ZX Spectrum 16k: Upgrade to 48k the hard way
I picked up a ZX spectrum for €10 at a market a few years ago, without any cables or power supply, plugged it in when I got home and it booted to the 'Sinclair Research' screen without any problems. A good find!
Eventually I wanted to play some old games but the keyboard was unresponsive when I tried it out, so back it went on the shelf until I could find time to take a proper look.
That time is today! There are a lot of excellent resources around for repairing and refurbishing Sinclair Spectrum computers and a lot of small suppliers that can provide any parts you need. I particularly recommend Spectrum for Everyone.
So, let's have a look at this rescued stray. The keyboard fascia is held on by duct tape and someone has obviously peeled it back without loosening the glue at some stage in the past.
The metal is bent out of shape and isn't going to be easy to restore. Otherwise everything looks good here; the paintwork, case and keyboard all look fine.
Flip it over and 3 of the 4 original feet are still there. You can see the perforations for the speaker at the bottom left, around the foot.
Wait, what's this - 16k? I'm not going to be playing too many games without the full 48k complement. You can buy a RAM pack that fits on to the edge connector at the rear, but they're mostly famous for falling off while in use.
The machine boots, the rf jack works and the case is in good order. This seems like a good candidate for fixing up. The keyboard is not responding but that can usually be attributed to a broken membrane as they are known to grow fragile over time. The main problem is that there is not enough RAM to run anything more sophisticated than Jetpac - but that is fixable too.
So here's the full kit for restoring the machine:
- RAM upgrade kit from Retro Spares Shop
- New keyboard fascia SellMyRetro
- Replacement membrane, also from SellMyRetro
- Flat head and Philips head screwdrivers
- The 16k ZX Spectrum
- Soldering Iron
- Penknife and Solder
- Receptacle to hold screws
The computer is held together with 5 identical screws. You have to be careful removing the cover as the keyboard membrane is part of the top but is also attached to the motherboard at the bottom.
In this case part of the membrane had disintegrated, explaining the input problems I was having.
Now we can get a closer look with the Keyboard out of the way. All the capacitors, the 9 blue cylinders in the photo, look good with no sign of leakage.
This is a two-step process, which is well described in the document that the Retro Spares Shop supplied with the upgrade kit.
- Solder the wire links that tell the memory controller which half of the RAM chips to use
- Slot the new RAM and TTL chips into the correct slots.
This is an Issue 3 Spectrum, so the wire links that I need to solder are under the heatsink. The earlier Issue 2 design has the wire links just to the right of the large ULA chip.
You can also see, in the highlighted red rectangle in the image above, where the new memory and logic chips have to go.
The heatsink is held on by a single flathead screw. Be careful though, there is a nut behind the motherboard which will fall out when you remove the single screw. It's also likely to get attached to the speaker magnet if you let it loose inside the case.
Here you can see the links that need to be bridged. Apparently wiring either side will do, as the modern memory has a very low error rate and the link decides which half of the chip is 'best' to use. In my case I will be joining 'TI' and '4'. The holes are filled with solder from the factory.
I don't have a 'desoldering pump' so I'm just going to use the iron to remove the solder.
I found it best to bend the wire link in place before placing it on the board. Leave some length in the legs so they go right through - they can always be crimped later.
I have cleared away the solder and driven the link through the PCB. You can see I have singed the board slighty in the first two holes - it's enough to have the soldering iron at a low setting to melt the factory solder.
As you can see, I have seated the RAM chips and also the TTL in the following marked slots.
- IC23: 74LS/HCT32
- IC24: 74LS/HCT00
- IC25: 74LS/HCT157
- IC26: 74LS/HCT157
They just click into place with the text on top oriented the same way as the existing chips. That pretty much concludes the upgrade process!
Replacing the membrane
Next I remove the old fascia, heating it gently with a hairdryer to soften the glue and then slowly prising it off.
This is a good opportunity to give the rubber keyboard a mild soapy wash. The old membrane is just underneath.
The tabs from the new membrane pass through the cover and slot into the motherboard. Replace the top slowly to make sure the new tabs fold gracefully into the case.
Everything is in place, now it is time to put the 5 screws back in.
And plug it in! Apparently it takes slighlty longer to boot with the extra RAM - I can't say I noticed the difference.
There is a more accurate way of checking; test the RAM with
PRINT PEEK 23733which will return
255 if the upgrade is successful. It returned
127 for me until I got the soldering right.
And here's the finished ZX Spectrum, cleaned, polished and turbo-charged!
How long did it take and how much did it cost?
It took about two hours all told, mostly because I'm not good at soldering.
|RAM upgrade Kit||£9.99|
About €50, so not much cheaper than buying a Spectrum on eBay, although the new parts should mean this one has a few more decades left to go. The people who sell refurbished hardware on eBay are definitely doing this for love rather than money.
The next step is probably just to plug my phone into the Spectrum to load some TAP files so that I can play games and then maybe get a DivMMC if I get really into one of them and want to save (or use a joystick).