ZX81 reversible internal 16k upgrade

his post is an upvote for Tynemouth Software's ZX81 reversible Internal 16K RAM upgrade. Their instructions are easy enough for even me to follow and don't involve cutting tracks.

This is the ZX81 I've had out on display and used whenever I wanted to. It's an issue 1 and was probably a kit judging by some very untidy assembly. It has a ZX8-CCB composite video mod and an external keyboard fitted. On board it has two 1k x 4-bit chips.

The ZX81 originally came with 1k on board. Thanks to a trick with compressing the display in ram, that was enough to type and run a small program but you soon felt the limitations. Back in the early 80s, the solution was a 16k ram pack which plugged into the back[1] and this is the way I've been using this particular machine.

These ram packs are notorious for 'ram pack wobble'. Even if fastened into place, you can still randomly find your work disappearing.

This is a very reliable solution using a more modern 32k chip (half of it is used, although I believe with more modification it's possible to use the full 32k). As you can see here, I had to pull the original ram chips from their sockets[2], add a socket for the new chip (the holes exist in the board, the new socket fitted around the smaller existing one) and add 4 wires. To avoid cutting tracks, we've bent four of the chip's pins out so that they don't sit in the socket.

That worked beautifully. I can now enjoy using this machine without holding my breath and tiptoeing around it, or having a heart attack if I accidentally knock it.

As this machine is already heavily modified, I swopped its ULA for a VLA81 modern replacement which has some display options broken out on dip switches (basically it emulates various versions of the ULA chip). This will allow me to use the working original ULA to fix another more standard zx81.

This is the 16k Hi Res version of BerZXerk. (Normally the zx81 only displays its character set.)

I wish I could tell my teenage self that here in 2020 I upgraded a ZX81 to 16k for about a fiver. They were very proud of their 2k zx81[1] (upgraded by a previous owner)

[1] 16k seemed very powerful at the time. My next computer, a Vic20, had 5k on board (3.5k available to BASIC) and I added an 8k ram cartridge.

[2] Luckily this particular machine has all of its chips in sockets, which is one of the reasons I like this particular one so much and feel more confident about using it.

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