Sinclair Spectrum 48k – Double Restoration

Background

I have a small collection of retro computers – an Atari ST, BBC Micro, Mac Classic and others. I’ve got a Spectrum +2 computer, like the one I owned in the 1980’s. However, I’ve never owned a Spectrum 48k computer. I’ve recently seen a lot of interest in the original Spectrum machine on Twitter, and was intrigued by their small size and good looks. How did they cram a working machine into such as small package? What was the infamous “dead flesh” keyboard like to type on?

So I jumped onto eBay and bought not one, but two 48k machines: the original rubber keyed Spectrum and the later Spectrum Plus. I enjoy fixing machines as much as I enjoy tinkering with them, so took a gamble with two machines sold as “untested”.

Buying a Spectrum on eBay

Looking at the machines available on eBay it looks like the machines fall into one of three categories:

  • untested” – the seller tells you “all the bits are here but I haven’t got x, y or z to test it”. Usually take this with a pinch of salt – they may well know the machine is broken. Or machines may come without accessories such as a power supply. It is reasonable that the machine may have parted ways with the PSU and other bits over the years, of course. I’ve seen such machines go for around £30-£50 depending on what accessories they come with.
  • working with a pile of games” – these machines are usually shown in a working state and come with a number of games (on tape) and perhaps a tape recorder. This is a fair option, but of course the machine may be on borrowed time if it hasn’t had any preventative maintenance carried out. Expect to pay £60 or more.
  • restored and guaranteed” – these Spectrums have had all the usually suspect parts replaced by an enthusiast who knows what they’re doing. Look out for a guarantee. Prices here start around £100 but may require an additional PSU. This is a sound option if you don’t want to get involved with repairing a machine.

There are a number of people offering repair services, so if you do end up with a faulty machine all is not lost. Prices are quite reasonable and parts are still available.

My Spectrum 48k cost £30, and the Spectrum Plus cost £33, plus shipping. Both came without any accessories.

Inspection and the ULA

The Spectrum 48k was in reasonable cosmetic condition – no marks on the case, although the keyboard faceplate had lots of small pits. It was easy to inspect inside, as it was missing all the case screws.

Inside, the keyboard ribbon cables looked worn and damaged, showing signs of age. The PCB is a thing of beauty – the issue 3 board does not have solder mask (green colouring on the PCB) on the top side, and all the components are set off against the cream PCB. There are very few components on a small board, compared to other machines of the era such as a BBC Micro. Here we find:

  • Z80 CPU
  • ROM
  • 8 small RAM chips – “lower RAM” – the lower 16k of RAM
  • 8 small RAM chips – “upper RAM” – 32k of RAM
  • ULA
  • PAL encoder IC (video encoder)

There are only a couple of standard 74 series logic IC’s, a few transistors and some passive components.

The reason for this compact size is down to the ULA – the Uncommitted Logic Array. Rather than having to build your glue logic out of 74 series IC’s, the logic was designed out of building blocks present in the ULA. Transistors and resistors on the chip could be wired together using a simple metal layer on the silicon chip to form the required design. There were also some linear components that could be connected to produce amplifiers, oscillators and handle I/O. Sinclair designed this interconnecting layer, which was applied by Ferranti who developed the process and made the chips. This was much cheaper than having a completely custom IC designed, from bare silicon, as the ULA could be used by other customers who designed a different interconnecting layer.

In this way, a great deal of digital and analogue circuitry was condensed down into a single IC. The Spectrum ULA handles the following functions:

  • reads the keyboard
  • controls the RAM
  • controls the CPU (CPU is paused whilst video is read out of RAM)
  • tape input / output for loading and saving programs
  • sound
  • video generation

If you’re keen to learn more about the Spectrum ULA, you should buy “The ZX Spectrum ULA – How to design a microcomputer” by Chris Smith. This is a fascinating book and describes in detail how the ULA works. To understand the ULA is to understand the Spectrum. As all the design details of the Spectrum ULA had been lost, Chris arranged for a chip to be de-encapsulated. It was photographed and he worked out the design and circuit operation. This was quite a feat.

Others have taken this work and been able to produce ULA replacements using modern components. One such device is the retroleum.co.uk Nebula. This allows you to replace a failed ULA without plundering another Spectrum for spares.

My original Spectrum has an issue 3 board manufactured in 1983, by inspection of the date codes on the IC’s. This was the most numerous board, with over 3 million having been produced.

Here is a link to a site where you can view all the Spectrum board variations. And some pictures and a description of board variations from the High Priestess of Spectrum.

The Spectrum Plus has a much larger case, has a more traditional plastic keyboard with moving keys, but contains a very similar PCB. In fact, it is just a newer iteration of the original Spectrum board, with the addition of a reset switch on the side of the machine.

My machine has an issue 4S board, made by Samsung in Korea. Production having switched from Dundee in Scotland where my other machine was manufactured. The Issue 4S has the following improvements:

  • improved ULA with a cleaner and brighter video output
  • improved RAM handling

Both my machines have a 6C series ULA. This replaces the 5C ULA found in earlier machines. The 6C greatly improved power consumption so they ran cooler. This helped to reduce device failures.

Cosmetically my Plus has a few small dings in the front of the case, and some very fine specs of white paint.

Pre-power checks

Before powering up a Spectrum, it is recommended that you perform some basic health checks using a multimeter. This can prevent further damage if problems are found. These are detailed in the following video by YouTuber Joules Per Coulomb.

I initially thought I had an issue with the voltage regulator on my Issue 3 board, but this turned out to be an erroneous measurement. I had left my new multimeter (which I was unfamiliar with) in the “auto range” mode. Setting it manually to the mega-ohm range, all appeared as expected. The reason for this is that the multimeter puts out a voltage on its probes when making resistance measurements. In these tests, we are performing some in-circuit tests on ” apparent” resistance. This resistance varies depending on the voltage present on the probes. The lower resistance ranges use a higher voltage. This is more likely to make transistors or diodes conduct, and therefore affect the readings.

These tests really are a bit of a “finger in the air” to see if the machine is faulty. Lower than expected resistance values suggest the power circuit or RAM chip(s) are faulty. If you power up a machine with a faulty power supply, you can take out the RAM. Also faulty RAM can cause the power supply to fail. You can end up in a vicious circle of destruction!

Both machines passed these checks. I have an ESR (equivalent series resistance) meter which is useful for checking the condition of capacitors, without removing them from the board. Spectrum capacitors (and indeed caps on many older devices) are known to degrade or fail, particularly as the original Spectrum is not well ventilated. The Spectrum Plus benefits from a larger case, with ventilation holes front and back to help promote airflow. Certainly the capacitors in my Spectrum Plus were in better condition (lower ESR). I replaced the caps in the original Spectrum to start with, and was interested to see if I could see any issues with the Spectrum Plus, who’s caps appeared healthy.

I used blue Vishay capacitors from CPC, as I wanted to keep the look of the original machine. Most modern caps are black and this spoils to look. However, I could only find blue caps in the 85 degree C rating. I would normally always use 105 degree C caps as these last longer.

The capacitors I used are as follows, some have a higher voltage rating than the original, but that doesn’t matter (and has the added benefit of lower ESR):

  • 1uF, 63V – CPC CA07195
  • 22uF, 25V – CPC CA07172
  • 4.7uF, 63V – CPC CA07193
  • 100uF, 25V – CPC CA07173

You have to order in multiples of 5. You can get packs with all the correct values for your Spectrum which may work out cheaper. For example, mutant-caterpillar.co.uk sell packs for each type of Spectrum PCB.

I had to cut off the capacitors as I struggled to desolder them and didn’t want to damage the PCB. One day soon I will invest in a desoldering station to make these jobs easier.

Replacement Power Supply

As my machine came without accessories, I had to obtain a new PSU. You need to be careful as the centre pin on the DC jack on the Spectrum negative, which is unusual.

I chose the following from tfw8bit.com. I’m a bit fussy when it comes to PSU’s as I’ve had cheap ones go bang or fall apart before now. I’ve also seen videos of PSU’s made from cheap flammable plastic that burst into flames under fault condition.

The unit I bought has been type tested by independent labs. They get samples and check the units are safe and meet relevant standards. And yes, I checked this model was listed on the test labs’ websites:

TUV test: https://www.certipedia.com/certificates/50325778?locale=en&page_number=3

UL test: https://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/showpage.html?name=QQGQ7.E183744&ccnshorttitle=Power+Supplies,+Information+Technology+Equipment+Including+Electrical+Business+Equipment+Certified+for+Canada&objid=1074197388&cfgid=1073741824&version=versionless&parent_id=1073992441&sequence=1

The cheap ones on eBay have a “CE” mark which means cock all, as it is a self-certification scheme. The “RoHS” mark just means that it doesn’t contain dodgy substances. They are probably fine…. But I don’t want to risk it…

With the power supply connected to each machine, I checked the relevant voltages were present around the board (+5V, +12V, -5V).

