The two X2 capacitors in the BBC Micro power supply are susceptible to failure due to age and can often go "pop" and let out a plume of smoke. Whilst this display can appear quite scary, it's not a big deal to fix.
In October 2010, my original BBC Micro started to make a rather acrid smell and I managed to turn if off before the capacitors actually went "pop" so I was lucky. Quite often you see BBC Micro's for sale on ebay where the seller declares the that their BBC was fine for half an hour while they were testing and then it went up in smoke. These capacitors are the reason. The problem is far from fatal and the BBC Micro can operate without these capacitors fitted as they are used exclusively as noise suppression. However, without them you may experience interference in other devices so it's worth replacing them rather than just removing them.
Dismantling the PSU is reasonably straightforward and replacing the X2 capacitors is easy if you're handy with a soldering iron but if you're uncomfortable with working on PSU's, there are places online that will service, repair or replace your PSU for you and it may be better that you contact one of those places rather than attempt the "surgery" yourself. If you are still thinking of tackling the swapping out the X2 capacitors yourself then it's also worth considering buying a revised BBC Micro Service Manual from RetroClinic as it has a full section dedicated to servicing the power supply unit.
At this point, it's worth stating that unless you're really confident about changing the capacitors, you should HEED THE WARNING LABEL...
After dismantling my PSU and de-soldering the two X2 capacitors, the picture below shows what I found. The capacitor on the left is the original 1982 X2 100nF capacitor rated to 250VAC. The capacitor on the right is the modern equivalent that adheres to new EU regulations and is therefore rated to 275VAC. The old capacitor has clearly given up and the resin case has split. I managed to switch off the BBC Micro before the capacitor had released any smoke, just in time I think!
As I mentioned, there are two X2 capacitors fitted in the BBC Micro PSU and the second is a smaller version of 10nF and as the photo shows, it had not yet failed however you can see stress lines and cracks in the plastic case so it's always worth replacing it at the same time. As you can see, the replacement capacitor is a WIMA rather than an EVOX RIFA capacitor. At the time, I couldn't find the correct rating of EVOX RIFA capacitor for the replacement so I opted for a WIMA capacitor instead.
Here's a detail shot of the smaller X2 capacitor showing those stress lines and hairline cracks that would eventually result in a failure.
Now, whenever I acquire a BBC Micro, the first maintenance job that I undertake when stripping the machine down is to replace these capacitors. It's worth it for the peace of mind you get knowing that the plumes of smoke have been staved off for another 20 or 30 years!
I managed to capture a BBC Micro with X2 capacitors failing so it should give you a good idea of what to expect if you've not changed out your X2 capacitors just yet.