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  Most recent updates on this page:

	12-June-2010: added several literature references for narrow AF filters.
	26-Mar-2010: added home built printer of Jan, ex-ON4ASZ/EA3DPB, and Dale, VK1/VK2DSH.
	17-Dec-2009: added articles and photo captions for other home-builts.
	7-Nov-2009: added photos of other home-builts.

    

[My Hell-printer]   [Other homebuilt printers]   [Hell/CW filters]

My Hell-printer. I built a mechanical Hellschreiber (receive/print only) in 1983. Most of the structure was made from double-sided PCB, precision-cut with a CNC machine during a summer job at the Dutch public telephone company (PTT) in 1982. The the printer spindle and the paper transport rolls were machined during a university internship at Philips Research Labs in Eindhoven in 1983.

My Hell-printer measures 15x8x12cm (lxhxd)


Click on photo for original without labels

My home-built Hell printer in 3D
(you will need red/green glasses to get the stereoscopic effect)

The circuitry for the tone detector, solenoid driver, and motor speed control (paper transport and spindle) is rather straightforward. The schematics below show the circuits that I used.

Detector and solenoid driver circuitry
(from Bastiaan, PA3FFZ)

Speed regulator for the spindle motor
(from Wireless World, August 1980)

                  Detector and solenoid driver                Speed regulator for the paper transport motor
(from Martien, PAØMJS)

Alternate detector designs are described here:

	"Radiomodem voor Hell, Morse en RTTY", Koos Fockens (PAØKDF), Electron, 10-12/1986  [in Dutch].
	"CW/Hell-skrift demodulator", Jan-Martin Nøding (LA8AK), Amatørradio, 1987, nr. 41, February, Nr. 41, pp. 41-43, Nr. 42, March, pp. 66-69

Paper transport speed calibration is easy: all you need is a stopwatch and a ruler or tape measure. Simply adjust the motor rpm to obtain about 47 cm per minute. Unlike the spindle in a Feld-Hell machine, my spindle is a single-thread (1-start) worm. It needs to turn at 7 columns per character x 2½ characters per sec x 60 sec/min = 1050 rpm. The Feld-Hell machine has a double-thread (2-start) spindle: it has two intertwined windings. Note that the number of threads is not related to the number of turns of each thread!  Both the Feld-Hell and my homebuilt spindle have two turns. The Feld-Hell spindle only needs to turn at ½ x 1050 = 525 rpm,  to get the same effect as a single-thread spindle at 1050 rpm.

Adjusting the spindle speed to 1050 rpm can be done several ways: 1) first get the electro-magnet to work properly (which you have to do anyway, hi), then feed the printer with "known-good" Hell-audio and adjust the rpm until the print becomes legible and the text lines are horizontal; or 2) use a tachometer.

I used method nr. 2 by building a very simple optical sensor (4 cheap components), attaching a strobe disk to the spindle shaft with a dab of glue, and measuring the sensor output signal with an oscilloscope. You can make a crude strobe card with a black felt marker, but such markers are really not black at all, and the ink does not have good reflectivity. Best is to print one on regular paper with a laser printer. Here is a file with strobe disks that have 1, 2, and 4 black segments and ±5 cm (2") diameter.

Small strobe disk attached to the spindle shaft

The sensor simply consists of an infrared LED and a phototransistor. No specific types (you can also use an IR diode + transistor in a single package, e.g., an SY-CR102). I shielded them with shrink tube (black on the 5 mm LED, blue on the 3 mm phototransistor in the photo below). As you can see in the photo, I bent the leads such that the diode and transistor are not parallel, but are trained at a point just in front of them (about 2-3 cm, 1").

      
My very fancy tachometer sensor

The largest swing in the sensor output signal (5 Vpp) was observed at a distance of 0.8-1 cm (±3/8") from the strobe disk. The scope image below indicates that the spindle is turning at 1500 rpm, and needs adjustment. A period of 10 msec (100 Hz) = 100 cycles per second, and 4 black segments on the strobe card means 100/4=25 rps which is 25x60=1500 rpm. The scope is set to AC-coupling.

