#ZX Spectrum

Let's have a look inside a ZX Spectrum Plus this time. It's basically the same as a ZX Spectrum, but with a (somewhat) better keyboard and a reset button. However, this machine caused a few surprises.

The first surprise was that instead of a serial number, the words "Upgraded" were printed on the case. Next to it there was a (broken) warranty seal from Sinclair Germany. It was completely new to me that Sinclair had actually sold upgrade packages for the ZX Spectrum.

Inside the case, I found an Issue Two board, which is a rare sight in a ZX Spectrum Plus. For an obvious reason: The Issue Two heatsink is too big for the Plus case, so the board was somewhat crammed into the case.

As I was going to replace the 7805 with a DC/DC converter anyway, this ugly sight of the twisted heatsink would be solved soon though.

I also found that the keyboard membrane got brittle over the years, and needed replacement with a modern replica.

Technical Check

The very first thing I do is the composite mod. It just takes a bit of wire and a few minutes of soldering, so it's worth to invest the time even if the Speccy should turn out to be irrepairably broken. A first check showed the start screen, so everything seemed to be allright first.

But then I connected the diagnostics cart, and the trouble started.

The diagnostics reported that all eight lower RAM chips were broken, and the LEDs on the cart showed that the -5V and 12V power lines were missing. A voltmeter confirmed that -5V was gone, and there were only 7V on the 12V line, so the onboard power converter was broken. Strange enough: When I disconnected the diagnostics cart, the system started again, although both voltages were still bad.

I checked the coil, but it showed no short circuit between the windings. So I replaced the usual suspects of a broken power converter: TR4, D15 and D16. I also replaced the 7805 with a DC/DC converter as planned, and recapped all the electrolytic caps. The -5V and 12V were good after that, and all diagnostic tests passed.

But now the screen had a strong green tint, the colors were pale, and there were visible horizontal lines.

On Issue Two boards, there are two pots for calibrating the white balance. One just needs to connect a scope to the composite line, and then adjust both pots until the signal noise is reduced to a minimum. However, I only managed to get the green tint a bit better, but it didn't disappear. Also the disturbing horizontal lines stayed. This was the best I could get out of the signal.

But why was the white balance perfect before I fixed the power converter? I later found out that the 12V are necessary for generating the color signal. While the power converter was broken, the 12V were missing, and so the display was presented in a perfect black and white. Since the start screen does not use colors, it just looked good on the first sight. When I restored the 12V line, the color signal was generated again, and the screen got tinted.

I first suspected the ULA, but the problem stayed when I swapped it with a known-good one. Then I swapped the LM1889N. The strange horizontal lines disappeared after that, and overall the colors got better, but were still not perfect.

The pots still couldn't restore a clean white. But then I found that VR1 wasn't really working well, and the signal was crackling when I was turning it. So I replaced it with a Piher one, which almost didn't fit because it is encapsulated. After that, I could finally calibrate the signal to have a minimum noise.

The result was a perfectly white-balanced picture, with the only green things being the passed diagnostics tests.

For a test, I reinstalled the previous LM1889N, and the color issues and the horizontal lines came back. So the problem with the green screen was a combination of a broken LM1889N and a broken pot.

The hardware part is done. Let's have a look at the keyboard next.

The Keyboard

The keyboard of the ZX Spectrum Plus is a bit special. On the connector side, there is no difference to the ZX Spectrum keyboard. However, the ZX Spectrum Plus has some more special keys, like cursor keys or a dedicated delete key. These keys need to do two simultaneous keypresses in the correct order. This is done by two membrane layers that are connected to each other. A keypress then closes the contacts on both layers.

These layers are interconnected at the top of the membrane connectors. So it is crucial to do a clean work there, and to make sure that the layers are properly aligned and securely fastened under the brackets. Do not overtighten the screws though. Remember that the plastic is almost 40 years old. 😉

After a bit of cleaning, I could then reassemble the machine.

And that's it. Now I finally also have a ZX Spectrum Plus in my collection.

