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The ZX80/81 16K RAM pack used two PCBs joined with a connector in the middle.

I'd initially assumed this was due to reuse of the original 3K RAM pack case (which has a single PCB and uses static ram) for ZX80 16K RAM packs and then the same case being used for the ZX81, however the case is significantly different and I think there wasn't enough space inside the older design.

I can't figure out why Sinclair didn't create a RAM pack with physical packaging similar to the Memotech design. A single PCB should be more reliable and the additional plastic cost of a wider case can't have been that much.

I remember there was an initial shortage of 16K RAM packs at the ZX81 launch, I originally ordered a ZX80 and received my ZX81 before the magazine reviews arrived but had to wait several months for a RAM pack, which leaves me wondering if there was some manufacturing issue.

Is there some piece of Sinclair history or electronic reason I'm unaware of?

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    I too had an excruciating wait between getting a ZX81 for my birthday in April, and getting a MEMOPAK for Christmas. Even the velcro strip didn't fix the dreaded wobble though. *8')
    – Mark Booth
    Jan 8, 2020 at 18:24
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    @Mark Booth , for what little it is (probably not) worth the Spectrum early external ram packs for the 16K models could also occasionally do this but not as frequently. I am afraid I have no sources to hand only memories (no pun intended) although I cannot recall if third party ones were worse or better in this respect.
    – AndyF
    Jan 11, 2020 at 17:00

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It was neither due history or electronic but for design reason:

The 16 KiB RAM pack was intended to be sold to existing (and new *1) ZX80 users as well. While both had their expansion port on back side, the ZX80's was all the way to the left, while the ZX81 got it moved over to the right side. Only a package not much wider than the connector would fit both equally well.

ZX80 vs. ZX81 placement:

ZX80 PCBZX81 PCB

(Pictures taken from Wikipedia)

This is as well why the connector itself is 'open' on both sides, unlike most other PCB plugs. It needed to fit both board cutouts equally well - and in addition, the opening of the ZX80 was a bit smaller.

The same goes for making it the 'straight' way it is, compared to Memotech's more elegant shape, as the ZX80 didn't have that wedge.

*1 - In some markets - like Germany - the ZX80 was sold for a short time in parallel to the ZX81 (at a reduced price).

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    If I recall correctly a similar "re-use" connection approach was taken with the ZX Printer ( not memory expansions ) to allow it to fit and work with the Spectrum ( ZX-82 :) ) as well as the ZX-81. What I cannot remember and its pointless anyway is if it was the ZX unit or the Alphacom one.
    – AndyF
    Jan 8, 2020 at 13:06
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    I'm so annoyed I didn't think of that! @AndyF The zx printer worked with both the '81 and spectrum (for some values of worked!)
    – PeterI
    Jan 8, 2020 at 13:19
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    @PeterI :) It still is a good question, requiring to think outside the box - aka, to look not just at the ZX81 .
    – Raffzahn
    Jan 8, 2020 at 13:30
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    I suspect these days anyone looking at a much needed memory upgrade on their ZX 80 / 81 would be very wise to investigate internal expansions instead where practical unless pure originality is a requirement.
    – AndyF
    Jan 11, 2020 at 17:03

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