Further to my earlier post welcome to UKVRRR
It's possible that there are some parts in the power supply parts are failing valves do fail but unless abused are remarkably robust.
To help some pictures of the inside of the set would be helpful along with the valve types used. Our German cousins were very fond of using valves such as the ELL80 and these do have something of a reputation.
Please don't feel offended if you already know this but make sure its unplugged when removing the back
Cheers
Mike T