I got this board of a Sinclair ZX Spectrum. It must have been a ZX Spectrum Plus model before, because there was this reset wire attached to it. There were also a few labels that were explaining the functionality of the components in German language, maybe for educational purposes.

I tried to run the diagnostics, but the module didn't even start, and the D0 LED was permanently dark. There must have been a short circuit somewhere on the data bus. But instead of repairing it, my plan was to make a completely new ZX Spectrum from as many new components as possible, with reusing only the ULA, CPU, LM1889N, the coil, and the RAM chips.

So I first removed the valuable components. The stripped original board was a sad sight, but the prospect of making a new Speccy from it made it less painful.

I checked the ULA in another Speccy, and it turned out to be fine. From the 16 RAM chips however, only 9 were still functional. This was much less than I expected. I'm having some of those old RAM chips in my stock, but they are precious and hard to find.

A New Board

The new replica board is made by PABB and can be ordered from PCBWay.

For the required components, I assembled a bill of materials. It contains as many new components as I could find, but some rare parts are long out of production. They can still be found as NOS parts at online marketplaces, or they can be replaced with replacement types or replicas (like the Retroleum Nebula or vRetro vLA82).

There are four wire bridges that configure the type of the upper RAM chips, and the brand of the ROM chip manufacturer. The correct configuration can be found in my bill of materials as well.

Instead of the modulator, I decided to use an S-Video mod and a 3D printed base plate. A simple alternative is to just solder an RCA connector to COMP and GND, and use it as a composite output.

After a lot of soldering, the assembly was almost completed. But before seating the valuable chips, I first checked that all three voltages (+5V, +12V, -5V) were present and within their acceptable tolerance.

The S-Video mod takes the place of the original modulator, but is not soldered in, but held by two screws. The screws also provide ground, so they must not be isolating. Three wires then connect the board with +5V, and the composite signal as luma. The chroma signal is connected to the positive end of C65, which must be removed first so the luma and chroma signals won't mix.

After that, the new board was finally completed and ready for a first test.

Bugfixing

But alas, this is what I was seeing when I powered it up for the first time.

The diagnostics showed no action on the CPU bus controls. My suspicion was confirmed when I checked the clock input of the CPU with a scope. It was just a flat line:

The CPU clock is generated by the ULA, but the clock signal was present there.

A look into the schematics shows that between the ULA clock output and the CPU clock input there is the transistor TR3, probably for amplifying the signal. Strange enough, the signal was still present at the right of R24, which is directly connected to the clock output, but at the left of R24 (which is connected to the base of the transistor) the signal was missing already. When I removed TR3, the clock signal appeared there too, so TR3 must have been the cause.

After a longer search, I found out that the Spectrum is very picky about the type used for TR3. The original ZTX313 is not in production anymore, so I used a BC548 first, which is said to be a replacement type, however not at this position. For TR3, the only recommended replacement type is the MPS2369, which is also a bit hard to find now. With that type, the clock signal was finally good (cyan: ULA clock output, yellow: CPU clock input).

And to my joy, the new Spectrum finally started up and showed the famous start screen.

The next step was to run a full diagnostics check. Now I got an error that the M1 signal was missing.

The M1 signal is generated by the CPU, and indicates the first of four machine cycles, which is the cycle where the next instruction is read from memory. The Spectrum itself does not use the M1 signal, but a few expansions like the ZX Interface 1 need it.

After replacing the CPU, all diagnostics checks finally passed.

So at the bottom line, all I could reuse from the old ZX Spectrum was the ULA, the ROM, the LM1888N and the coil. I was hoping for the RAM chips and the CPU, but I haven't been really lucky with them.

Test Run

Anyway, it was finally time for a test run. I connected the new Speccy to my computer, and used tzxplay to play the tape file of my favorite game, Starquake. It was loading and running fine. Also, the image quality of the S-Video output is excellent, and probably the best one can get from this old design. Only the ZX Spectrum Next has a better quality with its native, pixel perfect HDMI output.

