The unexpected cure

I was in a primary school today to be told –

“This class computer won’t start, must be the power supply or the power button”. The cure was even simpler.

Any ideas?

The problem was something called ‘residual charge’ – in this case probably caused by the scanner or the whiteboard box, or both. Some equipment holds a tiny charge when it is turned off.

In certain circumstances this residual charge is enough to convince the circuitry in the motherboard of the PC that 0V has not been achieved.  The effect of this is that when you try to turn the PC on, the circuitry thinks its already on.

The cure – disconnect the power cable from the PC for 10 seconds or so and the result is shown above! If that doesn’t work, you may need to also disconnect other connections from the PC (parallel port scanners were always a good laugh, but they’re few and far between these days).

Do you have any similar neat tricks?