512K SRAM internal upgrade

Discussion forums for users of the Cambridge Z88 Portable Computer
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Posts: 108
Joined: Thu Mar 11, 2021 12:34 pm

512K SRAM internal upgrade

Post by patters »

Hi all,

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.
The various historic guides for this upgrade mention that the new RAM will be limited to 128K until the additional address lines A17 and A18 are wired in. This was not my experience - perhaps this was the behaviour with contemporary PSRAM. I guess it was because they were left floating when I first tried to test. As a result I was concerned that I had a bad RAM IC. I needed A17 & A18 to be wired in for the machine to work at all. Without them I experienced a solid dark LCD display and one line of random pixels flickering. Kynar wire is a must for this job - it's small enough to go through the vias in the motherboard. The historic guides also suggest that you need OZ4 to recognise the RAM, suggesting that it won't work at all with OZ3. This is not really correct, it is recognised. What they mean is that only OZ4 will unlock the "expanded machine" behaviours with this additional RAM in Slot 0 (internal) as opposed to card Slot 1. Namely a larger PipeDream map, larger BBC BASIC memory, more UDGs, and a larger pixel graphics area.

I decided not to socket my RAM as I've become quite good at desoldering ICs, and I was fairly confident that the chip I had bought on eBay was genuine based on the quality of its surface etching. 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.

New SRAM.jpg
PSRAM removed.jpg
SRAM fitted - top.jpg
SRAM fitted - bottom.jpg
512K confirmed in OZ3.0.jpg
Last edited by patters on Sun Jun 27, 2021 11:09 pm, edited 10 times in total.
Posts: 17
Joined: Thu Apr 23, 2020 6:58 pm

Re: 512K SRAM internal upgrade

Post by Jbizzel »

Great job patters, and I can confirm that I have also carried out this upgrade today, following @patters steps. Thanks!
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