Sunday, 2 August 2015

Modifying a Sinclair ZX81 issue 1 - RAM

Modifying a Sinclair ZX81 issue 1

Upgrade the internal RAM to 16k bytes

This entry shows how I modified my issue 1 board ZX81. The pictures below show the steps I took.

The standard ZX81 as supplied by Sinclair Research Ltd only has 1k bytes of RAM (Random Access Memory). This is in the form of either a single 8 bit by 1024 bit (1k byte) SRAM chip (IC4, 4118) or two 4 bit by 1024 bit SRAM chips (IC4a and IC4b, both 2114 type). SRAM means Static Random Access Memory.

Now, if you are very careful, you can do useful things with only 1k byte of RAM. But you can do so much more with 16k bytes of RAM.

Back in the ZX81s heyday, the normal solution was to buy a memory (RAM) expansion and plug it into the expansion port (edge-connector) at the rear of the ZX81.

And you can still obtain used examples of the various different makes on the second hand market.

But some designs do suffer from RAM pack wobble. That is, a slight movement of the ZX81 and the plug-in memory sometimes causes a momentary break in one or more of the edge-connector contacts and your program either becomes corrupt or the ZX81 resets and deletes your program :-(

However, now that technology has moved on, you can get a single chip that can provide 16k bytes of RAM. And it is not that difficult to fit it to a ZX81 PCB (Printed Circuit Board).

You do however need some skills. Requirements are: able to de-solder, solder and to have a steady hand when cutting the PCB tracks. I used a hand held PCB 12V drill with a cutting blade fitted.

Before proceeding, it is wise to confirm that your ZX81 does in fact work. Then you know the condition and status of it before you work on it.

Once you have opened the ZX81 case by unscrewing all the screws (including the hidden ones under three of the rubber feet), you can unscrew the small screws that hold the PCB to the case.

Be very gentle with the keyboard membrane tails, as original membranes go brittle. Then any crack that forms will break the electrical connections that form the wires and some or all of the "keys" on your keyboard will no longer work.

In fact, it is likely that the cracks have already started long before you get a screwdriver to open the case. So buy a new keyboard membrane ;-)

Once the PCB is free of the case and the membrane, have a good look and see if there are any problems. For example dry solder joints or poorly soldered joints. Resolder any that look suspect [unless they are on the RAM chip(s) IC4/IC4a/IC4b in which case this/these are being removed].

If you are very good at desoldering, now is the time to put this skill to the test. Desolder the RAM chip(s) IC4/IC4a/IC4b. Be very careful not to damage the PCB tracks (traces) and pads. Although lifting a pad is not terminal, it does mean you have to repair the damage.

If however, you are less experienced, as it is unlikely you will need to reuse the old RAM chips, instead use some electronic side cutters to cut each pin (leg) of the RAM chip(s) at the bend where it goes into the plastic case of the chip. Then you can de-solder one pin at a time. This is far easier.

Top of PCB after old RAM de-soldered
Bottom of PCB after old RAM de-soldered

Top of PCB after tracks cut

Top of PCB after tracks cut - close-up

Bottom of PCB after tracks cut - see circled areas

Bottom of PCB after holes de-soldered

Top of PCB after holes de-soldered

Bottom of PCB showing signal details

Bottom of PCB showing wiring

Top of PCB showing wiring

Top of PCB showing SRAM socket

Top of PCB showing SRAM socket and wiring

Top of PCB showing temp +5V wire link (the uninsulated wire)

Top of PCB showing link L2

Top view, finished 16k RAM

The test program I used

Passed test okay