At end of the last year I was able to buy a ZX-Spectrum computer which did not work correctly. The main problem was that it could not provide proper video signal. The seller sent me a photo about the problem - see image on the right. I thought it could be one of three possible problems: video builder circuit (ULA), video memory corruption or power supply problem. If the ULA is wrong, then purchasing it is a difficult task. But fortunately the problem was with the -5V power - the second photo shows that the computer is working properly with the original capacitors, if I change the video memories to special static memory card which does not use -5V. After I 're-capped' all the electrical capacitors on the board, then the problem was solved immediately with the original DRAM memories.
And of course the keyboard membrane needed to be replaced too.