They are so easy to fix - providing you are prepared to attack the casing with a hack saw! Yes, it is glued shut!! The faults I have encountered is with the startup and output caps. When replacing, make sure the caps are rated at 105 deg C.
Try this. Disconnect all power sources. 1. Hold the 'option' key down. 2. Plug in the power source. 3. Keep holding down the 'option' key for about 5-seconds and then release. 4. Disconnect all power sources and wait for 10-seconds. 5. Plug in power source and power on computer. This has worked for me on countless occasions. Thanks to the guys at Stockley Park that told me this years ago.
The power supplies are dead easy to fix. 1. Due to their age, the fans normally seize up and cause all the caps to over heat 2. The primary stage blows up. Do not forget that a flat PRAM battery can cause power on failures.