• [Introduction]
• [The HTG-1]
• [Radios used with the HTG-1]
• [References]

©2004-2022 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: January 2022 (addd Fig. 27, added ref. 22, 25)

Previous page updates: November/December 2021 (inserted Fig. 2, Fig. 3, Fig. 9, Fig. 10; added ref. 19, 20, 21 + text, 22, 23; edited and re-arranged some other text, added Fig. 34); October 2021 (added refs. 14-18, added Fig. 25 + ref. 19); 3 October 2019 (added Fig. 2, added pdf for ref. 5A, 5B, 7).

INTRODUCTION

During WW2, the Royal Hungarian Army (RHA) used standard German Feld-Hell machines, possibly with Hungarian legends/labels. Overall, the RHA used Hellschreibers from 1939 until 1958.

Fig. 1: Field telephone exchange unit of the Hungarian army, with "Klappenschrank" switchboard and a German Feld-Hell machine

(source: ref. 3, 1941; Hungarian switchboard model 30-M with 30 lines, HIR = Honvéd (Homeland Defense Force) Infantry Regiment)

Fig. 2: German Feld-Hell machine (pre-1940 model) being used by a unit of the Hungarian Army

(source: collection of the Postal Museum in Budapest/Hungary)

Standard German Siemens-Halske Feld Hell machines were also manufactured in occupied Hungary by the Telefongyár company ("Telephone Factory"), starting in 1939. In Hungary, these machines are referred to as "39 M" (Model 39), see ref. 4.

During the fall of 2011, László Veres (HA7XK), brought to my attention the existence of a post-WW2 Feld-Hell model that was built in Hungary. Note that this is not simply a standard German Hell Feldfernschreiber. This model actually dates to ca. 1948. However, the design, including the electronic circuitry and construction, very closely follows that of the "standard Feld-Hell". It was also manufactured by the Telefongyár company - from 1948 to 1954 (ref. 7) - using left-over components from the war-time Siemens-Halske Feld-Hell production. The designator of this Hungarian model is HTG-1, where "HTG" = Hellschreiber Távírógép = Hellschreiber ticker-tape machine. Two of these machines are in the collection of the Museum of Military History (Hadtörténeti Intézet és Múzeum) in Budapest/Hungary (one is exhibited).

Fig. 3: Teleprinter training at the Signals Officers School, 1955, Hell teleprinter on the left, an STG on the right

(source: Figure 8 in ref. 5A)

The history of the Telefongyár Rt. (TRT, Terta) company goes back to the end of 1876, when János Neuhold officially registered his railway and Morse telegraphy equipment repair activities in Budapest/Hungary. During the following ten years, his repair business gradually expanded with production of telegraph equipment, a variety of small military equipment items, and railway safety/signalling equipment. At that time, the company name was Vasúti szerelvény és elektrotechnikai gépgyár ("Railway assembly and electrical engineering factory"). Around 1890, Neuhold received the first orders from the Hungarian national postal company for telephones. After taking his company public 1891, the name was changed to Neuhold Vasúti Szerelvények és Elektrotechnikai Gépgyár Rt. ("Neuhold Railway Fittings and Electrical Engineering Factory"). After a steady decline in business and unsuccessful refinancing, Neuhold abandoned the company mid-1894. He started a new company later that year: Neuhold és Társa Vasútfelszerelési és Villamossági Gépgyár ("Neuhold and Co. Railway and Electrical Equipment Factory"). In 1900, they were acquired by the German company Telephon-Fabrik AG vormals J. Berliner (TEFAG, "Telephone Factory Ltd. formerly J. Berliner", founded by József Berliner, Jakob Hanover, and Hahn Miksa). Starting in the early 1900s, the Budapest company received large orders for telephone and signalling devices for the Royal Hungarian State Railway. By then, the company was commonly referred to as Telefongyár ("Telephone Factory"). János Neuhold died in 1904, while on a business trip to the Telephon-Fabrik AG head office in Hannover/Germany. His son, Kornél Neuhold, took over the management of the company. In 1911, Telefongyár became an independent Hungarian company. Its activities cover the manufacture of telecommunication and telegraphic equipment, mechanical engineering, metal industry, electrical engineering and related products.

