New DEC museum entry :D (not the breaker sub-thread)

Tony Duell ard at
Thu Feb 22 20:15:11 CST 2007

> On 22/2/07 19:39, "Tony Duell" <ard at> wrote:
> >> I think it's gotta be a ground-fault that's causing Witchy's problems.
> > 
> > I am almost sure it is.
> Well, tonight I stripped the PSU down and there aren't any filtering caps,
> at least not the X type ones I'm familiar with. Instead the mains cable
> plugs into a honking big sealed tin box that terminates in a 2 inch earth
> wire and 2 spade connectors that the live and neutral hook up to. Said earth
> wire is bolted to the chassis and joined to a similar bolt holding the fan
> to the case.
> Anyone seen something like this 'mains plug in a sealed tin box' arrangement

Oh yes. That sounds like a mains filter unit. Inside there are several 
capacitors and inductors. If you're very lucky there'll be a schematic 
printed on the can somewhere.

> before? The thing's got to be 4 or 5 inches by 2.5 inches so whatever goes
> on in there is anyone's guess. If I was at work I may be tempted to unhook
> the earth wire that's bolted to the chassis but it's there for a
> reason....though normally I'd expect that to be there to ensure the psu
> casing was never live.

IOt may not do you any good to do that. Often the can is connected to the 
mains earth anyway, the wire is just there for a 'belt and braces' aproach.

What I would do as a temporary measure is to remove that filter can 
altoghter and wire a mains cable (or IEC plug) with earth to the PSU 
chassis and live/neutral to the 2 spade terminals (this leaves the 
breaker in-circuit, right?  I don't have a Pro in front of me at the 
moment, I can dig one out if need be). See if the machine still trips the 


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