A very common age related problem with the BBC computers is the failure of one or both of the X2 smoothing capacitors in the power supply and / or the failure of the "start up" capacitor.
If you have turned on an old BBC computer which shortly afterwards gives out a "popping" and "crackling" sound, followed by your room being filled with foul smelling, acrid smoke, then one, or both of you X2 capacitors has blown
! Alternatively, your BBC simple won't start up, then failure of the start up capacitor is also common.
The same manufacturer, type and part numbers as the original ones fitted are used, so they will fit perfectly in the power supply control board without having to bend legs, drill additional holes etc (as you have to with some replacement kits).
Even if just one of the X2's has blown (the most common scenario), all three of these capacitors should be replaced whilst you have the PSU stripped down.