Sinclair ZX Spectrum 48k+ Restoration

A while back I got my hands on my first Sinclair ZX Spectrum 48k+, yet another 1980s 8-bit computer to add to my collection. The machine was in very good condition and came with several accessories as well as its original box, however it was sold as “untested”.

The Sinclair ZX Spectrum 48k+ was one of the intermediate machines produced by Sinclair, and was the replacement for the ZX Spectrum 16/48k “rubber-key” in 1984 – they used the same mainboard so were functionally identical, but the 48k+ featured a larger case, a more traditional keyboard, and a reset button.

The Spectrum seemed to be all original, and featured a 1984 ISSUE 4S mainboard with 48 KB of factory-installed RAM – the ULA was socketed.

At the time of this restoration I still had an old TV with an RF input, so I was able to give the machine a quick test – it seemed to boot up fine, but I couldn’t do much else as most of the keys weren’t working, a common problem.

There are several steps which I like to take when performing a complete restoration on a Spectrum, which include fixing factory or age-related problems, and future-proofing.

As such, I planned out the servicing, preventative maintenance, and upgrades that I wanted to perform, as follows:
#1: Modify the RF video output to display composite video instead.
#2: Replace the original voltage regulator with a modern switch-mode equivalent.
#3: Replace the original connectors with new parts.
#4: Replace all the electrolytic capacitors on the mainboard.
#5: Replace the original keyboard membrane.
#6: Refurbish the power supply.

#1: Composite video modification:

The Sinclair ZX Spectrum’s only video output is modulated RF via an RCA jack, as it was designed to be compatible with old-school televisions as opposed to expensive monitors – however, this limits its compatibility with modern displays and reduces the quality of the video signal, due to the degradation caused by modulation and subsequent demodulation.

In the Sinclair ZX Spectrum 16/48k/48k+, it is possible to modify the RF modulator to output a composite video signal instead of modulated RF, which offers a much better image quality and is compatible with most modern displays.

Following this guide, I installed a composite video modification neatly and in a professional fashion inside the RF modulator, using a low-profile 100uF 10V electrolytic capacitor instead of a wire jumper – this blocks any DC offset on the video signal, improving display compatibility and reducing power consumption.

This involved removing the lid from the RF modulator, disconnecting the 5V power supply to the modulator PCB, disconnecting the video input to the modulator PCB, disconnecting the RCA jack inner pin from the modulator PCB, and redirecting it to the video input.

You can see my installation below. I’ve done it in a reversible manner, and it is impossible to tell from the outside that a modification has been performed.

I then re-tested the system, and it now seemed to be outputting a bright, stable, and crisp composite video signal which displayed well on my TV.

ZX Spectrum 48k+ displays correct image over composite video.

#2: Replace the voltage regulator:

From the factory, the ZX Spectrum uses a 7805 linear regulator (attached to the large heatsink at the top of the board) to regulate the 9Vdc input voltage to 5Vdc. Linear regulators are inefficient, unreliable, and produce a lot of heat, particularly under load – this heat builds up inside the case, increasing the interior temperature and subsequently reducing the lifetime of the electronics.

Modern, switch-mode, drop-in replacements for the 7805 are available, such as the Traco Power TSR-2450 – I removed the original heatsink (held in place by a bolt) and desoldered the original regulator, then installed one of these in its place.

#3: Refurbish the I/O (ports, edge connectors, etc):

I/O ports and edge connectors on computers see a lot of use which can mean that they get loose and unreliable, and they can corrode during use or in storage.

As such, I always install a new power input jack (a 2.1mm DC jack) and new “ear” and “mic” connectors (both 3.5mm mono jacks), all of which can be bought new from Retroleum. I also carefully clean both sides of the edge connector with a white eraser, and clean all of the IC sockets with contact cleaner.

#4: Replace all the electrolytic capacitors on the mainboard:

Electronic components generally don’t “age” as such, except for electrolytic capacitors, which are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics.