During WW1, the company produced military items such as guns, ignition devices, snowshoes, and boots, as well as light bulbs and small radios. After that war, Hungary was occupied by the Soviet Union, and the company was confiscated. Between WW1 and WW2, the company continued with railway signalling devices and telephones (incl. a handset under license from Telefunken), and expanded with batteries, radios, headphones, rail cart air brakes, fuel meters, audio recording equipment, and odd items such as potato-, tomato- and fruit-pulp thickening machines, zippers, spark plugs, refrigerators, and small boat engines. In 1928, the Standard Elektrizitätsgesellschaft (SEG, German part of the huge International Telephone & Telegraph Corporation (I.T.T.) conglomerate, see here) acquired a majority share in the company, but sold its share to the British Standard Co. in 1933. In 1939, the company was once again converted to produce war material, while telephone production continued. Early 1945, the company was placed under Soviet military command for about 6 months. In 1949, it was nationalized and renamed to Telefongyár Nemzeti Vállalat ("National Telephone Factory Company"), shortly thereafter simplified back to Telefongyár. Products included telephone and telegraphy equipment, telephone exchanges, movie projectors, audio amplifiers, radio equipment, manufacturing machining, audio tape recorders, TVs, and pipeline inspection equipment. In 1990, Telefongyár entered into a joint venture with Siemens-Hungary. It was privatized in 1991 and became a wholly-owned subsidiary of Siemens. Ref. 8, 9. Note that Siemens & Halske already established its presence in Budapest in 1887, with a tramway company. Siemens has been present in Hungary ever since, ref. 10 and ref. 14-18.

During the late 1930s, the Magyar Államvasutak (MÁV, Hungarian State Railways) conceived a nationwide longwave radio network for communication between the central office in Budapest, the regional/district head offices, and major hub rail stations. I.e., not to communicate with, or broadcast to, trains. Radio equipment was procured for 29 radio stations. From December 1940 through 1944, 18 of the planned stations became operational (see Fig. 2 in ref. 21A). Six frequencies in the 295-370 kHz segment of the long-wave band were allocated to the MÁV; 370 kHz was used as the principal frequency (ref. 21B). This telegraphy network was also used with the Hellschreiber system.

The German companies Telefunken and C. Lorenz AG supplied the original radio equipment (ref. 21A):

• Telefunken model AS33 longwave transmitter: 100-1000 kHz, 0.5-1 kW, modulation modes A1, A2, and A3 (keyed-carier telegraphy, tone-modulated telegraphy, amplitude-modulated voice); ref. 25.
• Lorenz longwave transmitter and receiver: 100 & 200 W, 167-1000 kHz. Exact model numbers unknown.

In 1948, MÁV started to rebuild the network and upgrade/modernize the radios (ref. 22A, 24), with:

• Transmitters from the Gép és Villamossági Szövetkezet (GVSz, "Machine & Electrical Cooperative", with Siemens engineers):a cabinet with a longwave transmitter (1000 kHz - 1 MHz, output 500 W), a short-wave transmitter (1-5 MHz, power unknown), and a shared power supply between them.
• Receivers, also from GVSz.
• German Luftwaffe war-time "surplus" aircraft radios FuG 10 SL ( = Sender, Langwellen = longwave transmitter) and receiver FuG 10 EL ( = Empfänger, Langwellen = longwave receiver), both 300-600 kHz, and made by C. Lorenz AG. The transmitter had an output power of 40 W (AM telephony) or 70 W (telegraphy). Ten custom transmitter/receiver sets were built for the MÁV. They comprised an SL, an EL, a power supply, a clock, a telephone handset, and an interface panel. See Figure 2 and associated text in ref. 22A.
• General-coverage receiver model E44, made by the Swiss company Autophon AG (see the radios section below). One such radio is shown in Fig. 4 below.
• Hungarian military shortwave receiver R/7 and the post-war military transmitter R-50 (built by Magyar Philips Művek Rt. (Hungarian Philips Works Co.). See the radios section below.

Fig. 4: An HTG-1 (removed from its carrying case) at the train station at Széchenyi Hill near Budapest

(source: ref. 23; the radio in the background is an Autophon E44, see the "Radios used with the HTG-1" section)

The MÁV longwave network was terminated during the spring of 1958 (ref. 22).

