My twenty years with computers – Part 4 – by Jakey G3JKY

The first real use I had from the Commodore 64 and its Datasette recorder was the calculation of distances between stations on VHF Field Days. Up popped the distance and the point in the RSGB scoring system, one point for up to 50km, three for 50-100 and so on. I had to agree that it was quicker than the Perspex sheet, scribed with concentric circles, laid over the map.

I have a vague memory of saving a long document on tape when we were having landlord problems in Lewisham. What I can’t recall is what program I used. The thing that does come back to me is that at an absent-minded moment I recorded something else over part of the document. The result of this was that the computer lost its way through the tape and never found an end.

It occurred to me that if I could erase the ‘interference’ the 64 might be able to retrieve the untouched parts of the document. So, remembering that the tape contained a series of very short paragraphs, I played the tape on the ‘Hi-fi.’ I noted the counter reading when the rhythm of the paragraphs vanished and again when it resumed. (A bit like measuring blood pressure perhaps?) Then I erased the section of the tape between the two readings. Much to my relief, the document had only lost a couple of paragraphs and the computer did find its way through the blank section!

Around this time I suffered the first power supply failure. The PSU supplies 5 volts DC and 9 volts AC. I imagine the AC is provided mainly to run the Datasette motor, but some DC is derived from it as well.

The original unit was a black lump with flying leads for input and output. It was “potted”  (filled with resin – a common practice) presumably to keep out moisture but unfortunately it also kept out anyone who might be able to repair it! (This could have been intentional) As might be suspected, it was the 5 volt regulated supply that failed.

I opted for a new unit. Some time later I discovered that a small company was apparently making a living by selling adapters to derive an isolated 5 volts DC from the 9 volts AC!

When the second unit failed, I continued to use it for AC and connected an old stabilised power supply to provide the 5 volts DC. This unit was so old that it relied on a gas-filled tube for its reference voltage rather than a Zener diode.  This may have been the source of the next failure, when the 5 volt rail went up to about 9 volts and did the C64 an electric oo-nasty, from which it has not recovered. Luckily, I was able to buy a second-hand 64 for an eighth of the price of the original!

The computer would have been saved if I had been cautious enough to add a crowbar protection circuit to the old stabiliser. In this circuit, excess voltage quickly triggers an SCR across the supply and blows a fuse or trips a cut out.

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Jakey – (G3JKY). From VS December 2006.

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