I bought the original board without any case. But luckily, there are replica cases, keymats, membranes, and faceplates on the market, so I could assemble a brand new outerior. Of course, I chose a transparent case, so the nice black mainboard could be seen from the outside. Well, at least a bit.

And there it is, an (almost) new ZX Spectrum in mint condition.

And yet another Speccy that I could buy for a good price. The seller said it was "untested", but I allege that he knew very well it was broken. It's fine for me as I mainly buy those things for the repair fun. 😁

The computer was in a sad condition when I got it. What's remarkable is that the machine was "assembled in Portugal". It's the first time I see this, and to be honest, it was one of the reasons why I wanted to have it. According to the very few information I found on the internet, those machines were intended for the Portugese and South American market, but some of them also made it to the UK and other European countries.

The faceplate was heavily bent, and a connector of the keyboard membrane was broken off. It seems that the previous owner tried to replace the membrane, but wasn't able to remove the faceplate.

That's the first hint that the machine wasn't "untested", but underwent a botched repair attempt.

I got the second hint when I tried to power up the machine, but found that it was completely dead, with all the voltages missing. The 5V is generated by an 7805 voltage regulator. It could just have died of old age. But considering the other hint, I rather guess that the previous owner has tried to power this machine with a standard 9V power supply. It has a reversed polarity, which kills the 7805 instantly, and usually damages the lower RAM chips and other components.

Let's have a look inside. There's an Issue 6A board inside, which is the final revision of the board. But besides that, there were no surprises. Anyway it's the first Issue 6A board I own, so I'm happy to have it.

The 7805 regulator is definitely broken, but I would have replaced it with a Traco Power DC/DC converter anyway. After I replaced it, the 5V line was back. To my surprise, the 12V and -5V lines were also back, so at least there was no further damage to the power supply.

I did my usual composite mod. Then I connected the computer to my monitor and powered it up to find out what else is broken. To my surprise the start screen appeared, and the Diag ROM also found that all RAM chips are working.

Okay, so much for the "repair fun" I was hoping to get. On the other hand, this board has a second custom chip, the ZX8401, also known as ZXMUX chip. If it would have been damaged, repair would have been a lot more difficult. Not impossible though, since the ZXMUX can be simulated by a few standard SMD chips.

Now that the Speccy was repaired, I continued with replacing the electrolytic capacitors. I also found and fixed a lot of cold joints at the lower RAM chips. The refurbishment of the board was completed after that.

Let's have a look at the case. The membrane connector was broken, but luckily there are new membranes available at retro shops. The previous owner tried to remove the faceplate, which is most often glued to the case. Most often, but not here. On this computer, the faceplate was just held in place by four brackets. All that would have needed to be done was to open these brackets and then easily pull of the faceplate.

Sadly, thanks to the botched repair attempt, the original faceplate was bent too much to be recoverable. It also had some visible scratches. I wished I could have salvaged it, but I decided to replace it with a new one instead. This time I took a metallic red faceplate, which looks as hot as a sunset in Portugal. 😉

And there it is, another ZX Spectrum for my collection.

The Superfo Harlequin is a ZX Spectrum 128K clone. It is special because even though it's a 128K Spectrum, it still fits into an 48K Spectrum case. It's also special because the ULA custom chip is replicated by discrete 74HC-type standard chips that can be replaced easily if one of them should get broken. It's just a small advantage though, because the RAM chips, sound chip, and Z80 CPU are rare by now.

I have ordered the Harlequin 128K Black Large DIY Kit at ByteDelight. It comes with all components that are required to build the main board, even those that are difficult to find elsewhere. There is also a Flash ROM chip enclosed in the kit, but it does not contain a Sinclair ROM image for license reasons. What's still required to build a complete Speccy is a ZX Spectrum case with keyboard, and a Flash ROM programmer for the Sinclair ROM.

Assembling

The Harlequin has only a single SMD component, and that one was even presoldered. All the other components are through-hole, so this DIY kit is even suitable for soldering novices.