THE HELLSCHREIBER TÁVÍRÓGÉP HTG-1

Fig. 5: Hellschreiber model HTG-1 (left) and the standard Feld-Hell

(all original unedited HTG-1 photos on this page: courtesy László Veres; used with permission)

Fig. 6: Hellschreiber model HTG-1 (left) and the standard Feld-Hell - paper tray cover opened

The amplifier box shows some major differences with respect to the standard Siemens-Halske Feld-Hell:

• The motor-generator turns at 3840 rpm (vs. 3600 in the S-H Feld-Hell).
• The nominal generator voltage is 150 volt DC (vs. 165 in the S-H Feld-Hell)
• There is a volume control for the modulated output pulses, not only for the input signal.
• Interfaces:
• There is no rectangular bakelite connector block
• There is no 12-pin round connector, but 3 pairs of binding posts (wire or banana plug) - more practical!
• There is a 3-position toggle switch for the interface configuration / operation mode.
• The electronics are based on four tubes of type 6AU6 and 6AU8 - instead of four RV12P4000.
• The monitoring/headset output ("Mithören" in the Siemens-Halske Feld-Hell) is not transformer-isolated; instead, it is connected to the anode of the pre-amplifier tube, via a blocking capacitor.
• There is no separate phone jack for a field telephone.
• The push-button to select the range of the voltmeter (battery vs. anode voltage) is a separate switch in the front panel. In the S-H Feld-Hell, the switch is integrated in the bezel of the instrument.
• There is no equipment label with serial number, year of manufacture, etc.

Interestingly, the "battery / power-supply" toggle switch - on top of the unit - has been retained.

Fig. 7: front of the amplifier box

Fig. 8: front of the amplifier box - annotated

Fig. 9: translation of the labels & legends of the switches and connectors

Hungarian is a difficult language - for foreigners. Basically, signal names and switch positions that begin with a "K" (for "Kemény") have to do with transmitter on/off "keying", those beginning with "M" (Modulált), with modulated tone pulses.

The only item that has a label on it, is the motor-generator:

Fig. 10: Label on an HTG-1 motor-generator

(source: courtesy K. Czirják (HA1UK), used with permission)

In the photo above, "Sz" ( = soroztszám) is a 6-digit serial number. It is actually a three-part number. The first digit indicates a version or modification level, "54" is the year of manufacture, and "095" is the actual serial number. Roughly 1500 machines were produced (ref. 7), but not more than nine are currently (2021) known to have survived (ref. 6).

The two diagrams below show the close similarity between the schematics of the standard Feld-Hell and the HTG-1. A detailed schematic of the HTG-1 is provided in ref. 1A.

Fig. 11: simplified schematic of the standard Hell Feldfernschreiber

Fig. 12: simplified schematic of the HTG-1

Fig. 13: top of the HTG-1 amplifier box - tubes, fuse, and toggle switch are easily accessible

Fig. 14: labels next to the vacuum tubes in Fig. 8 above (left to right)

The 6AU6 tube is a post-WW2  7-pin miniature pentode (RCA, October 1945). Ref. 2. It was typically used in AF and RF amplifiers, e.g. in television sets. Equivalents are the CV2524 and EF94, and it is similar to the EF85. Tungsram in Hungary also manufactured this tube (under license). The tube has a large transconductance (4000-5000 μmho, depending on anode voltage), but a relatively low amplification factor of only 36.

• Transconductance is a measure of the change in anode current to a change in grid voltage, with anode voltage held constant; units: mho (old; "ohm" spelled backward), siemens.
• (Static) amplification is gain, expressed as the ratio of the change in anode voltage for a change in grid voltage; units: mu (μ).

Fig. 15: the "6AU6" compared to the "RV12P4000"

The 6AU8 tube is a 9-pin miniature triode-pentode (RCA, 1947). The pentode part is used for the tone oscillator. The triode part is not used. Ref. 1A.

The RV12P4000 tube of the standard Feld-Hell has a large static gain (4000 max) as well as a large transconductance (2300 μmho). The Royal Hungarian Army (RHA) may also have used Tungsram tubes of type HP1220 in their German Feld-Hells, as a subsitute for the RV12P40000. The HP1220 is a high-frequency pentode with 12 volt heater voltage and 20x10=200 mA heater current.

