Careless Assembly

The computer had suddenly gone off with strange colours on the screen then it wouldn't reboot.

I removed the case and looked inside. There was a set of boards plugged into a motherboard designed for either an AT or an ATX case and this was an old AT case. Memory seemed to be a single DIMM and the processor on examination proved to be a "300 Meg" IBM type which is a Cyrix version of a Pentium. The "300" and similar codes for Cyrix, AMD and IBM processors indicate an equivalent rating figured in such a way as to give a higher number than its lower clock speed. Good for sales to the man on the street, compared with clock-rated Intel processors. The original computer had probably been something like a 486 and the new board had been shoehorned into the case. The hard drive and floppy drive cage had been too low for the new board because the processor with its chunky fan was directly underneath it. The upgrader had been reluctant to spend much time on the job and had half hacksawed and half bent away the lower edges... not a pretty sight. I removed all the boards and the IDE and floppy cables and fitted a simple VGA card, plugged in a monitor and powered up. There was the usual reassuring diagnostic activity from the hard drive but nothing on the screen of the monitor. Power rails were all nominal. Usually you get a bleep or two when there's a basic fault but this time the processor was dead. Plugging the processor into another machine proved it was OK though so I removed the memory which wasn't easy because it was firmly wedged under the hard drive cage (the side that hadn't been bent back). The motherboard had 72-pin SIMM slots as well as a DIMM slot so I tried a couple of 72-pin SIMMs. Still no result. I tried the DIMM, which I'd removed, in another machine to be rewarded by a loud bleep or two and no results. Looking at the top edge of the DIMM under a magnifier revealed the reason. The prolonged pressure of the edge of the hard drive cage had dented the top edge of the miniature circuit board. These boards are multi-layer things carrying several printed circuits bonded together. Some layers are ground and others are held at power rail levels. The dent in the top had shorted together a set of circuit tracks and caused the destruction of some of the memory addressing circuitry on the motherboard.

Rather than spend a lot of money restoring what was basically an old machine, the owner agreed to upgrade to a new ATX machine.

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