Diagnostic Checks

Both machines appeared to power up OK and display the Sinclair copyright message. I want to ensure my machines function correctly so I bought a device with a built-in diagnostic ROM, called a SMART card, from retroleum.co.uk. This can test correct operation of components in all Spectrums. It can also be used to load games using an SD card on the 48k Spectrums, and has a built in joystick interface for playing games. It is good value at around £20, but is a bare board without case (although perfectly expected for this price).

I used the SMART card to test the RAM, ROM, speaker and keyboard. It also has some video test patterns and other features that I haven’t yet explored. You can leave it cycling around the RAM tests. This means you can leave it overnight running tests and any failures will be captured. This is useful to pick up intermittent or heat related issues.

You need to be careful not to connect or disconnect devices from the Spectrum expansion connector while the machine is powered, otherwise you can damage the Spectrum.

Also, the edge connector is notorious for getting dirty. I cleaned mine with a fibreglass pencil.

The Issue 3 board passed all tests. The Spectrum Plus failed with a scrolling message saying “the RAM seems to be bad“. I suspected the edge connector as it didn’t happen every time I connected the SMART card. The connector was very corroded – this machine was probably stored somewhere a little damp, as the tin can for the RF modulator has also suffered. I used some fine wet and dry paper as it was so dull and mottled. This revealed a small amount of bare copper, which is not ideal, but better clear copper than oxidised tin. With a final polish with the fibreglass pencil, it worked OK and the RAM checks passed. I was unable to get into the other menus though.

Stuck Key and faulty speaker – Spectrum Plus

Booting the machine without the SMART card I could see BASIC keywords appearing on the screen, indicating a stuck key. The World of Spectrum website has a repair guide which detailed this as a known issue, which was resolved by slackening off a screw near the affected key.

I also found the keyboard membrane in a poor state – dirty, sticky connectors and some signs of heat damage along the ribbons. I ordered a replacement from dataserve-retro.co.uk

As a temporary fix I cut down the ribbons and was able to continue with the diagnostic checks. I also found the loudspeaker didn’t work. Tracing the signal using the scope I found the signal was OK and the loudspeaker faulty. I replaced this with a spare from mutant-caterpillar.co.uk

I replaced the keyboard connectors along with the membrane as they were impossible to clean and I suspected corrosion as seen elsewhere on this machine. Do follow the instructions – I didn’t and the bottom row of keys didn’t work. Once I read the instructions these keys work OK – although I do have to press them harder. I note others report this as a feature with new membranes, the explanation is that they are constructed differently and more the bottom row actually presses two layers of keys as the are effectively “shifted” symbols.

Improving the picture quality

These machines only have an RF video output. The composite video signal that is present on the PCB is modulated onto a high frequency carrier in the Aztec RF modulator (the silver can) and sent out the back of the Spectrum. This RF signal is noisy and suffers from ghosting. There is a simple modification to bypass the RF modulator circuitry, as pipe the composite signal onto the existing connector. A capacitor is used to couple the signal.

With this mod in place, a drastic improvement in image quality was seen. No more ghosting, noise and wavy lines. The Spectrum Plus still had some slight diagonal patterning on the image. However, I deliberately hadn’t recapped this unit. So I changed the two caps on the left of the board in the video circuit. This eliminated the patterning. I then went on to replace all caps. I don’t think that was necessary, but there were only ten an it only took 20 minutes.

The interesting thing is that the two capacitors in the video circuit actually have higher ESR on the replacement capacitors than the two that were removed. I would be interested to know why this was (leakage?).

Moral of the story: replace all caps on spec in a Spectrum.

The later 128k Spectrum models have an RGB video output which gives even better image quality. However, on my 14inch Sony CRT TV, the composite signal looks fantastic. Bright and punchy pictures, free of noise.

Reseating the ULA

I always like to reseat socketed components. They may have lifted due to heat, or the legs may be tarnished. I carefully removed the ULAs and cleaned the legs on both sides with my fibreglass pencil. The devices were carefully refitted, taking note of the correct orientation.

Thermal Greese and regulator improvements

I refreshed the thermal grease that sits between the voltage regulator and the metal heatsink. This helps promote good heat transfer. I found the issue 3 board did not have any thermal grease.

One modification liked by Spectrum repairers and enthusiasts is to replace the linear regulator and heatsink with a DC to DC convertor. This is much more efficient and produces far less heat. I don’t plan to use my machines extensively, and always like to try and keep them as original as possible. For this reason I also have not fitted a stick on heatsink to the ULA chip. Indeed, in order to fit this to the original Spectrum, you need to remove the ULA socket as their isn’t much space in the case. You’d really need a desoldering station to tackle this job, and I don’t have one.

