This is an old post, preserved for reference.
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You occasional find an old computer where the owner has said it blew up. Now you may think this is fanciful and related to the TV and movie attitude that computers blow up when they go wrong, or if you ask them too difficult a question. However, what they are describing is probably the mains filter capacitors failing. You get the same sort of low value capacitors filtering the mains on power supplies on computers like the BBC micro and Apple II.
ICL One Per Desk I recently repaired]
Funk-Enstorfilter 0.1uF + 2x2500pF XY + 2x100uH 250V ~ 2AOr a line filter with a 0.1uF capacitor across the mains and a pair of 2500pF capacitors between live and neutral, and a pair of 100uH inductors between input and output. The purpose of these filters is reduce noise generated by the PET being fed onto the mains. This would have been a problem in its day as there were still may simple radios and televisions which did not include power supply regulation, so were sensitive to mains noise, particularly on AM radio. These days there aren't many of those in use, and there is so much noise introduced by switched mode power supplies in computers and phone chargers etc. it's hardly worth the effort.
- black wire, neutral out of filter - blue wire, neutral to transformer
- blue wire, mains neutral in - blue wire, neutral into filter
- brown wire, mains live in - blue wire, live into filter
- yellow/green, mains earth - yellow/green to earth tag