Old as it is, and given the probability that during its lifetime it was on 24/7, weakened, faulty caps (and other components), especially in the power supply(s), would be high on my list of culprits.
And, no doubt, there will be a ton of them for all the various HV circuits.
Keep in mind that there are lethal voltages inside that box, even well after it is turned off, especially around the anode.
Just out of curiosity, are there any electrical sounding pops or "sizzles" while it is on?