Feld-Hell electronic circuitry and schematics

Posted on Feb 7, 2014

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. 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

Feld-Hell electronic circuitry and schematics
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coupled) inductors and a single capacitor (here: C18). The two inductors are formed by the tapped windings on one side of coupling/isolation transformer SG ("Summer-G bertrager"). So this transformer cleverly fulfills two functions. 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 46 dB (5. 3 Neper). The "Leitung" port is bi-directional: keyed tones are output to the phone line,...

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