053" shown on the equipment label suggests a significant production volume, as for the serial-number 641 of the TBs 24b shown above. The number may actually mean something like serial-number 053 of series 1, or even 53 of series 10.

Fig. 29: base-module "S 8 K" - lower paper tray pulled out

The keyboard is identical to that of model Tbs 24b, and has keys with metal rims. In the photo above, note the L-bracket shelf that sticks out to the right of the keyboard, between the keyboard and the paper trays. Its purpose is unknown; maybe it was experimental, maybe it was required to slide into a particular equipment rack.

Fig. 30: rear-view of this "S 8 K" base-module - cover removed

In the photo above, note the large number of (long) screws sticking out of the base of the keyboard-drum unit. The standard Feld-Hell has none of these screws. This may imply an experimental design standard. The character-drum of this "S 8 K" is identical to that of the standard Feld-Hell Tbs 24a, and not that of the 12-line drum of model Tbs 24b described further above.

The pinch-roller of the paper transport mechanism of this "S 8 K" has longitudinal grooves, unlike the flanged rollers of the Tbs 24a and 24b. I.e., the roller touches the paper tape over the full width, and could cause smearing of the freshly printed text. Again, experimental?

Fig. 31: printer module of this "S 8 K"

The motor-generator of this "S 8 K" is not mounted directly onto the gearbox with four bolts. Instead, it is installed onto four rubber anti-vibration mounts, with an embedded nut. These isolating mounts would have reduced the (considerable!) mechanical noise level.

The motor-generator is coupled to the gear box via the standard pin-coupling. However, the pin on the motor-generator side of the coupling does not pull a bar that is mounted onto the motor shaft. Instead, there is a disk. No ball bearing is visible.

Fig. 32: the mounting and pin-coupling of the motor-generator

(one of the four rubber mounts is missing)

The rubber motor-mounts are made by GETEFO, the "Gesellschaft für technischen Fortschritt m.b.H." [Corporation for Technical Advancement]. This company was founded in 1931 by Clemens August Voigt (1891-?), and specialized in rubber products. The GIMETALL department of GETEFO made rubber-to-metal bonded parts ("Gummi-Metallverbindung"). The company made motor mountings for vehicles and aircraft, spring suspensions, dampers, anti-vibration mounts for aircraft and tank radios, anti-vibration mounts for aircraft radio racks (e.g., Ju-88), flexible bases for whip antennas of military radios, seals for aircraft doors and hatches (e.g., Junkers), etc. Post-war, they made engine mounts for major car manufacturers such as Daimler-Benz, Porsche, BMW, Opel, Ford, and for motorcycles (e.g., Steyr-Puch).

Over the years, the company had a number of locations in Berlin: Woyrschstraße 13, Berlin; Potsdamer Straße 147, Berlin W 35; Genthinerstraße 38, Berlin W35; Gottlieb-Dunkelstraße 45-46, Berlin-Tempelhof; Ziegrastraße 15 Berlin-Neuköln; and Kaiser-Wilhelm-Straße 5, Berlin. At some point, GETEFO had activities in Schwarzach. The "yellow pages" of the telephone book of Berlin show no entries for GETEFO prior to 1935. Eventually (1952?), GIMETALL became part of (or merged with) Metzeler Gummiwerke AG, as Metzeler Gimetall AG. Metzeler was founded in 1863 in Munich as a trading and manufacturing company for products made of natural rubber (e.g., gutta-percha) and synthetic rubber (e.g., buna).

Fig. 33: the four rubber motor-mounts made by GETEFO

Fig. 34: listings in the "Gummi" (rubber) section of the Berliner Adreßbuch 1935 & 1943 (Part II, p. 243 & p. 289 resp.)

Fig. 35: registered (but defunct) logos of GETEFO and GIMETALL

Other than the equipment label above the printer, this particular machine only has the following markings: a red Luftwaffe acceptance stamp "BAL 327", and "Berlin 1944" in white. They are located on the inside of the hinged lid that covers the paper trays. It is unknown if the 1944 date is the year of manufacture. The Tbs 24b machine shown further above, was made in 1936...

Fig. 36: markings on the inside of the lid that covers the paper cassettes of the "S 8 K"

So... maybe... the TBs 24b was actually the "next generation" Hell Feldfernschreiber!

REFERENCES

Ref. 1: p. 3, 6 in "Die Entwicklung des Hell-Schreibers" by the inventor himself: Rudolf Hell; pp. 2-11 in "Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell - Gerätentwicklungen aus den Jahren 1929-1939", Nr. 1, May 1940:
Ref. 2: "Mechanische Schwingsysteme als einfache Bauteile für Signalübertragungen" [mechanically resonant AC-relays], W. Rauch, A. Ueberschuß, in "Frequenz – Zeitschrift für Schwingungs- und Schwachstromtechnik", Bd. 14, Nr. 11, November 1960, pp. 363-368