Restoring a Sinclair ZX Spectrum 48k for Charity

This April, I’m raffling off a refurbished ZX Spectrum 48k bundle for charity – I recently bought a boxed Spectrum that would be ideal for this purpose, but obviously for this to be “refurbished”, it required refurbishing. Details of the process are below.

I bought the Spectrum as partially tested from a chap in Hull – it was in good, if a bit dirty, condition, and seemed to be all original aside from a DIY composite video modification.

The board was a 1984 ISSUE 3B, with 48k of factory-installed RAM.

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

I buy most of my ZX Spectrum parts from Retroleum.

Composite Video Modification

The electronics seemed to be original with no signs of previous work, except for a DIY composite video modification – this is a common mod which makes the Spectrum output composite video over the RF jack, meaning the computer can be used with modern televisions and monitors.

The original video modification had been installed poorly, and in an irreversible fashion. I removed it and cleaned everything up, then installed the modification more neatly and professionally 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.

RF modulator interior following video modification.

Switching 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 – this builds up inside the case, increasing the inside temperature and reducing the lifetime of the electronics.

Modern drop-in replacements for the 7805 are available, such as the Traco Power TSR-2450 – I removed the original regulator and heatsink, and installed one of these in its place.

I/O Refurbishment

I/O ports and edge connectors on computers see a lot of use, and can corrode in storage. As such, 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.

Capacitor Replacements

Electrolytic capacitors are often used in electronics for decoupling and filtering, and are typically made from wound aluminium foil with a liquid electrolyte spacer – this electrolyte can dry out or leak over time, meaning that these kind of capacitors suffer from age-related effects which reduce their effectiveness (i.e. reduced capacitance, increased ESR, etc).

As such, I always replace all the original electrolytic capacitors with high-quality modern equivalents, which usually improves the video quality noticeably.

Another method of improving the video quality on the 48k (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.

Case Restoration (Including Keyboard Membrane)

The keyboard in the ZX Spectrum uses a membrane, comprising several plastic layers with printed conductive tracks, which make a contact when a key is pressed.

These membranes were cheaply produced and become brittle with advanced age, meaning that most of them end up failing with use or breaking on removal.

The original keyboard membrane tails, which usually crack or snap off.

Luckily, a range of modern-made replica parts (including cases, keyboard mats, keyboard fascias, and keyboard membranes) are currently available for the ZX Spectrum, from sellers such as ZX Renew – this allows even the most basket-case computers to be restored.

Therefore, I always fit a new keyboard membrane during Spectrum restorations. This requires removing the front faceplate (the thin sheet of painted aluminium with all the key descriptions on), which is easily damaged – the original was a bit dull and had a few scratches, so for a top-quality restoration I decided to replace this too.

PSU Restoration

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). Aside from that, the PSU casing and cabling just needed a good clean.

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

Testing and Repairs

The computer still booted OK following all of my modifications, and all the keys registered correctly with the new membrane. However – and I know this well – just because a computer boots, doesn’t mean it’s working properly. Thorough testing is necessary to verify operation.

Initially, the Spectrum would not work correctly with my SD card reader, a DivMMC Future from TFW8b (which I thoroughly recommend). This is usually due to a partial CPU failure (the lack of a valid M1 signal), which was the case on several 48k’s that I’d worked on previously. Sure enough, after socketing and replacing the original Z80 CPU with a known-good part, the SD card reader worked fine.

There is a lot of speculation as to how this failure occurs – some believe that this was a factory fault and that Sinclair bought part-working Z80s on the cheap, whereas others believe that this is a user fault that occurs when an expansion device is installed incorrectly, shorting the M1 line to 12V.

New Z80 CPU installed.

With the SD card reader working correctly, I could load some detailed diagnostic software – I usually use ZX Spectrum Dignostics, which validates the contents of the onboard ROM and tests all the onboard RAM, with an optional soak test mode. Initially, the upper RAM test failed, indicating a problem with the RAM IC at IC19 (corresponding to data bit 4).

Diagnostic test failure.

Given that the diagnostic software only indicated a single IC as being at fault, likelihood is that the IC was genuinely faulty – if more or all ICs were indicated as faulty, I would suspect an addressing problem before attempting to replace the RAM.

The ZX Spectrum has an unusual RAM configuration: 16KB of “lower RAM” for page and display memory is made up by eight 4116 DRAM ICs; 32KB of “upper RAM” for BASIC memory is made up by eight 4132 DRAM ICs. Sinclair bought 4132 RAM ICs because they were cheap – these were re-branded 4164 RAM ICs which had partially-failed factory testing. As such, they can be replaced by 4164 RAM ICs.

New RAM IC installed.

Sure enough, after socketing and replacing IC19 with a known-good 4164 DRAM IC, all the diagnostic tests passed, so it was time for some manual testing.

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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