Thanks for replying I really appreciate it, the board is from an old Acorn Electron Plus 3 which won't turn on my gut feeling is it's the capacitors as they are nearly 30 years old but just wanted to check what these two black things were as wondered if they had been damaged as one was discoloured. Have included a few more pics of the board as on checking the bottom of the board there are a few marks.
Worst case if all else looks ok will start replacing the 30 year old capacitors.
Beautiful clean board for 30 years old
Randomly changing parts is never a good way of trying to find a fault, an ESR meter on the capacitors would be a better idea - but not much use if you don't have one.
For a start you might try testing the two secondary rectifiers (the little round ones), as it's simple and non-invasive, you can read them in-circuit with the diode test range of a meter - if either are faulty it will read S/C (it's a fairly common fault on SMPSU's).
If not, then see if there's any DC voltage on the positives of those diodes (with respect to chassis), if it's low or pulsing it's more likely to be a capacitor issue.
Failing that, measure the voltage across the big capacitors at the top, there should be about 340V across those - if there is?, then check for voltage to the UC3524 chip, negative pin is pin 8, positive pin is pin 15, and the datasheet says a minimum of 8V and a maximum of 40V. If there's no voltage (or less than 8V) then there may be failed start-up resistors, which will probably feed pin 15 to start the supply.
The datasheet is rather vague, and only seems to give low voltage applications.