After repairing the PSU of IBM 5150 I was left with a couple of errors one of which was related to RAM. The main error indicating issues with RAM was 0400 201.
According to this article, 0400 indicates Motherboard Bank 0, Parity IC error. And of course, this one is soldered in.
But there is one way to test if it is faulty. It is called piggybacking. You basically put working IC on questionable IC, just check that IC pins are touching. I did not have 4164 IC, but from some other ZX Spectrum repair, I had KR565RU5 which is basically a Russian copy of the same IC. I did this and the memory error went away, so I just had to make proper replacement.
Next, I need some kind of adapter for the keyboard.