Case clean up

I stripped down the cases and removed some small spots of paint from the Plus using some 3-in-1 oil and a screwdriver. I should really have used a plastic tool here, and one key has a small scratch on it. You need to be careful which solvents you use on the Spectrum in case you damage the plastic.

To remove the faceplate on my original spectrum I just pulled it off, as it was already coming loose. You can use a hair drier to loosen the glue if required.

I washed the cases with a scrubbing brush, warm water and a splash of Flash liquid detergent. I just wiped the Spectrum Plus rear case, as it has paper labels.

I attempted to re-paint the white ZX Spectrum logo on the original machine using white Humbrol paint and a very fine paint brush. However, this looked terrible and I removed it with white spirit. Mutant-caterpillar sell a fine white paint pen, and ZXWife has a video on youtube showing you what to do. However, I decided I would make a hash of that as well, so decided to leave it.

The Spectrums were re-assembled and new rubber feet fitted to the original machine. The Plus has two foam pads on its retractable feet, and these were renewed. New case screws for the original Spectrum completed the job.

Tape test

I wanted to check both machines would load programs from tape. This tests another important feature of the ULA. I have an Alba tape recorder bought from Asda last year for only £10 new. Thanks to Chinnyvision for the heads up about this bargain on Twitter. It works well with the Spectrum, but you need to use a mono cable at the Spectrum end otherwise it won’t work.

If you want experience the sounds of tape loading but you don’t have a suitable machine or tapes, you can use a laptop headphone output, or that from a phone or tablet. Be warned though that some phones and tablets only output a very low level signal and programs will fail to load.

I measured the following levels from my devices using a scope:

  • Alba, cheap 2018 vintage, tape recorder – 6 volts peak to peak
  • iPhone – 0.4V peak to peak
  • iPad – 0.6V peak to peak

Neither the iPad or iPhone succeeded in loading files from MP3 files found on the internet.

However, both machines successfully loaded tape games using the tape recorder with no issues.

Keyboard testing

I wanted to put the keyboards to the test. The original machine was reported to have a “dead flesh” feel. The Spectrum Plus was marketed as having a more “professional” keyboard, with proper moving plastic keys. Although, as we have seen, it relies on the same keyboard membrane technology rather than proper key switches (as found on a “proper” keyboard of the era).

I located a short BASIC program from the original Spectrum manual. I typed this in on both machines. This program made use of the BASIC keywords that are printed on each key. This means you just press the relevant button rather than having to type in the whole word. Symbols and functions (sin, cos, tan, etc.) are reached by using various levels of ‘shift’ by holding down other modifier keys. This took some getting used to and was actually harder on the original Spectrum where they had used green and red colour on the symbols and keywords. This just added to the confusion. However, by the end of the exercise I got used to the system and could see it would speed up BASIC programming.

The rubber keyboard really wasn’t that bad. The odd positioning of the SPACE key on the right wasn’t too much of a problem. However, you certainly couldn’t touch type on either machine. I think the “dead flesh” label is a bit unfair, but it is certainly cheap and cheerful. As expected I found the Spectrum Plus keys to be an improvement, and the space bar is more familiar.

Conclusion

I have enjoyed repairing these two machines and comparing their differences. The Plus is simply a later iteration of the original Spectrum, although it looks much bigger. This extra size helps to accommodate the improved keyboard and also prevent heat build up in the unit. The image is also brighter, as provided by the improved ULA.

I was lucky that I didn’t experience any major failures – RAM faults appear to be common on these machines. I was especially lucky with my original Spectrum as someone had clearly been under the lid having a poke around. I’d also certainly recommend a retroleum SMART card for running diagnostic checks, as I can say with some confidence that I have working machines. I am also grateful to the enthusiasts supplying parts and home-brew boards to keep these machines running and in fine fettle.

I have added up the costs and I have spent, in total:

  • Spectrum rubber keyed model: £68
  • Spectrum Plus £67

This excludes the PSU which was £15, the SMART card (£20), and Humbrol paint!

The prefer the look of the original Spectrum 48k, and it is certainly one of the best looking machines in my collection, particularly with its new faceplate. It is currently sat on my desk and I’m working through a few old Spectrum games, starting with Chuckie Egg, one of my favourites. I use the divMMC Future to load games from SD card. This device works with all Spectrums and comes in a nice plastic case which looks good hanging out the back of the machine.

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