Giving the ZX Spectrum a Service

Categories: Retro computers

I’ve had my Issue 4B 48k ZX Spectrum for quite some years now. When I first got hold of it I replaced the keyboard membrane and did the mandatory composite video modification (taking the modulator input directly through to the output via a decoupling capacitor), and it’s been working away very happily ever since.

However, given this fellow is now over 35 years old I felt it was time to replace the electrolytic caps, replace the 7805 linear regulator with a modern switching replacement and try to improve the ‘jailbars’ (vertical lines on the screen which according to internet sources are down to ground coupling on the -12V line from the DRAM chips). Here we go…

Okay so this is the original machine before recapping. I’ve identified suitable branded (Vishay) electrolytics on and for reference added a parts list here and below:

Sinclair DesignatorDescriptionDigikey PartMFR’s refQty
C3, C28, C25, C50, C65, C80CAP ALUM 22UF 25V AXIAL56-MAL203036229E3CT-NDMAL203036229E36
C46, C27, C79CAP ALUM 1UF 63V AXIAL56-MAL203038108E3CT-NDMAL203038108E33
C74CAP ALUM 47UF 25V AXIALBC5237CT-NDMAL203036479E31
C44, C45CAP ALUM 100UF 25V AXIAL56-MAL203036101E3CT-NDMAL203036101E32
Sinclair ZX Spectrum Iss 4B Recap BOM

A couple of notes on this:

  • The 100uF, 25V caps C44,45 are a little too fat and short compared to the originals but do fit okay (see the images below)
  • This list excludes the polyester decoupling capacitors used I added later to the RAM chips and Recom 5V 1A regulator
  • Tip: when recapping a machine with axial parts I always try to align them so that the values are visible (facing upwards) for easier future replacement.

With the caps replaced and system re-tested the next task was to try and improve the composite image quality. My spectrum was suffering from the commonplace Jailbars (vertical shadows following the character lines). The recap in general had improved this but not totally fixed it. Apparently this is a common-place issue caused by the ground-plane design of the spectrum and switching noise from the RAM chips getting through to the video circuitry. Following advice on spectrum for everyone I set to work with some additional decoupling around the lower RAM chips.

After a little experimentation it looked like adding some fresh decoupling to the lower capactors had a noteable improvement but no further improvement with the upper caps. In the end therefore I settled to add some additional 220nF polyester caps in parallel with C5, C6, C7, C8. I decided not to remove/replace the originals but simply solder these in, which sit nicely along the edge of the board and fit in the case just fine.

These were simply parts I had in my capacitor stock, but an equivalent is this TDK part from DigiKey link.

Sinclair DesignatorDescriptionDigikey PartMFR’s refQty
C5, C6, C7, C8CAP FILM 0.22UF 10% 63VDC RADIAL495-75657-1-NDB32529C0224K2894
Sinclair ZX Spectrum Iss 4B Jailbar Fix BOM

As you can see here and in further images below, the picture quality isn’t perfect (as you can see from the characters contrast and sharpness are turned up high here) but it’s certainly comparable if not better than speccy’s looked when tuned into an old CRT TV on VHF channel 36, back in the 80’s. If anyone has any additional tips for improving jailbars (perhaps some L-C filtering around supply to the video generation circuitry) please leave feedback in the comments below.

Finally for this activity, replacement of the 7805. This has been on my to-do list for quite a while. Speccys are renown for generating a lot of heat through using a linear regulator attached to a rather ineffective bent piece of aluminium sheet. An external, finned regulator would have been better (toast-rack) given the limited airflow within the small case. However I’m sure Sinclair was keen to maintain the aesthetic of his small, elegant machine.

I tend to load my Spectrum up with various peripherals and depending on the peripheral design in many cases further load is placed on the 5V reg. Furthermore, as we know the ULA generates its own fair-share of heat, and at 35+ years old I want to keep the ambient temperature around this ageing fellow as low as possible.

For some years now I’ve been using these Recom/CUI switching replacements for linear regulators. I first started using them on a motorcycle data logger some years back where I needed a robust 3V3 supply but a linear LDO reg was generating far too much heat in my small enclosure. At $7+tax per part these are far from cheap but seem to be a robust solution link.

Sinclair DesignatorDescriptionDigikey PartMFR’s refQty
REGDC DC CONVERTER 5V 5W102-2178-NDV7805-1000R1
Sinclair ZX Spectrum Iss 4B Switching Regulator BOM

Removal of the 7805 by desoldering was straightforward and without any drama. I did most of the desoldering with the heatsink attached to help prevent overheating of the part (not that I will likely ever use it again but I tend to keep removed original parts for posterity). You can see below the board with the 7805 and heatsink removed and with the new switching regulator soldered in place. Tip: if you order the part listed above the legs are even pre-bent. This would unlikely fit properly into the spectrum case if fitted at a right-angle to the board.

I was concerned (having read some stories online of fitment of similar parts to ZX81/C64s) of damage or failure to boot being caused by switching noise. In the ZX81 case the user added additional 100nF decoupling caps in spare holes right adjacent to the regulator/power jack (link). In the C64 case the modder reverted to a linear regulator to prevent instability issues at swich-off as the input voltage collapsed (link). Fortunately, in this case no additional caps were necessary, and with no apparent issues at power down the Speccy works just fine.

Here’s the reassembled machine, next to the removed 7805 and chunk of aluminium. As you can see it’s working just fine and passing with my Russian diagnostics cartridge (see another post on that, coming soon) for over 1 hour in my workshop on what’s probably a 28 degrees C ambient temperature. All works fine and hopefully will continue to do so for a long time to come…