By now, we should all be familiar with the ubiquitous Hell Feldfernschreiber. The Feld-Hell has several designators, notably:
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Siemens-Halske "T typ 58" (Telegrafietypenschreiber 58) |
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Drawing Number Tbs (Typenbildschreiber) 24a-32 |
Until recently (June 2011), I thought that model 24a-32 was the only Feld-Hell model. Then Pierre Destexhe made me aware 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 suffix "b" suggests that it came after 24 "a". The machine documented here, was built in 1936.
The label on the front of the electronics box of Feld-Hellschreiber type 24b
The electronics box has a label that refers to it as being Type "Sv" ("Schreibverstärker" ?): A similar label on the front of the gear-box (between the motor-generator and the ink roller lever) calls it "S8K". The serial number of this particular machine from 1936 is 641 (same number on the electronics box, motor-generator, and gear-box). This suggest that at least 641 units were built.
The label on the electronics box of Feld-Hellschreiber type 24b
(label is located on the left-hand side of the electronics box))
The label on the gear-box of Feld-Hellschreiber type 24b
(label is located just above the lever arm of the ink roller)
The photo below clearly shows that Tbs 24b has very much in common with Tbs 24a.
Hellschreiber Tbs 24b
(unedited originals of the photos used on this page are courtesy Pierre Destexhe)
However, there are some significant differences between 24a and 24b. First of all, the electronics box:
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the interconnect block has no jack for a patch cable to a Feldfernsprecher (field telephone). |
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the interconnect block has no jacks for "Mithören" (monitoring, headphones), but does have jacks for "Sender" (transmitter). |
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there is no volume control knob, but a high-low gain toggle-switch. |
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there is no "900 Hz tone filter" on-off switch. The filter is selected via the configuration switch (see further below). |
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the voltmeter has a range of 0-300 VDC for the 250 VDC nominal anode voltage (instead of 0-190 VDC); the battery voltage range is the same (0-15 VDC). Note that the official maximum anode voltage of the RV12P400 tubes is 200 VDC. |
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there is no 12-pin round connector, but a plate marked "Anschlüsse für Prüfgerät und Ferneinschaltung". I.e., connections for test equipment (manufacturing, field test, repair & overhaul?), and for remote control. The latter probably is remote switching from "Bereit" to "On", via tone detection, to allow unattended receive operation. The connector is shown further below. |
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Per the schematic (at bottom of this page), model 24b has five tubes of type RV12P4000, instead of four. The final amplifier stage (the printer solenoid driver) has two RV12P4000 tubes in parallel. |
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The circuit card is not installed vertically at the rear of the box. Instead, it is installed horizontally, just below the tubes (valves). Component access is reduced. |
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There is no "Sammler - Netzgleichrichter" (battery -power supply) switch. |
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The fuse is not accessible from the outside. The box has to be opened to replace the fuse. |
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The mounting bolt (bottom left-hand corner of the box) does not have a slotted head (for a screwdriver or coin), but a wing-nut head, for easy manual operation. |
The front of the electronics box of Feld-Hellschreiber type 24b
At the center of the front panel is a 6-position rotary switch. 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.
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 follows:
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"Leitungsbetrieb" - operation over phone lines, 900 Hz tone pulses in both directions:
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"Funk Sendung tonlos, Empfang 900 Hz" - transmission via keying of unmodulated carrier (OOK CW), reception of 900 Hz tone pulses :
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"Funk Sendung 900 Hz Empfang 900 Hz" - radio transmission & reception of 900 Hz tone pulses:
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"Funk Sendung 2500 Hz, Empfang 900 Hz" - radio transmission of 2500 Hz pulses, reception of 900 Hz tone pulses:
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"Funk Sendung tonlos, Empfang 200-3000 Hz" - transmission via keying of unmodulated carrier (OOK CW), reception of pulses with 200-3000 Hz tone pulses:
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Disconnected (to be confirmed).
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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 TX) configurations, not during operation over phone lines....
