Spectrum ZX 48K

I was doing my weekly flee market thrift and I stumbled across something truly spectacular, a Sinclair Spectrum ZX 48K, an 8 bit home computer introduced in the UK in 1982! This thing is a stuff of legends somewhere in the range of Commodore 64.

Find of the day at a local flee market.

Some basic info:

• CPU: Z80A

• Speed: 3.5MHz

• RAM: 16KB or 48KB

• Rubber Keyboard

• Colour Graphics: TEXT MODES 32 x 24px GRAPHIC MODES 256 x 192px

• Sound (beeper — 1 channel, 10 octaves)

• Local price at launch: £125–16K version, £175 48K version

When I opened it up, I noticed that it had a bad keyboard membrane, which is the most common issue that happens to this 30+ year old machines.

The plastic deteriorates after the 30 years

Luckily they are also easily available. There are three main suppliers that still make them:

• Rwap (UK),

• Sintech (Germany),

• Retro Radionics (China)

I was also missing a power adapter, the Spectrum uses 9V power supply, modern switcher adapter will do just fine, however they need to be center negative, and I found one here. More about Speccy power supply in a future article.

When I was waiting for the parts, I also joined a very active community on Facebook. There they told me it would be a good idea to do some initial tests before powering the machine up. The very nice YouTube video describes the process in details:

Initial tests that are recommended to perform before the initial power on.

Basically you check if the power leads to the lower rams are OK and nothing is shorted and that the coil responsible for transforming the voltage is intact and not shorted.

After the spare parts arrived and I changed the membrane, I gave the housing a little scrub and we were good to go.

Newly installed keyboard membrane.

Before the first power up, I went to my electrical engineer friend to do the double checks, because he has a very well equipped laboratory. Everything checked up, so we plugged in the composite cable and the PSU.

I use AV IN on the TV, with the adapters that came with it and cinch connector. The socket on both devices is a normal 3.5mm aux, but the cables are normally not shielded and the picture quality is notably worse.

Success! It booted up and displayed the copyright logo.

Time to try some games! Since we didn’t have any cassette player, or any Spectrum games for that matter, I have downloaded PlayZX to my phone and hooked it to EAR input on the Spectrum. One LOAD “” latter this is what we got:

Spectrum ZX Exploding Wall game.

Now it’s time for some personal information about my new Spectrum ZX:

Serial number: D01–373056 Model: 48K Motherboard issue: 3B ULA: 6C001E7 CPU: NECD780C-1 CPU Date: 8411P8 RAM Manufacture date: 8423

Motherboard revisions

With Sinclair it was all about cutting costs, and there were multiple problems with the machine. There are multiple variants or board issues, here listed by Spectrum fans street cred and rarity:

• Issue 5

• Issue 1

• SRBP 3B

• Samsung 3B

• SRBP 4A

• Samsung 4S

• Issue 2

• Issue 4A/B

• Issue 6A

• Issue 3/3B

If you’re interested in details how one is distinguish from others I recommend this page. And if you’re after the schematics I would recommend this one.

Serial number under the edge connector.

What is also special about my unit is that it came heavily modded. It had a composite, 1.5mm aux, reset button and a ULA sink, which just shows that someone really tried to get everything it could from this machine but at the same time took good care of it.

Button between EAR (AUX input) and TV is a reset button, top connector at the far right is 1.5mm AUX sound out, bottom one is 3.5mm AUX composite output.

Reset is done by shorting across capacitor C27. Audio output is just connected to the speaker.

Composite mod.

To make full use of my new toy I have also ordered a DivMMC pro, basically a SD card to edge connector interface, with some extra features and a joystick port. Check out my post about DivMMCs for more details, I also did some comparison between the most common ones in the market.

Random fun fact 1: The keyboard had written BASIC commands on each letter, so each BASIC command only took 1 bit of memory.

Random fun fact 2: To reduce the price, the 32 KB extension(from 16k to 48k) used eight faulty 64 kilobit chips with only one half of their capacity working and/or available.