This is wonderful to read. I'm about to embark on getting my Sharp MZ-80K into good shape for the next 40 years. It still works fine but I bet there are a bunch of capacitors just waiting to die, and solder joints that are waiting for the perfect day to fail.
Usually that's good advice. But not always. If it's still working, you can take preventive steps before an easy-to-fix issue causes impossible-to-fix damage.
Silicon does degrade (and fails). But in my experience, most issues fall in 2 categories:
1) "Mechanical" problems. Connector problems, loose solder joints, corrosion, cracks in pcb traces, etc.
2) Power supply issues. Electrolytic capacitors are suspect #1 there (and they may not respond well to being powered after a looong time in storage).
Rules I apply:
a) If unsure how a machine looks inside (condition, mods etc): inspect internals before powering up.
b) Check that a machine is working before modifying it in any way. If not working: minimize steps to get to a working state.
c) Do mods in small incremental steps.
That way you can always backtrack to last working state.
The problem is that electrolytic capacitors can leak, even if not used. So, you can store a perfectly good computer and retrieve a ruined motherboard a number of years later. SMD capacitors are bad for this, the MZ80K predates all of that though, and I think only has a few through hole capacitors on the mainboard.
The power supply should also be checked, and in this case, it's safety capacitors which are the risk there, since some of the older ones (RIFAs for example) tend to be hygroscopic, and if they crack they end up blowing next time mains is applied across them. I've not had any problems with power supplies killing the machine with an over voltage, but i've heard this is also a risk.
Anyhow, 45 years or so is time for a recap, and it'll do another 30 years before it's time again...
Better advice with these older machines is "mess with it while it is still working". Even significantly more recent machines like the OG XBox pre-V1.6 can be badly damaged when the clock capacitor leaks onto the circuit board.
The problem with not doing some preventative stuff on these old machines is that in the event of a power supply failure (IIRC the MZ-80K has +12V, +5V and -5V) you can get overvoltage and cause damage to the chips.
