There is no PCB, just soldered a RAM on the back of the ROM and build a small dot pcb.
https://sinclairzxworld.com/viewtopic.p ... 302#p37302
This need at least 8-16k range not used by external RAM.
I can tell in more detail if needed.
I also have a rough idea what UDG4ZXPAND is doing. I assume it is possible to build that function with standard parts as well.
ZX81 issue 1 56k internal ram with battery backup, UDG, CHR$128 and WRX enabled
Minstrel 3 ZX81 clone 64k with battery backup, UDG, CHR$128 and WRX enabled
.. and thanks Max too.
I had read the posting you linked to. An admirably devious solution. Although I'm not averse in principle to soldering other components to IC pins on mobos that's so far been limited to getting base hardware working properly (e.g. spider mods, resistor arrays to Atari DMA chips, etc). Piggybacking a RAM on a ROM chip felt a step too far at this stage. It may be something to consider if I get yet another ZX81. It would be interesting to know what the FPGA (I assume there's one from one of Andy Rea's postings) is doing in detail but emulating it using standard chips, though a very fascinating intellectual exercise, would be overkill just for my case.
Fortunately UDG development under EightyOne can proceed apace. I'm also dipping into DesignSpark in idle moments for this circuit http://www.user.dccnet.com/wrigter/inde ... CHR$X4.htm
but under the circumstances that's out of interest rather than necessity. I could see it getting occasional use.