Sinclair ZX Spectrum 128k+3 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+3 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 eBay, and I managed to win it for a reasonable price.

The ZX Spectrum 128k+3 was one of the later machines produced by Sinclair, and was the first Sinclair computer that was completely designed by Amstrad – it was very similar to the ZX Spectrum 128k+2A, but featured a 3″ FDD instead of a tape deck.

The computer arrived safely, and appeared to be in original condition except for the addition of a phono jack sticking out the rear case, which I assume was used as a composite video output.

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

The first step was to remove this ugly modification, which required dismantling the computer – the ZX Spectrum 128k+3 is relatively simple to pull apart, with five screws on the underside of the case, and two screws alongside the FDD.

The phono jack cabling was attached to the upper side of the PCB, so I didn’t even have to pull the board at this point – desoldering the two cores was a simple task.

Now that the machine had been returned to stock, I planned out the servicing, preventative maintenance, and upgrades that I wanted to perform, as follows:
#1: Service the internal 3″ FDD.
#2: Fix the audio distortion issues associated with the factory ZX Spectrum 128k+3.
#3: Fix the RGB SCART blanking issues associated with the factory ZX Spectrum 128k+3.
#4: Replace all the electrolytic capacitors on the mainboard.

#1: Service the internal 3″ FDD

The ZX Spectrum 128k+3 has a 3″ FDD, which is a proprietary Amstrad drive which uses uncommon 3″ floppy disks. These drives are belt-driven, and use a rubber belt which is particularly prone to ageing – it’s very likely that the belt would have failed, so I didn’t even bother to test the drive before I pulled it for servicing.

The drive is relatively simple to remove, and is only held in place by a screw, a grounding cable, a power cable, and a parallel data cable.

With the drive removed, it’s time to service it – this involves replacing the original drive belt, cleaning the read/write head, and lubricating the stepper rails and spindle.

To do this, I used a commercially available service kit which included everything required.

I won’t cover the process I followed for servicing, as this is already documented online.

#2: Fix the audio distortion issues associated with the factory ZX Spectrum 128k+3

Sinclair machines are well known for their quality issues, both in design and manufacture, and the 128k+3 is no exception – when designing the Z70830 PCB, design errors in the audio circuitry (specifically the amplification stage) cause heavily distorted audio.

These design errors were fixed in the 128k+2B (Z70833 PCB), and the redesigned audio circuit can be applied to the 128k+3, as well as early 128k+2A machines which also feature the Z70830 PCB.

I won’t cover this process in a lot of detail, as this is already documented online, but it involves the following modifications:
– Replace R42 for a 1.2kOhm 0.25W
– Replace R43 for a 5.6kOhm 0.25W
– Replace R62 for a 1.5kOhm 0.25W
– Replace R63 for a 2.2kOhm 0.25W
– Replace R64 for a 6.8kOhm 0.25W
– Replace R67 for a 15kOhm 0.25W
– Replace R72 for a 820Ohm 0.25W
– Install R204 (10kOhm 0.25W between the base of TR5 and ground)
– Install R206 (390Ohm 0.25W between the emitter of TR5 and ground)
– Remove ceramic capacitor C37

To do this, I used a commercially available resistor kit from Mutant Caterpillar Games.

The modifications required the removal of the PCB, which is held in with a few screws.

The audio modifications are quite fiddley and take a fair amount of time to implement, but they are well worth doing – here is a video of my machine following the modifications, and here is an excellent comparison of the audio quality before and after the modifications.

#3: Fix the RGB SCART blanking issues associated with the factory ZX Spectrum 128k+3

Sinclair machines are well known for their quality issues, both in design and manufacture, and the 128k+3 is no exception – when designing the Z70830 PCB, design errors in the video circuitry (specifically the RGB SCART blanking signal) prevent RGB mode from being selected correctly, leading to noticeable ghosting on the RGB video output.

I won’t cover this process in a lot of detail, as this is already documented online, but it involves piggybacking a 1kOhm 0.25W resistor across R44 to bump up the SCART blanking signal to around 1.5V (and into the 1V to 3V range required).

The modification was quick and simple to implement, and could even be done without removing the board.

Testing showed that the modification was successful – the ghosting has been eliminated, and the RGB video output looked excellent.

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

I replaced the electrolytic capacitors on the mainboard whilst it was removed – this didn’t take long at all, as there are only a handful of capacitors on the board.

I used a commercially available capacitor pack from Retroleum.

Conclusion

After all this work was performed, I did some finishing up: I thoroughly cleaned the mainboard, 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; I thoroughly cleaned the PSU, and fitted a new UK plug (3A fused).

Then, it was time for testing: all keys registered correctly; RGB and composite video outputs worked OK; internal FDD loads disks reliably and quietly; 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!

I also built my own switch-mode PSU for the ZX Spectrum +2A/+2B/+3 line of computers, which is surprisingly easy and cheap to do – the details are available here.