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Joined: Jan 1970
you could take the switch out of circuit and see if all is well.
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According to the HP85 service manual, the power switch is a double-pole unit that breaks both the live and neutral mains wires. I assume you realised this, and are checking each section separately.
For testing you could try shorting out each section of the switch with a jumper wire (take care -- this is mains you're working with). If the machine then works (of course the power switch won't do anything if it's shorted out), then you know the switch is faulty.
Again, from what I can see in the service manual, each half of the switch has 2 wires of the same colour, so to short it out you just link those 2 wires of the same colour together, and keep the 2 groups separate.
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If the switch is flakey, you can probably make it work again just by spraying it with contact cleaner. Most switches like this have sealed housings. I have successfully fixed many of them by drilling a small access hole in the side of the switch with a hand (pin vise) drill. I try to locate the hole away from probable internal workings like pivots or bearings. I drill in an upwards direction so that drill shavings do not fall into the switch.