Assuming an 80's-era computer that is in good condition and used regularly, what, if any, routine maintenance is needed? For example, do I need to do anything to keep the floppy disk drive(s) in good condition? Or the power supply?
Handling of 80s machines is not different from any modern computer:
Keep 'em in average humidity and remove accumulated lint once in a while.
Not smoking is also a plus. Not so much because of appearance, but cigarette smoke contains a lot of oily and sticky substances that build up layers inside that reduce airflow and seal off chips.
In fact, old machines are less prone than modern ones with their tightly designed, large throughput airflow.
Thinking of it, forcing chips back into their sockets might be another maintenance move. Depending on socket quality, chips have a tendency to creep slowly out of their sockets. A tiny bit every thermal cycle. Whoever pressed on some chips in old machines will notice that they often can be moved down a few microns ... sometimes more (*1). Then again it's not really neccessary until there are any problems.
For long term maintenance it might be good to look inside every now and then (ges with cleaning) and have a look for unexpected change - like shades arround batteries (if equipped with such) large capacitors and power connectors. When the machine is used often, maybe also liten (and check) the power switch for burnings or shades. While sizzling sounds aren't really a sign of danger, they may hint about burn on power contacts. (Thanks to TEMLIB and traal for the hint)
But there are also things you should not do.
Foremost not using so called cleaning disks. I've seen more heads destroyed by their abrasive workings and/or chemicals used, than I've seen any benefits. Do it like we worked with mainframe disks in the 70s & 80s: take a torch (the electric kind please), look for visible traces and only if clearly dirty clean them. Not using these fast scratching disks, but clean fine linen cloth and alcohol. Slowly rubbing in circles and checking again for change. It's helpful to use a little flat wooden stick (like they come with iced lollies) and sew the cloth up as a sleeve to pull over that stick. This way you're able to move the cloth straight and without too much distortion. The tools we used, were much like this, just premade.
*1 - Some may remember that the failure of the Apple III was not only due to low performance, missing compatibility and high price, but also by way too cheap sockets. Sockets that ejected their chips not measured in decades but in a few weeks - leading to the invention of the Apple drop repair move: lifting the III by 4-5cm (2 inch) and dropping it. If that (temporarily) removed symptoms, a round of chip pressing was needed.
Well in the mid 80'ies the common
i8080 based computers in my region, like PMD85, had a lot of IC sockets which tend to jump the ICs out of it slightly causing instabilities of the computer. So the routine was to take all the ICs from their sockets and push them back in. That also removed some oxidation from the pins and the computer would work again for at least half a year.
Later on with the Z80 and better memories, the number of ICs used dropped significantly and other problems surfaced like connector oxidation. So the same was applied for all the cartridges and connectors on all the computers/devices at that time. For example, the D40/D80 had a nasty wide connection cable connected directly to the bus of ZX 48K, and without occasional removal it did mess things up from time to time.
Alcohol cleaning of all the connectors and pins was done when all the above did not help. (I do it till today with my PC's, some of which are 10+ years old and still working).
PS. My Didaktik Gama 89 built in 1989 is still in working condition (last time I checked).
I used to clean the FDD head with a soft tampon and alcohol (maybe once in 2-3 years) and the drives are still working! Some drives loss the head load spring force due to plastic degradation (of the spring housing and lock, the spring is usually OK) so I got one drive reinforced with a plate and some screws and it is also still working...
The sad thing is that the old computers were built to last 20+ years and the newer modern ones are unusable after 2-5 years. That much the progress had lead us to ...
The dust is usually not a big problem in old computers as that was introduced with rotary fan cooling systems which works like vacuum cleaners (they suck in all the dust from the room and stick it to the PCB and ICs) which was not commonly used for ICs until
i80486 and computers infected with this was usually due to wrongly designed PSW fans. For such computers occasional dust and dirt removal (once per year) is a good idea... If you do not do it you may be surprised once you start... Once we found bugged
i80486 with passive cooling (a ~3.0 cm bug was cooked in on the surface of the CPU for god knows how many years :) )
When none of the above helps there is a possibility of cold-soldered joints which will eat up the tin around a soldered pin to the point that it is no longer electrically connected or represent a resistance or even capacity or thermal joint causing troubles. Once I got a
3.5" FDD where I could take out the power supply connector from the PCB by hand... The solution is to suck out the old thin and apply soldering-flux, new tin and solder again ... Sometimes even just new soldering helps. You can identify such joints by black spots or holes in the tin near the soldered pin. Ideal soldered joint should be smooth and shiny (silver color).
Another common electrical problem is bad electrolytic capacitors. They can usually be identified by degraded shape (they are a bit enlarged, usually on the topside of the cylinder) or by the fact that they are a bit loose (you can dangle with them a bit without applying big force or bending their pins). They need to be replaced. They are usually located near PSW (beware, those are kicking long after the device was switched off !!!) or heat source (beware, they can be hot !!!).
Some types of capacitors go bad over time. Re-caping power supplies and other parts of old electronic kit is common preventative maintenance.
- Electrolytic caps have various failure modes which will prevent normal function. Worse, they can bulge and leak. Leaking electrolyte can corrode PCBs.
- Tantalum caps can fail spectacularly. Old tantalums should be replaced as well.
- As I understand it (I'm no expert), ceramics don't go bad nearly as often as other types.
There's a ton of more informed info than I can give searching the various classic computing and vintage radio forums about how and when to replace caps in vintage gear.
Electronic Contact Cleaner (i.e. http://www.penntoolco.com/96-004-450/?_vsrefdom=adwords&gclid=EAIaIQobChMItIePpNvL2AIVxpd-Ch1A1QdXEAYYASABEgLmzPD_BwE)
And compressed air are your friends.
Reseating chips (and forget spellcheck, it IS a word) is a great point just dont press too hard, they are more brittle then you'll be used to as heat treating chips has come a long way.