Date   : Thu, 16 Nov 2006 21:40:19 +0000 (GMT)
From   : info@... (Sprow)
Subject: Excitement, disappointment. Capacitors.

Hi,

In article <455BA59E.1040006@...>,
   Alex Taylor <astravan@...> wrote:

Good news on your repairs!

> Another note on capacitors: I've seen a few eBay listings and postings 
> in this group, about exploding power supplies. Loads of my Beebs have 
> 'exploded' and work perfectly - the same happened to my Apple IIe. 
> There's a filter capacitor that's prone to exploding violently if it's 
> not been used for a long time, however I've found that it's not an 
> essential component and the PSU will work fine without it. It's also 
> quite cheap and easy to replace if required.

The exploding cap (there 2 on the BBC B supply: C1 & C2) is there mainly to
form a filter with the common mode choke T1 to stop switching noise being
conducted back onto the mains. There are quite strict limits on the harmonic
content you can put back on the mains in case it interfers with other nearby
appliances or you're using power line carrier comms for example.

The failure mechanism (and cause of the smoke!) is when the polypropylene
dielectric breaks down and you get a mains short which will blow the fuse.
If you work out the electric field across a foil/plastic/foil plate with
250V across it it's not surprising it gives way...

As they're UL approved parts they don't burst into flames and they'll self
heal as the dielectric cools - often you don't even need to remove the
component.

They're about 50p each to replace, but can be a little tricky to find the
same pin pitch (C1, 22.5mm, 100nF; C2, 15mm, 10nF),
Sprow.