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
ZX81 RAM compatibility for replacement
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Re: ZX81 RAM compatibility for replacement
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
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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
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ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.