I can read from the various posts in this thread that you all think it's a walk in the park to fix these old 80's computers, oh boy...you guys may know a couple of common things such as dry soldering and drying capacitors, but there's a lot more to fixing those things than you might know.
One of the most common faults of the 80's was the ROM/RAM circuits, they where often clusters of 2/4/8 kilobyte ram chips (often 4164 etc.), and finding dead ones requires a couple of "old skool tech skills", one of the simplest one is the "thumb test", is one of the Ram chips very hot (you could of course use a bottle of cold-spray, I don't know what it's called in your country...but to us it was just Cold spray, this is essentially a spray that sprays super cool air because of a chemical process when in comes in contact with air, the surface will be really cold, forming ice crystals) and then you can see clearly which surface is getting hot fast. Another method is to use the oscilloscope to see if anything is out of the ordinary (you need to know how it looks as an image first, the voltage changes because of the logic communication will form an image, and if you know how it looks when normal, this is also a method we used.)
You can also use a logic tester, this is an instrument that can monitor the traffic in those logic circuits, you can set it to the speed of the actual logic (usually 1 to 20 MHz, depending on the computers speed) and see if everything is okay.
Another common flaw back then, was broken prints...over some time, these boards gets really hot, and this stretches the metal on the PCBs, and broken connections is some of the hardest things to find.
Another typical flaw is design flaw, over time...we needed to change I/O chips on certain models simply because it was so badly designed that they would eventually go bust, they where very sensitive too...so many of the DIY'ers out there who made their own Fast-Loaders/Robotics connected to the I/O ports would regularly blow these chips.
Pity I live in Scandinavia, I'd love to retire doing this