Why does the solenoid-driver stage have two RV12P400 tubes in parallel? Most likely because the printer solenoid requires a large drive-current. This is consistent with an interesting aspect of the electronic circuitry: for printing of 900 Hz tone pulses, there is no tone-detector! The amplified 900 Hz pulses are fed straight to the printer solenoid. The printer solenoid is actually transformer-coupled to the output of the driver tubes.
Tbs 24b: AC-coupling from the input interface all the way to the printer solenoid
Tbs 24a: AC-coupling up to the detector, 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). Actually Rudolf Hell himself 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. It appears reasonable that a series-resonant relay requires a significantly larger current drive than a single solenoid. 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). RV12P400 tubes have an anode current rating of 3 mA and an anode dissipation rating of 1.5 W. In model 24a, the driver tube already operates at its limit: 4 kΩ solenoid resistance, 180 Vdc 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.
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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: |
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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 |
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!
The tone-detector and grid-bias circuitry for operation with 200-3000 Hz tone pulses
No tone-detector for operation with 900 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 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.
The simplified schematic of Feld-Hell model Tbs 24b (12 July 1935)
(click on image or here to get full size [8 MB])
Behind the aforementioned cover plate marked "Anschlüsse für Prüfgerät und Ferneinschaltung", there is a bridged resistor connection, six 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.
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 hook, to facilitate pulling hem out.
The side of the electronics box, with connectors for the motor-generator
One of the two rectangular connectors - with pull-out ring
The top of the electronics box
The rear of the electronic box
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 function and usefulness remains to be verified.
The keyboard of 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.
An other difference is that in the Tbs 24b, the character columns are scanned top-to-bottom. This is consistent with the 12-line font of the T empf 12 "Presse Hell" printer and associated senders (1934-1940). Tbs 24a scans its 7-line character columns bottom-to-top, which is consistent with T empf 14 "Presse Hell" printers (starting 1940). It is (still) unknown at what speed the characters were sent (2.5 or 5 characters/sec). The fast, resonant printer relays described above, are not needed for tone pulses that are at least 4 msec long (ref. 1) - as is the case in Feld-Hell model 24a (8.16 msec). However, they were required for printers for the 12-line font: 1.4 msec pulses (144 pixels, 5 characters/sec). The font of the Tbs 24b has 110 pixels. Whether the Tbs 24b has a speed of 2.5 chars/sec or 5 chars/sec, the pulses are shorter than 4 msec (3.6 and 1.8 msec respectively). Hence, a fast printer relay was required.
The circumference of the character-drum of Tbs 24b
The reconstructed font of Tbs 24b
(some characters look strange or incorrect (e.g. "Y"), but the depicted font is that of the actual drum; as the number of columns is even, characters such as T and I are not centered on the pixel mosaic.)
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.
The slip-contact arrangement of Feld-Hell 24a
The printer unit (spindle + electromagnet) is 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.
The printer unit of 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" ("British") 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.
Spindle of the Tbs 24b Spindle of the Tbs 24a
Spindle of the Tbs 24b Spindle of the Tbs 24a
(right-handed, top-to-bottom) (left-handed, bottom-to-top)
The 12 VDC motor-generator turns at 3600 rpm, like model 24a. However, the generator of model 24b outputs a nominal anode voltage of 250 VDC (50 mA), instead of 165 VDC (25 mA). The higher anode 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.
The motor-generator of Tbs 24b
Label on the motor-generator of Tbs 24b
There is no hinged cover for the paper trays. Hence, the frame of the keyboard does not have a release button for such a cover. The paper trays have a release button on the right-hand side, instead of the front.
Paper trays of the Tbs 24b
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.
The Tornister carrying case of the Tbs 24b
The 3-color "Buntfarbenanstrich" 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 initative of the German private industry and the German government. The purpose was to standardize specifications of, e.g., colors, and hence promote industrial rationalisation. 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.
©2011 F. Dörenberg N4SPP
