For a while I worked opposite the Timex factory where the Sinclair computers were made. Apparently the staff were supposed to faultfind and repair any boards from the production line which didn't work. Instead, what they often did was to throw the faulty board out of the window and onto the flat roof of the building.
When contractors eventually went up there for building repairs, a huge pile of ZX Spectrum boards was found. They were given away, and we got a few. Many turned out to be still repairable despite lying for a year or two in the Scottish weather. Of course there were no cases, just the boards.
The ZX81 used static RAM, but the Spectrum used early dynamic RAM which required a -5v rail. This was generated by a little onboard inverter. If the inverter failed your entire RAM bank drew overcurrent and fried itself.
Since RAM chips were not exactly cheap in those days, that little inverter was the cause of a lot of grief. The later RAM chips did away with that arrangement.