Thanks all for your tips - I've replaced line filter capacitors in Apple II PSUs before, so might crack open the 9x0 PSU and check these out along with the capacitors.  I'm so tempted to just fire it up - the crackling sound kind of settles after a few minutes (but still "cracks" every 10 seconds instead of the initial "crack crack crack" when first plugged into 240V).  But I'd consider that a foolish move knowing it hasn't been touched for possibly a decade in storage.

It has unknown RAM capacity as I've yet to fire it up, the banks are full but all modules look of low capacity (1 - 4MB sticks?)

JB