Home-made Hellschreiber made by VK2DSH
(e-mail edited by SM6MOJ)
Click for larger image (300 kb).
I have started to write an article
describing my Hellschreiber, so photos will come soon. The idea came from Murrays ZL1BPUwebsite, and it seemed like a fun project - which it was.
The printer uses a helix made from a brass cylinder with two turns of steel wire wrapped around
it. The helix is driven by a DC motor from an old DEC LA120 printer that I
junked years ago. The motor was fitted with a shaft encoder that is used to
control the speed of the helix. The pixels are printed onto the paper tape
by an arm which pushes the tape onto the helix which has a thin film of ink
applied by a sponge rubber pad. The arm is operated by a 3000 type relay coil
switched by a high voltage FET driven by the output of the demodulator.
I can adjust the level at which the magnet actuates and thus control (to a
limited extent!) the contrast of the printed text and the amount of noise
that is printed. Paper feed is by means of a pinch roller driven by a stepper
motor from an old 5.25 inch floppy disk drive. The rotation of the helix
triggers the stepper motor every revolution, so at the end of each column the
paper is advanced. Altogether it's fairly noisy in operation....
The TX side takes characters from an old TTY keyboard via a 65c02 based
single board computer and formats them for transmission. Output from the
SBC drives the modem and PTT for transmission. The software was writen
in 65c02 assembler and is about 3kbytes long(!) I coded the standard 'feld
hell' font. The software can send automated CQ and also allows a several
buffers to be sent, all fairly basic compared to other systems.... So the
machine is a funny collection of bits and pieces but it works quite well,
I have been surprised how well the printer works, even under quite noisy
band conditions.
The object of the exercise was to build the machine using as much 'junk box'
material as I could. In the end I spent about $50 on it, $10 just for a
can of spray paint! The main problem with it so far has been the amount of
paper tape that it uses.... Also the helix needs to be improved, the wire
moves slightly which causes the height of the printed characters to vary,
a machined helix would be much better.
Close-up of the print mechanism
Click for larger image (360 kb).
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