The Sinclair ZX range of computers were among the first affordable computers and kick-started the UK’s home computer revolution in the 1980s. These machines relied on consumer television equipment for their display output and, as such, they featured an RF modulated output so that users could simply “tune-in” to them on their television sets.
Modern TVs use digital tuners and thus are no longer compatible with the analogue output of the Sinclair ZX computers. However, most TVs – even the most modern models – still feature composite video inputs and it turns out that it’s very easy indeed to modify the video output on these computers to be compatible with a TV composite video input. As well as providing compatibility with modern television sets, this modification also improves the computer’s video output quality. This post will detail the procedure for modification of the Sinclair ZX Spectrum Computer, but it should be fairly similar on other Sinclair models of that era.
As shown in the basic block-diagram below, the basic principle of video generation and display for the ZX Spectrum Computer is as follows:
- Generate composite video output from memory map
- Add chrominance (colour)
- Generate UHF with RF modulator
- Send down cable to TV
- Recover composite video with TV UHF demodulator
As can be seen, the modulation/demodulation process is actually quite wasteful because we start with composite video, modulate it at the Spectrum video output, and then we just de-modulate it at the TV end to get composite video again! This was convenient at the time the computer was designed because it allowed users to display the computer output on standard analogue tuner TVs, but with most modern TVs featuring composite video inputs it’s pointless – we can just cut the mod/de-mod process out and connect standard composite video out straight from the Spectrum Computer directly to the TV composite inputs!
Completing the modification
The modification is very straightforward. All you need to do is disconnect and remove the RF modulator, and then connect the machine’s composite video compatible output directly to the RCA connector via a decoupling capacitor. Here is the process broken down into stages:
1: Remove the front cover
To remove the front cover, release the 5 screws on the bottom of the machine. The front cover will then lift away, but BE CAREFUL of the keyboard flat cables – you will need to disconnect these before you can lift the front cover all the way off. They can be a little bit tight but with care they will simply pull out. After this you should be able to remove the front cover completely. The item we are going to remove is the RF Modulator unit which is enclosed inside a metal screening can at the top left-hand side of the unit, shown in the photo to the left.
2: Remove the RF Modulator top screening plate
Next you will need to remove the top plate from the RF Modulator screening can. It simply prises off – a flat blade screwdriver will help you. Be careful not to slip and cut yourself though!
With the Screening can removed you will be able to see the RF modulator circuit inside. We are going to remove this circuit board completely in the next steps.
3: Disconnect the Modulator 5V supply & composite video feed
The RF Modulator has two connection cables from the main PCB, as shown in the photo. One is a 5V power supply and the other is a composite video feed. These need to be de-soldered from the main PCB. The easiest way to do it is to heat the joint from the top side of the board and then, with soldering iron still applied to the joint, carefully pull the cable through the joint with a pair of pliers. Be careful when you do this as there is a danger of flicking up molten solder – unless you wear glasses then basic eye protection should be worn.
4: Disconnect UHF feed resistor from RCA Connector
Now you need to disconnect the feed resistor from the RCA connector shown in the photograph. You can either desolder it or just snip it off with a good pair of wire snips. The choice is yours! If you’re going to snip it off then make your cut close to the RCA connector itself – this way you can save the RF modulator circuit complete and it will be easy to reinstall in the future in the unlikely event that you decide to put it back.
5: Remove the RF module from the Main PCB
Now it is time to remove the RF module completely. To do this you will first need to remove the Main PCB from the bottom casing. This is very easy to do – there is just one screw securing the Main PCB to the bottom casing so release it and the Main PCB will come away easily.
After this you need to desolder the RF module anchor points which are shown in the photograph. It can be a little tricky to desolder these because they are connected to a huge ground plane which – in conjunction with the screening can itself – tends to sink the heat of your iron away from the solder joint itself. If you have an adjustable temperature on your iron you will want to turn it up to full for this particular job. It is quite difficult to completely free the joints of solder so you will probably find that the removal process is a case of heating the joint and carefully prising the module free a little bit each side at a time.
6: Remove RF Modulator PCB
With the RF module removed it is time to remove the modulator PCB. This is very easy – there are four anchor points as shown in the photograph. Desolder these and the PCB will come free. You will then be left with an empty screening can which you’ll want to solder back in to the board so that you can re-use the RCA cable.
7: Fit decoupling capacitor
With the empty screening can fitted back onto the PCB as shown in the photograph you’re ready to fit a decoupling capacitor between the machine’s composite video connection point and the RCA connector. All you need for this is a 100uF 10V capacitor. I used a 16V part because it’s all I had in my parts bin – it works fine. At these kinds of capacitances you’re probably going to be using a polarised part (most likely an electrolytic like the one I’ve used). In that case connect the positive end of the component to the RCA connector and the negative end to the main PCB. On an electrolytic capacitor like the one shown, the negative lead is designated by a marking on the component body itself.
NOTE: The decoupling capacitor isn’t really that essential. It may improve picture quality for certain models of television set – it depends how the composite video input circuitry works – but on other models you probably wouldn’t notice any difference. If you don’t have a suitable component around you and you can’t be bothered to order one then you can just try connecting a cable straight from the composite video point on the Main PCB directly to the RCA connector itself. It should work.
Time to test it! Don’t bother putting it all back together just yet – connect up the power and connect the RCA connector to your TV composite video input using a standard RCA cable. If all is well (and it should be) you will see a nice clean video output!
I hope you enjoyed this simple modification. If you try this mod yourself then I welcome comments – please feel free to join the discussion! I will leave you with a video I made of the modification that I made it while I was still on a voyage of discovery with it myself – you may still find it useful.
Here are some links to other good sources of information on modding your Spectrum: