Sinclair ZX Spectrum 128k+ “Toastrack” Restoration

I’ve owned several different Sinclair ZX Spectrum computers over the past couple of years, but I hadn’t got my hands on a 128k+ at this point, so I set about looking to get one for my collection. Eventually, a machine in good, boxed, but untested condition with peripherals popped up on Facebook Marketplace – after an hours’ drive up to Newcastle, I put my bartering cap on and managed to get it for a reasonable price.

The ZX Spectrum 128k+ was the last Spectrum produced by Sinclair before their takeover by Amstrad, and was affectionately known as the “toastrack” due to the large voltage regulator heatsink on its side – it was an upgrade to the ZX Spectrum 48k+, featuring 128KB RAM, 32KB ROM, an RGB video output, three-channel audio via a AY-3-8912 chip, MIDI compatibility, an RS-232 serial port, and an external keypad.

The computer was in very good condition, and appeared to completely original.

After checking that the output voltage on the PSU was as expected, a quick power-on test plugged into the RGB port revealed that the computer seemed to boot up fine.

Initial testing – computer boots normally, with noticeable “jailbars” on RGB video.

As the machine seemed to be working okay, I planned out the servicing, preventative maintenance, and upgrades that I wanted to perform, as follows:
#1: Replace keyboard membrane.
#2: Replace 7805 voltage regulator with an uprated version.
#3: Replace power connector and tape connectors.
#4: Fix the video output issues associated with the factory ZX Spectrum 128k+.
#5: Fix the audio balancing issues associated with the factory ZX Spectrum 128k+.
#6: Replace all the electrolytic capacitors on the mainboard.

#1: Replace the keyboard membrane

The ZX Spectrum 128k+, like the 48k and 48k+ before it, uses a membrane keyboard 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 128k+ 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.

Brand new 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.

#2: Replace 7805 voltage regulator with an uprated version

The ZX Spectrum 128k+ comes from the factory fitted with a 7805 voltage regulator, rated at a 1A peak output – the computer draws closes to 1A, so running it with peripherals can push the 7805 to its limits. It is therefore recommended to replace it with a 78S05 voltage regulator, which is is rated at 2A and will therefore run cooler and more reliably.

This is a relatively simple task, as the voltage regulator in the 128k+ is attached to the mainboard via a cable with a push-fit connector. The regulator is attached to the heatsink via a bolt – it is important to apply new thermal paste between the new regulator and the heatsink, to ensure that heat is transferred between them efficiently.

New voltage regulator installed.

#3: Replace power connector and tape connectors

I/O ports and edge connectors on computers tend to see a lot of use, meaning they get loose, and they can also corrode in storage. As such, on Spectrum restorations I always install a new DC power jack and new EAR / MIC 3.5mm mono connectors, and carefully clean both sides of the edge connector with a white eraser.

These modifications require the removal of the PCB, held in by a few screws.

#4: Fix the video output issues associated with the factory ZX Spectrum 128k+

The ZX Spectrum 128k+ (and upon extension the 128k+2, which was a hasty Amstrad clone of the 128k+) suffer from various design flaws which affect their video quality.

Specifically, the audio signal is not only attached to the RF output, it’s attached much earlier in the video chain – this leads to video distortion on the composite and RGB video outputs. By disconnecting the output of C126 and reattaching it to the emitter of TR11 (via R130), the audio signal will be removed from the dedicated video outputs, but will still be available on RF – this leads to a very significant quality improvement.

Detaching audio from the video signal on the 128k+ (image credit: ByteDelight).

Another method of improving the video quality on the 128k+ (specifically, to reduce the “jailbar” effect on the background) is to increase the filter capacitance on the 12Vdc power rail, by replacing C7 and C8 with 1uF ceramic capacitors and the 22uF electrolytic capacitor at C28 with a 47uF electrolytic capacitor.

#5: Fix the audio balancing issues associated with the factory ZX Spectrum 128k+

Sinclair machines are well known for their quality issues, both in design and manufacture, and the 128k+ is no exception – when designing the 128k+ PCB, design errors in the audio circuitry (specifically the balancing stage) cause the 3-channel audio from the AY-3-8912 chip to be significantly quieter than the beeper audio from the ULA.

It is possible to amend this issue – I won’t cover this process in a lot of detail, as this is already documented online, but it involves replacing R115 (just left of the ULA)  with a 1.65kOhm resistor to equalise the balance.

#6: 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, these should be replaced as a part of preventative maintenance.

I replaced the electrolytic capacitors on the mainboard whilst it was removed – this is a fiddley process and can take quite some time, but is well worth doing.

I used a commercially available capacitor pack from Retroleum.

After the full recap and implementation of the video fixes described earlier, the RGB video output is incredibly crisp, and is barely recognisable compared to before – this just goes to show that these steps are well worth doing.

Conclusion

After all this work was performed, I did some finishing up: I thoroughly cleaned the mainboard with compressed air and an ESD-safe brush, all its ports using contact cleaner, and all its edge connectors using a white eraser; I thoroughly cleaned the case inside and out using Cillit Bang and a microfibre cloth, and replaced missing foam pads on both of its pop-out legs; I replaced the two leg springs with 3D-printed equivalents, as one of the originals was broken; I thoroughly cleaned the PSU, fitted a new UK plug (3A fused), and replaced its two missing feet.

Then, it was time for testing: all keys registered correctly; RGB and composite video outputs worked OK; reads and writes correctly from/to an external tape recorder; power LED worked; reset button worked; worked correctly with DIVMMC Future; 128k ROM/RAM tests passed OK; 48k beeper audio and AY sound chip audio worked; all joystick inputs worked.

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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