The scope indicates 1500 rpm
 

The ink-roller is made of several round felt pads that I stacked, glued together, and drilled a hole through the center. The pads are simply felt bumper-feet for furniture (the ones with adhesive backing are the easiest), available in several forms at your local Do It Yourself store .

I use Pelikan-brand endorsing ink. It is thick and comes in a tube. The label says that it is intended for felt pads. Just what I need. You just may want to use gloves when handling this type of ink! And don't use water-based ink; not only will it cause the spindle to rust, the capillary properties of the paper will cause the ink to be dispersed and make the print fuzzy.

Some other options are Pelikan dark violet "Stempelfarbe ohne Öl" (endorsing ink without oil). This is liquid ink that comes in a small bottle. Another (slightly) thicker liquid ink is Edding T25 (e.g., black); this is basically professional drafters ink.

Here are the descriptions of some other homebuilt Hell-printers and Hellschreibers:

	"PAØMJS maakte zelf een mechanische hellontvanger", Dick Rollema (PA0SE), Electron, nr. 10, October 1980, pp. 557-559 (courtesy Gerard Wolthuis, PA3BCB)
	"Selbstbau eines Feld-Hellschreibers", Eckhard, DG9JO, RTTY, No. 2, 1981
	"Schrijftoestel voor ontvangst van hell- of morsesignalen", Martien, PAØMJS, Electron, 5/1982
	“Mode HELL, zelfbouw (deel 1-2)”,  Bastiaan Edelman (PA3FFZ), CQ-PA, 10/1997, pp.  348-349 and 11/1997, pp.  385-387    new some more details are described here by PA3FFZ; personal communication, June 2008, in Dutch
	"Hellschrijver voor zelfbouw (deel 1-5)", (description of home-built Feld-Hell of Martinus Rooth (PA0MPR) and A. Rooth-Bemms (PA0ARB)), Bastiaan Edelman (PA3FFZ), CQ-PA, 2/1994, pp. 32-34, 3/1994, pp. 63-64, 4/1994, pp.  97-98. 5/1994 , pp. 121-122, 12/1994, pp. 322-325    new
	"A home - built, direct printing telegraph system. *The Feld-Hellschreiber*", Dale Hughes (VK2DSH), Amateur Radio Magazine (WIA),  Vol. 68, No. 11, November 2000
	"De XYLCD - Deel 1 & 2", 128x256 LCD with micro-controller that can be programmed to be a hardware Feld-Hell receiver, see photo below), by Wim, PAØWV, CQ-PA, 2008, nr. 11 & 12 [in Dutch]. See photo further below.

The Hellschreiber printer of Bastiaan, PA3FZZ
(home-built spindle is etched from brass rod; motors are tape transport motors from a VCR (high torque and easy to control speed, unlike VCR video-head motors)

 

Homebuilt printers by PAØARB and PAØMPR (SK)

(on display during the 2003 annual Hell-meeting hosted by Cor, PAØVYL,

at the Jan Corver Amateur Radio Museum in Budel; original photo courtesy Gerard, PA3BCB)

Homebuilt Feld-Hell by PAØARB and PAØMPR (SK)
(modified typewriter keyboard; instead of character drum: single stationary character-disk with 7x14=98 contacts + diode matrix (±800 diodes) + rotating contact; printer helix made of 2 turns of heavy-gauge steel spring (5 mm ID), stretched to 10 mm (for 10 mm wide paper); ECF80 triode/pentode)

The Hellschreiber printer of Jan Smeets (ex-ON4ASZ/EA3DPB)
(paper transport motor is a 220 Vac squirrel-cage motor; printer helix is driven by a 24 Vdc motor, with (coarse) speed control via a rheostat/potmeter); the printer's electro-magnet came from an electro-mechanical pulse counter (mfr: Hengstler); prints on standard 11/16 inch wide Telex punch tape)

