Everything is repairable, given enough effort and expense given to it. In general these are fairly reliable but obviously not in your case - maybe they're just getting to that age where they are going to start failing now. There were a few different PSUs used in these machines, all the same formfactor but some are Delta, some are TDK and I think some are Dyna Comp or Astec, so there's isn't a single answer to 'How to repair it' - it will depend on the PSU.
Have you got a model number / picture of the label and some pics of inside it to hand?