There were 4 64k x 4-bit Integrated Circuit chips in the Amstrad-era Sinclair ZX Spectrum +2 and +3 models. These are known as the +2A, +2B (both +2 "black"), +3 and +3B (also both black cases).
I say "Amstrad-era" as these were produced by Alan Michael Sugar's Amstrad company, when Sugar bought the rights to the Sinclair name for computers only (he didn't buy the Sinclair name). This is as opposed to the Clive Sinclair-era computers ZX80, ZX81, ZX82 (Spectrum), Spectrum 128k "Toastrack" and QL.
I do not include the Amstrad 128k grey in this question, as that is based on the Toastrack, whereas the +2A, +2B and +3 are new designs "under the hood".
The 128KiB RAM in the +2A/+2B/+3 machines come from 4 x 65,536 x 4 = 1,048,576 bits = 131,072 bytes = 128KiB.
Examples of such 64k x 4-bit ICs are: the Texas Instruments TMS4464 (a.k.a. SMJ4464) or Samsung KM41464A, Micron MT 4067 883C and the IC marked AMS x 4 (as marked - I do not have any more info other than a suggestion that this is an Amstrad chip).
My question has four parts:
- Are these chips 100% usable and fully functioning in all models - the +2A, the +2B, the +3 and the +3B?
- Are there are any issues with any of these chips with any of the different PCBs under these models (e.g. Z70833 Issue 1, Z70833 Issue 4, Z70830 Issue 1)?
- Do the standard factory modifications to the PCBs affect the behaviour of ICs?
- Can these chips be mixed on the one machine (e.g. 1 x Samsung, 1 x Texas, 1 x AMS, 1 x MT)?