The Z80 and most of the 47xx logic chips are still manufactured, though. (Small RAM and ROM chips may become a problem, in the near future.)

But then even if you have a small time electrical repair peep come in and pop a chip from a socket and drop in a replacement, what do you do if it still doesn't work anymore?

In that scenario you need to find someone completely familiar with the old model of computer to break out the multimeter and oscilloscope. That's gonna be a huge pain.

And in the meantime none of their stuff works, though I suppose they can rest on the existing library of punch cards they most certainly have. Or do it manually again.

Even in the worst of cases, these were popular machines and you can buy one at ebay.jp about anytime. It probably becomes much more of a problem with hardware closer in time. E.g., it's a 486 machine, the network is probably ARCNET, because industrial, where do you get any hardware that fits in? (Contrary to its 486 siblings, the 8-bit machines are still around, because they were cultural significant, and there are folks playing around with them and preserving the knowledge of keeping them running. Finding the maintenance manual for a late 1970s machine is easy as compared to finding one for a, let's say, 2001 run-off-the-mill computer.) Also, you'll be re-entering any data there is, things will probably get lost, because the project stalls. Meanwhile, while you are re-planning, selecting the new hardware, comissioning an external team to furnish the new software, eventually test it and roll it out, any production connected to this is on halt…

This isn't the only place using such equipment, and as long as there is a profit to be made someone will supply the parts. The only components that can't sit forever on a shelf as NOS are (off the top of my head) capacitors and production of those isn't going anywhere.

For non-electronic parts: that's why you have a machine shop.

I used to freelance HW/Firmware development for a small company that retrofitted old machine tools with modern motion controllers. There is a surprisingly large industry out there ready to repair or retrofit whatever old controls are on your old machines.

As long as they haven't been sitting on the shelf since the 1950s, any capacitors you have lying around will be fine.

The electrolyte isn't milk. It doesn't go off.

It does dry out though. Electrolytic can caps are not hermetically sealed.

Over about 50 years, maybe.

Put it this way, I've replaced maybe a dozen "bad caps" in equipment in 35 years of repairing high-end musical kit, six of which were in one 1970s synthesizer made from very cheap parts.

But, I've done very well out of the fashion for "re-capping" equipment because I charge 300 quid before I even start to consider clearing a spot on the bench for stuff that's been "re-capped" before they bring it to me to fix.

Yes, I am aware of the re-capping nonsense. I was noting that caps do not live forever.

> The only components that can't sit forever on a shelf

You're never going to find electrolytics that have "gone bad" sitting on a shelf.

Define 'gone bad'. They definitely gain ESR.

Recapping is mostly nonsense, there are however some devices out there with electrolytic caps that are known to leak. Some old Amiga and Macs have these issues for example.

From there it seems to have spread out into a "recap everything" craze that is just silly.

Yeah, exactly, and because it's so popular it's become the "go-to" for any fault - "oh it must be bad caps!" - and then you end up with perfectly good equipment getting ripped apart and cheap shitty Aliexpress electrolytics dropped in to replace perfectly good ones, with lots and lots of really poor soldering, dry joints and lifted pads everywhere. Oh and heaven help any ECOs made to the board, "oh someone had circuit-bent it and added all this blue wire to the back of the board so I took all that off", gawd help us.

The real fault is an 0.1μF disc ceramic, one of those little brown ones, that's gone leaky and now thinks it's a 1k resistor. Not that you'd know, with all the other iatrogenic faults the damn thing now has.

Awful. Just awful. The "it was circuitbent so I tidied it all up" thing was a Prophet 600 that I had to write off, it was so badly damaged.

> at some point finding parts and people able to repair those will be hard or impossible.

Maybe, maybe not. If the machine uses the right "legacy" parts (a Z80 is a great example, but so is anything belt- or gear-driven, etc.), then it's a very good bet those parts and the people who are familiar with them will remain available for much longer than you or I will be alive.