This question already has an answer here:
Having managed to source a TRS-80 Model III that I'm told hasn't been powered on for quite some time (and I suspect this is a massive understatement), I'm worried that the first thing that might happen when I apply power is that there will be a large flash and/or bang, followed by not much at all :-)
Although it is a specific machine, I'm happy if some answers are generic, so as to cover a wider audience (or is it visience when you're reading rather than listening?).
So my question is hopefully a simple one. Given its unknown condition, the high liklihood that it may not power up correctly, and my innate paranoia about destroying something I just bought, what should I check before I even plug in the power?
The TRS-80 Model III is a pretty robust machine but there is one common problem -- the RIFA capacitors on the power supply. These were paper-type and very often short out producing lots of smoke though generally don't cause any damage to the machine itself. There is an excellent page devoted to that procedure:
Also note that some care must be taken when opening the machine as it is easy to damage the CRT neck when doing so.
There are other common problems that are troublesome but not dangerous to the machine. Stuck brightness/contrast knobs. Keys that don't work. Disk drives that need lubricating. I highly recommend this video as a how-to guide:
First, do a visual inspection. Make sure it's reasonably clean, no liquid damage etc. Check for foreign objects and possible shorts.
Then check the power supply. It's easier with external types but some internal ones can be disconnected from the main board and tested individually. At the very least, check for shorts between the supply rails and ground. Also visually inspect capacitors for leaks.
If your country has fuses in plugs make sure its the right fuse, generally the smallest available. 3A is more than enough.
Then finally carefully power it up and be ready to switch it off again if it smokes.
You can do more if you have the right equipment and time to go over it in detail, but that should generally be enough for most machines.
This is not a full answer, feel free to improve it. Let's start with the basics: Use ESD protection when touching any PCBs. Clean the PCBs.Inspect for any physical damages. Inspect all capacitors to see if any is leaking. Check any batteries. If possible, check the function of the power supply when it is not connected to any PCBs.