Fragments of some QSOs printed with Jan's machine
 

The Hellschreiber sender/printer of Dale, VK1DSH/VK2DSH - built 1999
 

(ACSII keyboard from a TTY terminal, a Single Board Computer (SBC) with a 65C02 processor (source-code is available from Dale), spindle motor (with optical encoder) from a DEC line printer, paper transport stepper-motor from a 5¼" floppy disk drive, electromagnet from a telephone relay, 16-char LCD, electronics for motor speed control, tone generation, and tone detection/magnet driver; prints onto standard 11/16" telex paper tape. All details are in Dale's article referenced above)

Close-up of the printer mechanism

The XYLCD project of Wim, PAØWV
(the LCD measures approximately 12½x7 cm / 5x2¾", including the rim)

Hell/CW filters. At some point, I built a very sharp AF filter for my printer (-3  dB bandwidth is 150 Hz, -60 dB is 680 Hz!). The filter output impedance is that of land-line POTS: 600 ohm. This matches the audio input impedance of Hellschreibers, as they were also intended for use over telephone land-lines. The design is from Klaas, PA0KLS (ref. 1).  It uses five standard 88 mH toroids, surplus from telephone companies; 88 mH loading coils (in various forms, sizes, materials) have been used extensively on long telephone lines worldwide since the very early 1900s. In 1951, the 135 mH value was introduced (in addition to 22, 44, and 88 mH). In phone systems, these (series) loading coils do not permit data communication (POTS-modem, (A)DSL, etc.) beyond voice bandwidth.

The 88 mH telephone toroids have become rather hard to find. There is a very good alternative: the Toko 10RB series. I bought two sets of five at a very reasonable price at JAB Electronic Components in the UK, for use in a filter per ref. 9 Note that that you can find inductors of the same value for considerably less money elsewhere. However, you will find that they do not work in these filter applications: their DC resistance is much too high - at least five times that of the Toko inductors!

Articles about this and other passive narrow AF-filters:

	Ref. 1: "Laagfrequent filter voor Hell en CW", in "Reflecties door PA0SE", Electron, 6/1981
	Ref. 2: "CW-Filter mit 200 Hz Bandbreite", E. Kantz, Funkamateur, Nr. 8, August 1981, pp. 402-403
	Ref. 3: "Die Frequenztrennung", Funktechnische Monatshefte (FTM), Heft 5, May 1942, pp. 66-68
	Ref. 4: "Laagfrequentfilter van ON4ASZ voor telegrafie en hellschrijven”, in "Reflecties door PA0SE", Electron, January 1992, pp. 5-6
	Ref. 5: "Audiofilter - realisiert nur mit L und C" [ON4ASZ/EA3DPB filter], Frank Sichla (DL7VFS), CQ-DL, 3/1999, p. 223
	Ref. 6: "CW-Signalregenerierung ganz einfach" [ON4ASZ/EA3DPB filter], CQ/DL, 9/1999, p. 735
	Ref. 7: "Audiofilter - einfach, aber gut" [ON4ASZ/EA3DPB filter], CQ-DL, 12/1999, p. 984
	Ref. 8: “Passive (LC) narrow band AF filters” [with ON4ASZ filter], Pat Walker (G3VA), in “Technical Topics”, Radio Communications (RadCom), March 1998, pp. 62-63
	Ref. 9: “Een passief laagfrequentfilter voor de ontvangst van telegrafie”, Roelof Bakker (PA0RDT), Electron, January 1997, pp. 15-20 (incl. PA0KLS CW/Hell filter). Here is a small Excel spreadsheet that I made for calculating the filter component values.

A razor-sharp passive CW/Hell filter
(additional circuitry on the right is an LM386 AF amplifier (bottom) and a TL082 input buffer amp)

CW/Hell filter design by Klaas, PAØKLS
 

Interfacing the CW/Hell filter to an RX and Feld-Hell
 

Band-pass characteristics of my filter
(spec curve in red, measured curve of the bandpass filter in my Feld-Hell is shown as reference (curve shifted to 1 kHz))

©2005-2010 F. Dörenberg N4SPP

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