http://bitcycle.org/retro/zx81/internal_RAM/
Pictures included, that should make it easy to locate solder points etc. Description is complete and verified for accuracy.
Highlights:
- RAM chip goes into a socket, without first bending pins sideways. So very easy to swap RAM chips. All changes are made on the board / IC socket itself.
- 10 cm. wire in total does the job.
1: /RFSH -> A11
2: +5V -> A12
21: +5V -> A10 (LK1 -> LK2)
23: /WR -> A13
Most 16 or 32K RAM mods show a pull-up resistor on pin 27 (/WR). I have long suspected, and recently verified it's not needed for a basic 16K mod, so it's not included here. All hi-res software I've thrown at it, works fine regardless. What would it do for a basic 16K mod anyway? The Z80 drives the /WR signal always afaik, so a pull-up on there serves no purpose. Unless power is failing and you want to keep the SRAM bits as they are... (read: battery backup).
I believe this to be the 'cleanest' 16K mod I've seen to date. And for the 32K RAM modders out there: no reason you couldn't use this as a starting point. Pin 26 is where it's at...