I thought it would be useful to post here that I successfully carried out a 512K SRAM internal upgrade, and that it definitely works with the Alliance AS6C4008-55PCN part. This is pretty much the only remaining 512K SRAM in a PDIP-32 package that is still sold new, without resorting to recycled chips or ones of unknown provenance from Chinese eBay sellers. It can be acquired for around £10 at the time of writing.
A few specific tips:
- Desoldering ICs with a cheap spring-loaded plunger solder-sucker isn't very difficult, it just needs a bit of practice and the right technique. I had thought the chance of actually being successful doing this would be low, but I recommend watching this YouTube video by GadgetUK164 for a very good explanation of the technique. Hats off to him for taking the time to pass on the practical skill to others, and for not simply spending hundreds of pounds on a vacuum desolder station like other most retro YouTube channels.
- My own best tip for this is to hold the iron in your dominant hand and the solder-sucker in the other. Have the sucker in position but tilted while you heat the spot with the iron. I find it's useful to hook the rim of the nozzle around the pin next to the one you want to extract since it's normally large enough to cover two pins while flat against the PCB. Once molten, pivot the sucker back to vertical swiftly and press the button. Don't be afraid to get the tip of the pump right in there next to the iron.
- It's very difficult to reinsert the keyboard membrane tails once the work is done. If you try to push them in square on they just buckle and risk shattering. They really do need to be fed in at a sideways diagonal angle and then swung in. The keyboard sockets on the motherboard are side-feeding unlike the ZX81 and Spectrum ones, and they have components in front which makes it more difficult.
I decided not to socket my RAM as I've become quite good at desoldering ICs. This meant that I didn't have to trim away any of the plastic reinforcing ribs on the underside of the keyboard. I simply bent up and clipped the tip off pins 1 & 30, and wired them consistently with the historic guides. No other changes were necessary.