My twenty years with computers – Part 2 – by Jakey G3JKY

Having corrected the errors in my Commodore C64 Manual, I was able to try some of the simpler programs and make them run properly. After a while converting Fahrenheit to Centigrade and suchlike lost some of its appeal, especially as the program had to be retyped every time I wanted to play.

Some means of storing the program was required and the obvious method was to buy the Commodore Datasette tape recorder. At this point I applied the “Hubbard Principle,” (known only to the Clifton Radio Society,) which involves looking in the cupboard to see if there is anything that can be pressed into use. I had a spare cassette recorder and Elector Magazine for January 1985 gave a circuit for connecting the C64 to such an item. Problem solved, I thought!

The circuit included a double op-amp, an attenuator, some clipping and a couple of transistors performing some switchery to make the computer think it was seeing the real thing. I made it up on a PCB, one of my best-looking constructional jobs. Maybe if I had built the usual birds’-nest it would have worked, but in spite of peering at bits of it with the firm’s best ‘scope, I could not persuade it to function.

Perhaps I should have recalled my time on the London Transport bus radio network, where Route 11 used Marconi telemetry with Storno radios. Marconi used one cycle of 1.2KHz to transmit an “O” and two of 2.4KHz for a “1”. Storno telemetry used 1.2 for an “O” and a cycle and a half of 1.8KHz for a “1”. This meant that some telemetry units just would not work with some of the radios, due to phase shift somewhere in the system at 2.4KHz. The result was that the “Os” and “1s” were not synchronised.

Back on my homemade interface board, it seemed unlikely that everything would behave as the author described. So I admitted defeat and bought the Datasette. I spent many happy (?) hours trying to make this device work properly with the C64. A little knowledge, real or imagined, is a dangerous thing, and led me to devise means of coaxing it into life, but not by using the method given in the User Manuals. As the guarantee for the computer was nearing its end, I decided to swallow my pride and return it to the maker for investigation.

This was probably the first sensible thing I had done! When it was returned, everything did work in the way the Manual said it should.

It has only just occurred to me, twenty years later, that there was a slim chance that the Elektor circuit MIGHT have worked if the computer had been 100%. One snag with anything as complicated as a computer is that it is almost impossible to test every function before accepting is as serviceable.

To be continued.

Return to the index of Vital Spark articles.

Jakey – (G3JKY). From VS September 2006.

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