Other components (capacitors, resistors, potmeters, possibly also the transformers) in the HTG-1 Hellschreiber were made by the company REMIX Radiotechnikai Vállalat [Radio Technology Company] in Budapest/Hungary. The company was founded in 1932 and changed its name to simply "REMIX" in 1937.

Fig. 16: rear of the HTG-1 amplifier box - cover removed

As in the Siemens-Halske Feld-Hell, all components have a small sticker with the component number from the schematic. However, here, the signal numbers are not screen-printed onto the circuit card, but stamped into the card. Resistors and diodes have a plastic sleeve.

As in the standard Feld-Hell, the vacuum tube sockets are mounted in a heavy U-shaped profile, to which the circuit card is also attached:

Fig. 17: the HTG-1 circuit card module, removed from the amplifier box

Fig. 18: the other side of the circuit card, with two audio transformers

Fig. 19: looking into the HTG-1 amplifier box from the rear - circuit card module removed

(the green and blue electrolytic capacitors are modern replacements)

Fig. 20: looking into the HTG-1 amplifier box from the rear - right-hand side of the box

(the blue electrolytic capacitors are modern replacements)

Fig. 21: looking into the HTG-1 amplifier box from the rear - left-hand side of the box

(the green electrolytic capacitors are modern replacements)

Fig. 22: looking into the HTG-1 amplifier box from the top - circuit card module removed

Fig. 23: side of the HTG-1 amplifier box with two connectors for cables to motor-generator & keyboard-drum

(same as standard Feld-Hell, though the latter also identifies each connector (and matching plug) with a colored dot)

Fig. 24: keyboard and printer-head of the Hellschreiber model HTG-1 - same as Feld-Hell

Fig. 25: Minor differences on the left-hand side ( = gear box) of the machines

(source: courtesy Istvan "Pista" Kovari (HA2EBA))

Fig. 26: Close-up of the standard Hell printer module of the HTG-1

Fig. 27: Voltmeter of a 1954 HTG-1 (left) and of my 1938 standard German Feld-Hell machine

(note the difference in the anode voltage range (140-160 vs. 150-185 Vdc) and the opposite red/blue color markings)

Fig. 28: carrying case of Hellschreiber model HTG-1

Like the standard German Feld-Hell machines, the HTG-1 has a 1-page instruction sheet on the inside of the lid of the carrying case. Unlike the German machines, it is printed on an aluminum sheet, instead of on paper:

Fig. 29: 1-page operating instructions

(source: courtesy Istvan "Pista" Kovari (HA2EBA), used with permission)

See ref. 19 for the full-size sheet and an English translation.

RADIOS USED WITH THE HTG-1

The Hungarian State Railways (MÁV) used the HTG-1 with several receivers and transmitters. As shown in Fig. 3 above, one of those radios was the model model E44 "all band" receiver. It covers the frequency range of 100 kHz - 37.5 MHz, and was made by Autophon AG in Solothurn/Switzerland. Ref. 20A, 20B, 20C, 20D. The model was ordered by the Kriegstechnischen Abteilung (KTA) of the Swiss Army in 1944, designed that same year (hence Empfänger 1944 = E44) and produced from 1945 to 1951. Only 138 E44 were produced. It measures 40x32x31 cm (WxHxD). The unit comprises eleven tubes of type Philips "Miniwatt" D1F, a pentode known for its rather poor reliability. The associated power-supply is the E44Z, with built-in audio amplifier and loudspeaker. The receiver was used with a simple long-wire antenna, via transformer coupler AT44. The E44Z and AT44 are not shown in the photo.