I spent the rest of the day with getting the components out of their bags, locating their correct location and then soldering them in. The ByteDelight kit was carefully assembled. Every component comes in separate bags per value, and are enumerated in their optimal order for assembling. It's literally just soldering by numbers. 😄

The most boring part was to solder in all the 51 sockets. The DIY kit came with standard sockets, but I generally prefer turned pin sockets, so I used that ones instead.

The kit also contains the crystal that is needed for an NTSC setup, so you can choose between a PAL and NTSC machine. The board itself is pre-configured for PAL though. For an NTSC machine, a few traces at the bottom side of the PCB need to be cut.

Flashing the ROM

The DIY kit comes with an AMD AM29F040B Flash ROM. It is large enough to contain up to 8 ROM images. A DIP switch selects the image to be used. The pre-flashed image contains a Diag ROM, some other software, but no ZX Spectrum ROM for license reasons. The board itself also supports original Spectrum 48K and 128K ROMs, as well as 27C256 and 27C512 EPROMs.

ROM files can be found on the internet. I decided to keep the first six Flash ROM banks, and use bank 7 for a Spectrum 48K ROM, and bank 8 for a Spectrum 128K+2 ROM.

For flashing, I use the XGecu TL866II+ programmer and the minipro open source software. First I read the original content of the Flash ROM:

minipro --device 'am29f040b@DIP32' --read harlequin.bin

Then I made a copy of the first six banks. It's easy with the dd command. With a block size of 65536 bytes, the banks can be selected with the skip and count options. To keep the first six banks:

dd if=harlequin.bin of=harlequin-6banks.bin bs=65536 count=6

After that, I use cat to compile a new binary. Note that each bank must be 65536 bytes large, so if a ROM image is smaller, it must be duplicated (or quadruplicated):

cat harlequin-6banks.bin \
  48k.rom 48k.rom 48k.rom 48k.rom \
  128k+2.rom 128k+2.rom \
  > harlequin-new.bin

The new image can then be burned to the Flash ROM:

minipro --device 'am29f040b@DIP32' --write harlequin-new.bin

With the Flash ROM inserted into the Harlequin board, it was finally completed and ready for a first start. Unfortunately the maker of the Harlequin board saved a rectifier bridge, so it's still important to take care for the correct polarity of the power plug. Like the ZX Spectrum, the Harlequin needs a power supply with a 5.5/2.1 mm barrel plug with center negative. Most power supplys on the market are center positive.

Even though the Harlequin has a lot more chips than an original ZX Spectrum, it is very frugal. It only consumes 1.7W at 9V, while the original Speccy consumes 4.8W. On the other hand, the Harlequin does not need 12V and -5V to run, so these voltages are not generated. This might be a problem for a few very exotic expansions.

The Case

The DIY kit only comprises everything that is needed to assemble the main board. What's missing is a case with keyboard, and a power supply. The board has the same dimension as an original ZX Spectrum 48K board, so you can use original cases (e.g. the standard "rubber key" case or the ZX Spectrum Plus case), or buy a new replica case with new membranes, keymat and faceplate. The latter case is more expensive, but you get a brand new case in return, and you can pick from a large variety of colors.

I decided for a white keyboard, and a transparent case so one can still admire the beautiful Harlequin board even inside a closed case.

The Harlequin has a separate RGB mini DIN connector. It is made in a manner so it won't interfere with a classic case. However you might want to use the RGB connector as it offers a much better image quality. Shops like ZX Renew offer special Harlequin cases with a cutout for the RGB connector. If you want to use a classic case, you might need to cut out a bit of the beautiful old case to access the connector.

Since we are talking about making holes into old cases: The Harlequin has a built-in joystick interface. If you want to, you can cut out a space for a 9 pin Sub-D male connector, and wire it to the board. I refrained from making a cut into my beautiful Harlequin case, and use a classic Kempston joystick interface instead.

Let's Play

The simplest way to load software into the Harlequin is by the Mic/Ear port. There are smartphone apps and also a lot of tools that can generate the sounds to load TAP or TZX files, so there is no need to dig out the old tape recorder and audio cassettes.

