Return to Z88 Upgrade Page 2

On my series 4 board, see below, the ROM (on the left ) is in the lower part of a 32 pin socket, whilst the 28 pin RAM chip is directly soldered in. The existing ROM is thus easily removed by being levered out of it's socket with the aid of a small straight edge screw driver. If you look closely at the top of the RAM, you can just see in the photo, the white lines, of the top of a 32 pin chip outline, together with the four extra holes, currently filled with solder. The latter holes are fully connected, so the board was designed for a possible upgrade to the largest memory available at the time, which was 1Mb = 128K chip. Only 32K was installed, purely on cost grounds, the larger capacity chips could have almost doubled the cost of manufacture! However, it is for this reason that OZ version 3, the last in production machines, readily recognises 128K, but not 512K, for which you need a version 4 ROM, as well as modification of the main board.

Now to the nitty gritty, that is desoldering the RAM chip. The easy route is, to snip off the pins of the chip, and then with a soldering iron, melt the solder, and pull out the remains of the pins with a pair of tweezers. The more difficult, is to attempt to desolder, using a solder pump. It is probably best to support the board vertically, then heat each pin , from the chip side, at it's base. Wait until you see the solder melt on the base, then place the nozzle of your solder sucker / pump completely over the end of the pin and trigger it. With luck, all of the solder will be removed. At the same time remember to remove the solder from the 4 extra holes! Once all of the holes are clear, put in place the 32 pin, turned pin, low level DIL socket. You really do need the turned pin socket. The cheaper, compression fitting has a much higher profile, which with the chip fitted, will stop the keyboard sitting level. Before soldering it in, make sure that you have put it in the right way round. The indentation in the top bar of the socket should be lined up with a similar indent in the white silk screened chip outline. It doesn't really matter, but we might as well get every thing in it's correct position, you then don't have to think too hard when putting in the new chip! Do use a soldering iron with a small sharp head, and make sure that you see the solder flow into the hole, as we don't want any dry joints. Take care not to use too much solder, as we don't want to create any short circuits with other pads / tracks.

We have now prepared the main board, so let's put in the new chips and complete the job!