Date   : Fri, 17 Nov 2006 07:54:39 +0000
From   : astravan@... (Alex Taylor)
Subject: Excitement, disappointment. Capacitors.

Sprow wrote:

> Good news on your repairs!

Thanks! Still a few more to go though (the randomly crashing one and the
three that start up to a monotone), and still a few dodgy CUBs left.

> 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.

Ah! I didn't know that. I think I'll make sure that they're fitted and
working when I inevitably sell a few of my Beebs.

> 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...

After I inspected my Master ET's PSU, the capacitor had blown out of the
side, with enough force to bend the metal leg over to 45 degrees.

> 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.

The last one that blew up, I just continued to use the computer normally
while it poured out smoke. because I knew what was happening to it. 
However it got quite a reaction from the two 13-year old lads I was 
showing BASIC programming too!

> 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).

The PSU in the Master ET upstairs has a capacitor with the wrong lead
pitch on it, but they were long enough to bend across into the holes.

-- 
Alex Taylor