One, if not the most, common problem with the ZX Spectrum is related to the lower memory. That is, the eight 16Kbit chips along the bottom of the PCB. Unlike the 32K extension, this lower memory, that comprises addresses 4000h to 7FFFh in the processor address space, is made from 4116-type DRAM chips.

This type of memory would be cheap at their age, but comes with a number of disadvantages. Mainly, the need for three separate power sources: +5V, +12V and -5V. The first one to power the logic, and the two others, for polarizing the substrate.

What's more awfull: according to the datasheet, these power sources must be applied in a specific sequence. To be concrete: the -5V power line must be applied before the others, and have to be removed the last, to ensure long-term reliability.

Hence, it's not a surprise that frequently, a failure on the DC-DC circuit that generates +12V and -5V is followed by a failure in one.. or all the 4116 chips.

Repairing this malfunction by using 4116 parts is perhaps the easiest form, assuming you have a good stock of these chips. The problem is that doing this is like changing a damaged keyboard membrane with another one of the same type: it will last some, but not too much time.

There's number of methods to replace the lower RAM with 4164 / 41256 chips. Normally they are some kind of "dirty" hacks, because the 4116 pinout doesn't match 4164 or 41256 pinouts, and therefore, the new replacement cannot use the same layout traced for 4116 chips. That means that, in the end, chips are often "piggybacked" on top of the old 4116, sharing signals, and wire-wrapping those that are not common, or are placed at different possitions. It works, but it's not clean, nor ready to a further repairing in case there's a malfunction in the replaced memory bank.