My father started with a ZX-81 (a TS-1000 with just half memory). He later went to buy a Spectrum. And he always thought Sinclair was better than Amstrad... until we started collecting, then he found the crude reality and he now understands why the keyboard or the ULA break so easily...
Congratulations for your repair, just be cautious, these computers are really fragile.
Fragile?
If you had the original ZX81 it would be approximately 45 years old now.
You could have the finest computer man ever built and even that would probably have failed components, leaky capacitors, oxidized contacts, etc and not work anymore after four decades. --- It would probably have been designed better, documented better, and be easier to repair. But wear and tear plus sheer age would eventually lead to some sort of failure.
The membrane keyboards in early Sinclair computers are perhaps a bit on the cheap+flimsy side, but if any of the originals still work at this point it's either a miracle or they weren't used much.
As for the ULA chips dying, overheating is a reliable killer of pretty much all silicon integrated circuits.
Even modern microprocessors and chipsets will fail if they run too hot for too long. And that's considering bigger cases with air space, vents, heatsinks, active cooling, thermal throttling, temperature sensors, etc.
When you move backwards in time to earlier generations of electronics/technology, there is inevitably higher power consumption and more waste heat produced.
And while older chips may be a little bit more resilient in some ways, running at high temps in a case with almost no air flow is a recipe for an early death.
