bluejay

I also tended to observe that a vintage rig may fail at several points in different ways. And a whole thorough going over the machine is a good idea. All socketed chips removed and cleaned. All important caps examined. All connectors buffed up and spit polished. Drive mechanics greased with CD-ROM grease. Power supply connectors cleaned and tensioned if needed. Maybe even replace the drive data cables if they've been played with a lot - remember they are press fitted and the ribbon cable is not soldered. So that could be hanging by a thread or have oxidation. You would also clean the slots. Check the interface cards for corrosion. Route cables away from internal RF noise. Check the CMOS battery. And much more.

IIRC if this is your machine:

https://en.wikipedia.org/wiki/Compaq_Portable_II

..I could spend a whole month of evenings giving it a tune-up. A lot of tiny detailed manual labor, you know. Prep it for the next 2 decades. I mean that's what it takes. But I don't do much of that these days anymore - my interest having switched to software.

Many vintage motherboard repairs I've done were for things like the wrong voltage having been applied to a port which blew out the protection or a periphery chip. Or a chip that was being influenced to operate incorrectly by contamination or shorts or another marginal chip that remains functional. Sockets a big thing too. Shit like that.

Yeah, I have 2 of the multi IO cards and neither work. I've tried deoxit on the card slot and the connectors, and tried both of the 16 bit ISA slots. Didn't work. I'll have to take the whole thing completely apart and see what I can do to fix it. Meanwhile, I left my c128 off for too long and it decided to die as well. I managed to fix it partially but I think I may have to replace a few sockets. The thing originally came with sockets that have contacts only on one side. It also needs some new thermal grease. But a c128, unlike the Compaq, has no moving parts other than the keyboard so it shouldn't need as much maintenance.