Repairing filtering capacitors on faulty BBC Micro PSU

Burnt out 0.1uF 250v X2 rated capacitor

30 year old computers often have a few drawbacks, especially when the equally old components inside them start to malfunction. There is a known fault with the BBC Micro’s power supply that can cause one or both of the filtering capacitors to fail. I mean, it’s not a particularly drastic fault if it happens after 30 years but it is kind of alarming when your computer starts spewing copious amounts of evil smelling smoke.

After a bit of Googling I found another person who has had this happen, and a website explaining how to repair the fault. So after a trip to Maplin to buy part number N69CN (MP X2 275V 0.1uF cap) and N65CN (MP X2 275V 0.01uF cap) I set about repairing the PSU.

The repair is reasonably simple – remember capacitors store charge and ones inside PSUs can be lethal. Also extracting the PSU board from its metal housing is very tricky, I found removing the mains input socket helped. If you’re going to power the PSU on while it’s sat on your bench, use one of those mains voltage testers or electrician’s screwdriver to check nothing is shorted and live – the heatsinks shouldn’t be live, but the large transistor’s metal can is.

I can now get on with the important task of playing some games