Date   : Sun, 30 Oct 2005 22:25:10 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Guess age from serial number?

Annihilannic wrote:
> Anyway, last night I was happily typing in the Lunar Lander programme
> from the BBC User Guide with the beeb sitting on my lap when suddenly
> pfffffft! and some foul smelling white smoke started floating out of the
> speaker holes... I switched it off and turned it over, and more white
> smoke came out of the vents underneat... I guess my leg was covering the
> vent under the power supply.  It had been on for an hour or so... could
> that be the only reason, or is the PSU just showing its age?

*usually* when you get lots of white smoke and that noise it's the noise 
supression capacitors that go aross the mains input on most power 
supplies. I can't remember if the beeb has them or not, but I'm willing 
to bet that it does. Look for rectangular yellow capacitors near to 
where the mains enters the power supply - if one's failed it'll be 
obviously cracked open, often leaking a bit of brown goo.

When one fails they generate a *lot* of smoke! For some reason they 
usually seem to fail as you described too - not when the machine's 
switched on, but sometime later. We're finding a lot of them dying these 
days in vintage machines of all types.

They're there primarily to stop the device (the beeb) from contaminating 
the mains with lots of noise and upsetting other mains-powered devices, 
which is why the beeb carries on working fine without.

I'd still replace them though - modern ones which will hopefully last a 
lot longer are available from Maplins and the like.

> Could this PSU be one of the ones you were talking about?

It was probably just that capacitor's time to go - as I say, we're 
finding a lot of them blowing in old machines these days. I'm not sure 
if the failure's simply age-related, or due to the parts being 
under-spec (often they're rated at 250V which might be a little close to 
mains input for comfort, and years of running close to the limit kills 
them. Modern replacements seem to always be rated at 275V instead)

> I did have a 5-pin DIN to stereo RCA audio cable plugged in at the time,
> I hope the two missing pins are the ones used for motor control on the
> special 7-pin DINs.  A perhaps foolhardy assumption I made when plugging
> it in to my laptop to use as a recording device (works a treat).

It's hard to outright kill a switchmode power supply like the one found 
in the beeb. Even if outright short something such power supplies tend 
to just go into shutdown mode.

cheers

Jules