I'm not sure if you're familiar with using a multimeter but if you are try checking the voltages of the red, yellow and brown wires of the lead which connects the power supply board to the main PCB (there are 4 red wires and 7 black ones, but all the reds are joined together, the same for the blacks). The reds should measure 3.3V, yellow 12V and brown 5V. If not then the power supply is suspect.
Remember that the 33% of the circuit board nearest the mains input will be live mains voltage, so be careful if it needs to be said.
Also while you have the back off, see if any of the capacitors on the power supply board are bulging and consider replacing them if they are. You can get kits from various suppliers to do this, e.g. relkit 33e from satcure.com but you need to have a soldering iron and familiarity with using it.