[Suns-at-Home] Sparcstation 5 power supply

[Charles Lindsey]

> Message: 1
> To: Suns-at-home <suns-at-home@net-kitchen.com>
> Date: Thu, 23 Aug 2007 17:30:47 +0100
> From: "Charles Lindsey" <chl@clerew.man.ac.uk>
> Subject: [Suns-at-Home] Sparcstation 5 power suppky
>
> I have a SS5, ans managed to short out the +5v supply. Now it is dead.
>
> But I looked in vain for any fuse that might have blown, or cutout that
> might need resetting.

OK, I eventually got it working, so will relate the whole story in case it  
is of use to anyone else.

Indeed, there are no fuses (except one on the outgoing mains supply to the  
monitor). And indeed much of the circuitry is "live" (and protected only  
by such fuses as may exist in the external mains supply) even when the PS  
is switched "off".

There are a couple of transformers, and also various toroidal inductors  
wound with thick copper wire which I had assumed were filters to keep  
external noise out of the computer and noise generated by the computer out  
of the mains (and generally to keep the FCC happy). However, there is one  
big one, near the 'mains' end of the PS, that has several windings on it.

Now there is a clear demarcation between the "primary" and "secondary"  
parts of the PS, and this particular toroid is clearly on the "primary"  
side, except for one two-turn winding which connects to the secondary side  
of things (presumably part of some feedback loop, since it is of much  
thinner wire).

I remember, when I shorted out the supply, seeing a 'glow' in the region  
of one of these toroids, but it all happened too quickly to be sure. Now,  
on examining this two turn winding, the colour of its insulation seemed  
distinctly "off". And the odd thing about it was that its two wires were  
tightly bound together by a plastic clip before their connection to the PC  
board.

So maybe, if a large current flowing through there had caused the  
insulation to get hot, it would have got even hotter inside the clip, and  
maybe allowed the two wires to short together.

And so it turned out. Removed the clip, separated the wires, and Lo! the  
PS was working again. Effectively, that plastic clip had created a bizarre  
kind of 'fuse'. Bug or feature?

-- 
Charles H. Lindsey ---------At Home, doing my own thing------------------------
Tel: +44 161 436 6131                       
   Web: http://www.cs.man.ac.uk/~chl
Email: chl@clerew.man.ac.uk      Snail: 5 Clerewood Ave, CHEADLE, SK8 3JU, U.K.
PGP: 2C15F1A9      Fingerprint: 73 6D C2 51 93 A0 01 E7 65 E8 64 7E 14 A4 AB A5