Fig. 30: The Autophone receiver model E44

(source: ©2021 Ulrich Fierz (HB9AIK/VK2ANI), www.hb9aik.ch, used with permission)

The HTG-1 was used with at least the following Hungarian military radios:

• The R/7 transmitter ("adókészülék", "adó") and the R/7 receiver ("vevőkészülék", "vevő"). Ref. 1, 12. These radios date back to around 1939, and were manufactured by Telefongyár Rt. (Terta), like the HTG-1. They covered the frequency range of 3750-8600 kHz, and operated on 4.5 and 132 volt (from a battery, or an R/7a pedal-powered generator).
• The Hungarian Air Force used the HTG-1 with the R-40 and R-50 transmitters, for communication between bases. Both were CW/AM transmitters and had a "hookup for Hellschreiber". Ref. 13.
• The R-40 had an output power of 100 W (CW telegraphy or Hellschreiber) or 25 W (AM phone). It covered the frequency range of 2.5-20 MHz with three bands. The R-40 was built by Telefongyár - Magyar Adócsőgyár [Hungarian Transmitter Tube Factory] during 1952-1955. This transmitter was also used in radio trucks.
• The R-50 CW/AM transmitter had an output power of 1.5 kW (CW/Hell) or 250 W (AM tone/phone). It covered the frequency range of 2.75-15 MHz. The R-50 was built by Magyar Philips Művek Rt. (Hungarian Philips Works Co.) in Budapest during the early 1950s. This company was established by the 1939 merger of Vatea Rádiótechnikai és Villamossági RT (Vatea Radio Technology & Electrical Co.) and Philips Rádió és a Villamossági Rt. (Philips Radio and Electrical Co.). It was nationalized in 1949, and renamed Magyar Adócsőgyár. In 1966, it was absorbed into the Egyesült Izzólámpa és Villamossági Rt. (United Electric Incandescent Light Factory Co.), which became part of General Electric (GE) in 1989.

Fig. 31: HTG-1 with R/7 transmitter and R/7 receiver

(source: ref. 1)

Fig. 32: R/7 transmitter

(source: radiomuseum.hu, accessed 25 March 2019)

Fig. 33: R/7 receiver

(source: ref. 12)

Fig. 34: The 25/50 W (AM/CW) transmitter model R-40

(source: radiomuseum.hu, accessed 25 March 2019)

Fig. 35: The 250/1500 W (AM/CW) transmitter model R-50

(source: Katonoi rádiók magyarországon - Military radio in Hungary, accessed 21 November 2021)

REFERENCES

If you have a copy of ref. 1, 4A, 14, 17, 18: please contact me!

