Testing Power Supplies! Re: Norsk Data Nord-10/S restoration

[Tony Duell]

> 
> >>>>> "Joe" == Joe R <rigdonj at cfl.rr.com> writes:
> 
>  Joe> Testing power supplies before powering up a system is just plain
>  Joe> paranoid! (And I'm trying to be polite here!) There are a
>  Joe> thousand parts in modern computers that can cause just as much
>  Joe> damage!!!!!!!! 
> 
> I disagree -- at least in the case of linear regulator supplies, which
> is what you'll normally find in a classic computer.
> 
> A very simple and very nasty failure in a linear regulator is a short
> in the pass transistor.  If that happens, you get a much higher output

Worse are the non-isolated switching regulators, like those DEC PSU 
bricks. They have a higher input-output drop (typically about 30V DC on 
the input side for a 5V output) and if the chopper transistor shorts, it 
all appears across the output terminals!

> voltage, and there is no current limiting.  It's a fair bet that this
> will fry a large fraction of the semiconductors in your system.
> 
> If the supply has a "crowbar overvoltage protection" circuit in it,
> that's a different matter.  But testing a linear supply is trivial.
> So why not do it?

I'd rather not trust a protection device if I don't have to. Sure I like 
crowbar circuits, and they have saved my chips a few times, but I don't 
like to depend on them.

-tony