There are several simple precautions that are always worth taking when powering up a vintage microcomputer after long periods of storage or non-use. The minimum, simple steps, should include:
- Place the computer on an electrically safe workbench, preferably one that includes a grounding strap for the user. A wooden table is an OK substitute, just avoid static discharge from your body.
- Open the computer for visual inspection. Dust inside the case and around any ventilation ports (i.e. PSU fan) should be blown out using compressed air. Grime can be removed using isopropyl alcohol (70%) and a clean rag.
- Look carefully at the component side of the motherboard and any cards for anything abnormal. Common anomalies to check for are leaky electrolytic capacitors, damaged or improperly inserted chips, empty sockets (missing chip?), and any evidence of component overheating, like blackened, charred area around a resistor or cap, or on the PCB.
- After properly sorting out any of the above, if needed, you should gently press on each socketed component to ensure it is completely and correctly seated in the socket.
- Assuming it can be done without exposing you to dangerous voltages, disconnect the internal PSU from the motherboard, then power on the PSU only from AC power. Watch and listen carefully for any arcing or sparking so you can power it right back down quickly. If it looks OK, then wait several seconds and check for any unusual or acrid odors from the PSU. If it appears stable, you should check the PSU voltages with a multi-meter next. Obviously, checking voltages will require you to obtain some basic troubleshooting guide so you know which pins to check and the specified voltage you should see. See answers to this question.
- If above is good, reconnect PSU. Remove any peripheral cards that are not essential to subsequent testing, then power up the main board and CPU. Again, watch and listen carefully and check for odors or other anomalies so you can power it right back down.
- Ensure any fans that are installed are working and that the system or PSU is getting adequate airflow.
- If good so far, you can power back down, re-assemble the case, and start testing the machine with video displays and other peripherals.
This might sound like a lot of steps, but barring necessary repairs along the way, it can be done in 5 to 10 minutes.
You mentioned the floppy drive in the Commodore 128D as well. All the same simple checks above for the main board should be done for the floppy drive motherboard too. Frequently, the read/write head on a floppy drive will need to be cleaned before it can access disks. This is done by wetting a cotton swap with isopropyl alcohol and using that to gently clean the delicate head area. Also, if you keep the Commodore case open during operation, you can visually observe if the head is moving laterally across the disk. Sometimes it can get stuck or sort of sputter if the track it moves on needs lubricating after a long storage. I normally use a small amount of silicone lube for the metal tracks, and you should attempt to remove any old lubricant before applying the new. Of course, there can be many other failure modes for floppy drives, but these two simple things have gotten many drives working again for me. On 3.5" drives, the moving parts associated with inserting and ejecting the diskette are also prone to needing old lubricant cleaned off and new lubricant applied.