These capacitors typically comprise aluminium windings insulated by a liquid electrolyte, which can dry out over time and negatively affect performance (even failing dead short), or leak out and cause corrosion to the PCB and surrounding components – as such, they should be replaced as a part of preventative maintenance.

As such, I always replace all the original electrolytic capacitors with high-quality modern equivalents, which usually substantially improves the video quality – this takes some time on the Spectrum as there are several capacitors and the board can be quite fragile, so care should be taken.

I usually remove all of the capacitors at once using my desoldering station (a Duratool D00672), then install the new ones one-by-one whilst taking particular care to ensure that the value, voltage rating, and orientation are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll blow up during use.

You can’t always trust the markings on the PCB silkscreen, as sometimes mistakes were made in the design from the factory (take the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original.

Another method of improving the video quality on 16/48k/48k+ machines (specifically, to reduce the “jailbar” effect on the background) is to increase the filter capacitance on the 12Vdc power rail, by installing 220nF ceramic capacitors in place of C5 – C8.

I used commercially available capacitor packs from Retroleum.

#5: Replace the keyboard membrane:

The keyboard in the Spectrum uses a membrane – comprising several plastic layers with printed conductive tracks, which make a contact when a key is pressed – which tends to become brittle with age, and either becomes unreliable or stops working altogether. I always replace these as a matter of course, no matter their condition.

Accessing the keyboard membrane requires dismantling the computer – the ZX Spectrum 48k+ is relatively simple to pull apart, with eight screws on the underside of the case, and two keyboard membrane tails plugged into the mainboard.

The keyboard membrane itself can then be accessed by removing all the screws on the keyboard backplate, and undoing the screws on the two tail clamps.

Modern-made keyboard membranes are available to purchase from various retailers – take care to buy the correct type, as the early 16/48k “rubber-key” uses a different membrane from the 48k+/128k+. The new membrane can then be installed – I won’t cover this procedure in detail, as it is already documented comprehensively online.

The new keyboard membrane installed.

Once the membrane was installed I tested it, and all keys now registered correctly.

#6: Restore the power supply:

Functionally, the original PSU was working fine – I always install a modern mains plug (3A fused) on any PSUs that I use, and check the output voltage(s). It was also missing two rubber feet, which I replaced – aside from that, the PSU casing and cabling just needed a good clean.

I’d usually recommend using a modern PSU with any vintage computer, as the originals can be prone to failure – however, the ZX Spectrum takes an unregulated 9 Vdc input which is regulated internally, meaning a PSU failure would be unlikely to damage the computer, so an original PSU (with a modern plug) should be safe to use.

It is normal for these unregulated 9 Vdc PSUs to output a voltage greater than specified (typically around 15 Vdc unloaded), even under load, as this will be roughly linear to the AC input voltage – according to its datasheet, the 7805 voltage regulator can output 5 Vdc at up to 1.5A and handle an input voltage of up to 25 Vdc.

Testing and Conclusion

Just because a computer boots, doesn’t necessarily mean that it’s working properly. Thorough testing is necessary to verify operation.

After all this work was performed, I did some testing: all keys registered correctly; the composite video worked OK; the internal speaker worked OK; the machine would read and write correctly from/to an external tape recorder; the machine worked correctly with the DIVMMC Future; all 48k ROM/RAM tests pass OK; all joystick inputs worked OK; reset button worked OK.

I also installed a black metal heatsink on the ULA, which runs rather hot, using thermal tape – opinion is divided on whether or not these actually help to dissipate more heat from plastic-cased ICs, but either way it can’t hurt, and it looks really neat too.

The case needed a new set of four rubber feet and two new foam pads on the pop-out legs, all of which are available to purchase on Retroleum – aside from that, it cleaned up really well with a good scrub using Cillit Bang and a microfibre cloth.

Another restoration complete – happy days!

Published by themightymadman

A conscientious, intelligent and committed graduate engineer, with excellent interpersonal skills, an eye for detail and a keen interest in hardware design, mathematics, and software development.

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