• Ref. 1: "A HTG-1 távírógép műszaki leírása és kezelési utasítása" ["HTG-1 Telegraph Specifications and Operating Instructions"], István Mag (ed.), issued by the Hungarian Ministry of Defence, 1954; 67 pp. (courtesy László Veres, HA7XK)
• Ref. 1A: HTG-1 wiring diagram (Figure 19 in ref. 1)
• Referenced in "Egy különleges masina, a HTG-1 Hellschreiber adó-vevő" [pdf], ["A Special Machine, the HTG-1 Hellschreiber Transceiver", pdf], article by Miklós András Lázin (HA5CBM) on the Budapest Amateur Radio Club website, 30 January 2017. Retrieved 25 March 2019.
• Ref. 2: "6AU6 - 6AU6A", Datasheet, Tung-Sol Electric Inc., Electron Tube Division, August 1962, 4 pp.
• Ref. 3: "Bilder von der ungarischen und finnischen Nachrichtentruppe", Pleger, p. 230 in "Deutsche Nachrichtentruppen (Die F-Flagge)" [Zeitschrift für die Nachrichtentruppe und Truppennachrichtenverbände des Heeres, der Luftwaffe und der Waffen-SS - Nachrichtenblatt der Nachrichtenkameradschaften des NS-Reichskriegerbundes], Vol. 17, Issue 7, July 1941
• Ref. 4: articles covering the standard Siemens-Halkse Feld Hell built by Siemens/Hungary during WW2, referred to as "39 M" ("Model 39")
• Ref. 4A: "39 M. Tábori távírógép műszaki leírása és kezelési utasítása (1942)" [Model 39 - Technical description and operating manual for field tape-teleprinter (1942)], Magyar Királyi Honvéd Híradó Kiképzőtábor Parancsnokság [Signal Corps School of the Royal Hungarian Army], 1942, 30 pp.
• The cover page of this document is here.
• Ref. 4B: "A 39 M. tábori távirógép részletes kapcsolása" [simplified Feld-Hell schematic; lit. "Detailed wiring of Model 39 field ticker tape"]
• Ref. 4C: p. 48 in "A vezetékes híradó anyaggal kapcsolatos tapasztalatok" ["Experience with wired news material" ], Vermes Zoltán, in “Honvéd” ["Hungarian Army / Soldier"], Vol. 2, Nr. 9, September 1947, pp. 44-50. Source: adt.arcanum.com, retrieved 26-Nov-2021.
• Rough, unimproved Google translation to English, with transcript of the original article is here.
• Ref. 5: "Távírógépek a katonai híradásban" [Military Signals Tape-Teleprinter], Béla Vörös:
• Ref. 5A: "Távírógépek a katonai híradásban I." ["Telegraphs in military news coverage - Part 1"], Vörös Béla, in "Haditechnika" ["Military Technology" magazine], Vol. 25, Nr. 2, April-June 1991, pp. 48-54.. Retrieved 3 October 2019.
• Ref. 5B: "Távírógépek a katonai híradásban II." ["Telegraphs in military news coverage - Part 2"; does not cover Hell printer], Vörös Béla, in "Haditechnika" ["Military Technology" magazine], Vol. 25, Nr. 3, October-December 1991, pp. 40-45. Retrieved 3 October 2019.
• Ref. 6: personal email exchange with Lázin Miklós András (HA5CBM), 5 December 2017.
• Ref. 7: "A magyar középgépipar - Hadiipar és haditechnikai termelés Magyarországon 1945 és 1980 közöt" [The Hungarian Medium-Sized Machine Industry - Military and Military Technology in Hungary between 1945 and 1980], Germuska Pál, Hungarian Academy of Sciences PhD Thesis, 2014, Argumentum Publishing House, 2014, 613 pp., ISBN 978-963-446-735-9. Retrieved 3 October 2019. [pdf]
• Ref. 8: "The Transformation of the Defense Industry in Hungary", Yudith Kiss, "brief 14" of the Bonn International Center for Conversion, July 1999, 34 pp. [pdf]
• Ref. 9: "History of the Telephone Factory from 1876 to 1991", Google-translation of the "factory history" pages of telefongyar.hu; retrieved 4-Oct-2018.
• Ref. 10: "Milestones in the land of the Magyars - Siemens celebrates 130 years of success in Hungary", siemens.com global website; retrieved 4-Oct-2018 [pdf]
• Ref. 12: "A Magyar királyi Honvédség (1922-1945) rádióeszközei: II. ( R/6, R/7, R/7-a, R/7-E, R/7-L, R/8, R/8-a, R/9, R/10)" [pdf]; ["Radio equipment of the Royal Hungarian Army (1922-1945)]
• Ref. 13: "Hungarian Air Force Communications Equipment", CIA Information Report, 3 October 1957, 3 pp. Document nr. CIA-RDP80T00246A037700360001-4. (sanitized / declassified / released 12 July 2010). Accessed 25 March 2019.
• Ref. 14: "A Siemens-Hell rendszerű tábori írókészülék kezelési utasítása" [Siemens-Hell-based field printer equipment operating instructions], Magyar Siemens-Schuckert Művek Villamossági Rt. [Siemens-Schuckert Hungarian Electricity Works Inc.], Budapest, 72 pp.
