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How To: ZX Spectrum Composite Video

ZX Spectrum emulators are all very well, but it’s a bit like listening to a much-loved vinyl LP on your digital-playback-device-of-choice. Sure, it’s clearer, faster to access and more convenient, but there’s something lacking; a level of authenticity has been lost.

Firing up my ‘everyday use’ ZX Spectrum+ immediately takes me back to my youth. It’s a more tactile, visceral experience. Yes, I’m sentimental and unapologetically so.

Now the 1980s world of Spectrum wasn’t all plain sailing. There are definitely things I could do without and would not miss at all. So, these posts cover a few updates I’ve made to my Speccy to bring it into the 21st Century.

Part One: Video Output

There’s a number of problems here. Firstly, the video output form the ZX Spectrum was always a hit horrible (but no more horrible than any other outputting device of that era). Secondly, the RF signal it produces is on the way out. Analogue signals are no longer operating in the UK and it’s only a matter of time before all available TVs no longer carry an analogue tuner. Even if your flat-screen LCD TV can tune into the Speccy, many models struggle with the signal for all manner of technical reasons. Even if you get a weak-but-steady image, the ‘smart’ on-board software will happily conclude there is no signal there and throw up a blank screen (this is particularly true of ZX81s).

Luckily, these are easy problems to overcome provided you know which end of a soldering iron hurts. Internally, the ZX Spectrum produces a perfectly good composite signal which is then fed into the RF modulator. Amazingly enough, it is just as simple as by-passing the modulator altogether.

This process takes about 15 minutes and results in a standard composite signal from the same phono plug which pretty much any TV display out there will be able to handle now and for years to come. Even then, composite to HDMI convertors are cheap and readily available.

People have made much better videos and posts about this process than I have here and I strongly recommend you Google about and get a good idea of what you’re going to do. Nevertheless, here’s a brief run-through.

Before starting, here are the caveats: This is not anything for which I will take responsibility. It’s your Speccy, it’s your soldering iron and it’s your duty to double-check everything here. If in doubt, don’t do it. Also, this modification only applies to the ‘original’ 16k, 48k and ‘+’ Spectrums. The 128 models and upwards have built in RGB capability.

You will need:

  • Screwdriver (Crosshead No 1)
  • Soldering Iron, Solder
  • Steady Hand
  • Some single-core wire or a 100uF 10V electrolytic capacitor (16V is fine)

The capacitor is optional. On certain displays it will produce a steadier signal, on others it’ll make no difference at all.

You’ve got your static strap on, yes? Good.

With a brave heart, remove all the screws from the back and carefully separate the keyboard from the motherboard. There are two thin ribbon cables you will need to gently pull away from their connectors. Put the keyboard to one side, away from the cat.

The RF modulator is easy to spot; it’s the silver box at the top left of the motherboard where you plug the TV in. Its lid is not secured by screws and can be gently prised off.

 

The RF Modulator
The RF Modulator

Inside the modulator the centre pin of the phono plug is connected to a resistor with a plastic sheath covering it. De-solder the resistor from the centre pin and gently bend it away.

On the left of the RF modulator are two bare wires that connect it to the motherboard. These both need to be cut mid-way from the point they enter the box. Bend the one nearest the corner of the box (furthest from the back of the Speccy) out of the way, we’re not going to be using it.

Now get your piece of wire or capacitor ready. Most modulators used in the Speccy have a spare hole on the side next to the cut wire, we’re going to make use of this.

Orientate your capacitor if you’re using one. The negative pin is marked on the side the of the body. The capacitor must be soldered in the correct way around.

Feed the capacitor’s positive wire through the hole and carefully solder it to the centre pin of the phono plug.

Solder the negative wire to the wire you cut earlier; the one that fed through the adjacent hole.

A rather poorly laid-out shot of the modification
A rather poorly laid-out shot of the modification – Click to embiggen
Another perspective
Another perspective

That’s it. Make sure the cut wires and de-soldered resistor are isolated from the rest of the circuit. Carefully replace the top of the modulator and re-attach the keyboard ribbons (be very gentle with these). You may wish to test your work before screwing everything back together. Connect a suitable lead between your Speccy and TV and fire things up. All being well, you’ll find a rock-steady signal on your ‘Video’ or ‘Composite’ input. Re-assemble your newly future-proofed device.

Yay! Steady as a rock.
Yay! Steady as a rock.

There are many variations on this modification and I encourage you to Google around. My method is one of the less elegant ones but avoids removing and soldering directly on the motherboard or permanently disabling the modulator. To reverse the process, remove the capacitor, re-solder the two external wires and re-solder the resistor.

Here’s some links for more information:

http://www.jammarcade.net/zx-spectrum-composite-video-mod/
Good all-round explanation

http://samsmods.blogspot.co.uk/2011/10/zx-spectrum-48k-composite-video-mod.html
Zen-like modification that removes the RF modulator

http://www.brianhoskins.co.uk/website/spectrumcompositemod/
Another demonstration of using the capacitor

Google for more

 

 

Published inVintage Tech

3 Comments

  1. It’s good to see more people dragging their Spectrum computers into the 21st century 🙂

    The Spectrum was actually my first computer and I credit it with the birth of my interest in computer programming.

    I have been meaning to get down to Bletchley Park for a number of years. Perhaps this Summer I can make it happen!

    • PJ PJ

      That would be great. If you do, let me know and I’ll show you ’round.

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