I am using my tzxtools. The tzxplay command plays back TZX and TAP files to the standard audio output. I connect the sound card output to the mic/ear connector using a classic phone jack cable.

Since the Harlequin is a full-featured 128K clone, it also comes with an AY-3-8912 sound chip and even a stereo output. So the first thing I did was loading a game that makes use of that soundchip for in-game music.

When I started to refurbish old computers in 2021, I couldn't imagine that it was so much fun. 😁 The other day I bought another ZX Spectrum. According to the seller, it had some strange artefacts on the screen and also stability issues, so it was sold as defective. When I tried it at home, it was even worse. I just got a black screen on a white border.

Inside the case I found an Issue 2 board. The previous owner has added a composite output on a separate connector. As the age of TVs with tuners is definitely over, there is no need to keep the modulator output. I will do my own composite mod instead, and remove this ugly cable that was hanging out.

The manufacturing dates of the components tell an interesting story. This computer has probably been manufactured around the end of 1982. However, all chips that are related to the upper 32KB RAM are socketed, and some were made in 1983. I guess it was originally built as 16K model, and has been extended to the full 48K a year later. As the only chip on this computer, the ULA was made in 1984, so maybe it had been replaced around then.

My main suspicion was that the ULA was broken, so I put it into one of my working Spectrums, and was happy to find it in working order. The problem must be somewhere else.

The usual first step is to check the voltages. And bingo, the 12V line had around 7V, and the -5V line was flat. This sounded very familiar, and a look at the coil confirmed my suspicion. The coil had a purple color, and a short between the primary and secondary winding. I guess the coil was already pre-damaged when the Spectrum was sold, causing the artifacts because of poor voltages on the lower RAM chips. When I powered up the computer at home, I eventually killed it.

Well, it's not the first time I had to deal with a broken coil. I unsoldered it, rewound it, and replaced the semiconductors that usually get grilled as a result. Then I powered the system again, and found that all voltages were back to normal. Success!

I put the ULA back into its socket, so I could check what else is broken. And (to my displeasure, to be honest) the computer just came up and was working again.

What a spoilsport! I was hoping to have some more repair fun with that machine. 😉

Okay, what next? I started with replacing the electrolytic capacitors with fresh ones. Then I found something strange: A wire link was missing that was supposed to be there.

That link is important. The upper 32K RAM chips are actually 64K RAM chips, where one half of the memory turned out to be faulty after production, so they were sold with half the size for cheaper. The link configures which half of the memory is to be used. There is no pull-up resistor, so keeping it open is not a valid option. It might cause the upper RAM to randomly flip between the working and faulty memory half. I doubt that this computer has ever been working stable after it was modified to 48K. This link has just been forgotten by whoever did the modification.

The RAM chips are TMS4532-20NL4. The trailing 4 indicates that the upper part of the memory is to be used, so I added a link between the center hole and the "+5V" hole. A trailing 3 would require a link between the center hole and "0V".

I soldered in the link and replaced all electrolytic caps with Vishay ones. I also replaced the 7805 voltage regulator with a Traco Power TSR 1-2450. This modern DC/DC converter is a drop-in replacement that needs no heatsink, and is small enough to still fit into a classic ZX Spectrum case.

Issue 2 Spectrum boards have two variable resistors, VR1 and VR2, for color calibration. With the aid of an oscilloscope, calibration is a matter of a minute. I connected the scope to the composite video output (or to the video input of the modulator), and then adjusted both resistors until the signal was looking as smooth as possible. There is a blog article at Spectrum for Everyone that gives more details about the calibration.

Finally, I ran the ZX Spectrum Diagnostics tool. All tests passed, even those of the upper RAM.

Another repair job well done. 😄

So there is my 3rd ZX Spectrum. Above all, I like the exceptionally good condition of the case. It seems that the computer has barely been used in its 40 years. The keys and faceplate actually look pristine, and there are also only very few and small scratchmarks.