ZX81 RAM compatibility for replacement

[Fabrizio69]

Hi all,
I have a ZX81 with white screen. The cursor appears when I use the external 16k memory so should be a RAM problem.
On the board there are the couple of MCM21L14P30 and MCM21L14P45, they differ only for the access time (30ns and 45ns).
Which chip of ram can I use for replacement? All kind of 2114 are good? Are there others full compatible chips? Those on the board should be low consumption (max current 30mA, if I remember well). Let me know, please, I would like to restore the 1k memory since I have the 16k external one.

Thanks in advance
Fabrizio

[1024MAK]

The MCM21L14P30 is a low power version of the MCM2114. The MCM21L14P30 has a maximum access time of 300ns NOT 30ns.

The MCM21L14P45 has a maximum access time of 450ns NOT 45ns.

A data sheet for these is available here

Any 2114 SRAM chip with a maximum access time of 300ns or quicker should work including the non-lower power versions.

The low power versions are better, as in both the chip and the 7805 voltage regulator will run cooler compared to using the non-lower power versions. But you can use the non-lower power versions without problems. The 7805 voltage regulator has more than enough capacity. Sinclair themselves used whatever they could get for a reasonable price, hence the wide variety of chips used (as can be seen in photos of ZX81 boards).

I do strongly recommend that you use sockets. As 2114 SRAM chips have been obsolete for a long time now, hence any you get will either be "new old stock" or "pulls" (removed from existing boards). So unless you buy from a seller that has tested them, you may have to buy a number of then and try each in turn.

Mark