• The cover page of this document is here.
• Ref. 15: "A Telefongyár története (1876-1991) és vevőkészűlékeinek ismertetése" ["History of the Telephone Factory (1876-1991) and description of its receivers"], Kóger Lászio, Simoncsics Lásló, in "A Rádiótechnika Évkönyve 2008" ["Yearbook of Radio Technology"], pp. 4-18.
• Ref. 16: "A Budapesti híradástechnikai ipar kezdetei - A Telefongyar első évtizedei" ["The beginnings of the Budapest telecommunications industry - The first decades of Telefongyar], Tóth Imre, in "Tanulmányok Budapest Múltjából" ["Studies of the past of Budapest"], Vol. XVIII of "Budapast várostörténeti monográfiái" ["Monographs on the history of Budapest"], 1971, pp. 309-332. Source: library.hungaricana.hu,  accessed 12 October 2021. [pdf]
• "Die Anfänge der budapester funktechnischen Industrie - Die ersten Jahrzehnte de Telefonfabrik" [2-page summary in German], pp. 333-334 in the above document
• Ref. 17: "A Telefongyár története 1876-1976" ["The History of the Telephone Factory 1876-1976"], Sarolta Czakó, Károly Jenei, Telefongyár (publ.), 1976.
• Ref. 18: "A Telefongyár helye és szerepe a magyarországi műszaki fejlődés történetében" ["The place and role of the Telephone Factory in the history of technical development in Hungary"], Faics Lajos, Siemens Telefongyár Kft (publ.), 1997.
• Ref. 19: "Kezelési utasítás", 1-page operating instructions for the HTG-1, original Hungarian version (courtesy Istvan "Pista" Kovari (HA2EBA)) with English translation appended.
• Ref. 20: articles about the Autophon E44 receiver
• Ref. 20A: "Swiss HF-LF receiver E44", Ulrich Fierz (HB9AIK/VK2ANI), in "Signal - The Journal of the Vintage and Military Amateur Radio Society", Issue 59, May 2021, pp. 3-10. Source: www.hb9aik.ch, accessed 11 November 2021. [pdf]
• Ref. 20A: "Autophon MF-HF Empfänger E44 -  Part 1" [in German language], Ulrich Fierz (HB9AIK/VK2ANI), in "History Technic Journal" (HISTEC, Schweizer Zeitschrift für historische Technik"), Nr. 2, 2021, pp. 15-19. Source: www.hb9aik.ch, accessed 13 November 2021. [pdf]
• Ref. 20B: "Autophon MF-HF Empfänger E44 -  Part 2", Ulrich Fierz (HB9AIK/VK2ANI), in "HISTEC Journal", Nr. 3, 2021, pp. 15-19. Source: www.hb9aik.ch, accessed 13 November 2021. [pdf]
• Ref. 20D: "E-602 (E44)". Source: armyradio.wiki, accessed 7 November 2021. [pdf]
• Ref. 21: "XX. Század 1926-1950" ["20^th century 1926-1950"], Chapter 6 (pp. 317-414) of "A magyar vasúti távközlés rendhagyó krónikája" ["Unusual chronicle of Hungarian railway telecommunications 1846-2000"], János Pap, 2019, 940 pp. Source: bgok.hu, retrieved 8 November 2021.
• Ref. 21A: "A vasút hosszúhullámú rádióhálózata" ["The longwave railway radio network"], SzT, pp. 384-387.
• A rough translation to English is here.
• Ref. 21B: "Hell távirók hosszúhullámú rádiórendszeren" ["Hell Telegrams on the longwave radio system"], RM, SzT, pp. 387-388.
• A rough translation to English is here.
• Ref. 22: "XX. Század 1951-1970" ["20^th century 1951-1970"], Chapter 7 (pp. 431-560) of "A magyar vasúti távközlés rendhagyó krónikája" ["Unusual chronicle of Hungarian railway telecommunications 1846-2000"], János Pap, 2019, 940 pp. Source: bgok.hu, retrieved 4 January 2022.
• Ref. 22A: "Megszüntetésre ítélték a MÁV hosszúhullámú-rádióhálózatát" ["MÁV's longwave radio network ordered to be terminated"], SzT, HZ, pp.455-456.
• A rough translation to English is here.
• Ref. 23: post by "Unterfeldwebel" on 6 June 2016 in the "HTG-1 Hellschreiber adó-vevő masina" ["HTG-1 Hellschreiber transceiver"] blog on roncskutatas.com "WW2 military history portal", accessed November 2021. [pdf of that page, translated to English by Google]
• Ref. 24: "Korszerüsitik a MÁV távközlő berendezéseit" ["MÁV's telecommunication equipment is being modernized."], "Amerikai Magyar Szó" ["American Hungarian Word" newspaper], Vol. VI, No. 42, 17 October 1957, p. 9. Source: library.hungaricana.hu, retrieved 12 November 2021.
• Ref. 25: "0,5/1,0 kW - Langwellensender Type As 33/Al 34", Telefunken product brochure, W. B. 187D (2000), February 1939, 2 pp.. Source: www.cdvandt.org. Accessed 3 January 2022.

External links last checked: March 2016 unless noted otherwise.