Category: ZX Spectrum

Building a ZX Spectrum Clone in 2021 – Populating the board.

Acknowledgements – ZX sizif 512K Project – all credits to Eugene Lozovoy.

After receiving most of the parts, I started populating the board; I applied solder paste for the SMD parts using the stencil that came together with the PCBs, baked it in the reflow oven, and handed soldered SMD parts on the bottom side with hot air.

The power circuitry is populated on the board next to test the (ULA) CPLD, the most crucial part of this board. Sadly the primary regulator was missing from my package, and I have to reorder it from China. To continue testing, I inserted 5v from my bench power supply to realise I had the polarity reversed; after accessing the damages, the CPLD and buffers were all dead.

I don’t have another CPLD on hand, so I have to wait for the next shipment.

The CPLD and Regulator came in the mail about two weeks later, and I was able to populate and test them, the bare circuit works, and I was able to detect and program the CPLD on the JTAG chain.

The rest of the parts go onto the board except the PS2 keyboard interface, which I have no intention to use. All main ICs are mounted on sockets as I am unsure if the 2nd hand chips work.

I programmed the Flash ROM next without any incident.

I worked through the night to get everything up on a Friday evening as I was excited.

The video encoder needs some tuning; after I plugged in everything, I managed only to get a stable black border and white screen, looks like the CPU, RAM, or ROM is not working.

magenta screen

Somehow one of the transistors broke and rendered a magenta screen. I replaced it, and the screen colour went back to normal.

The next thing regarding the CPU is to check the clock lines; no clock means no execution. Indeed, the CPU is not receiving a clock signal; tracing upstream to the 28 Mhz oscillator, it was not running.

The 28Mhz Crystal I bought from China doesn’t have any specifications, so I must guess some of the values to make it work.

The circuit that feeds the clock line is a typical pierce oscillator circuit, so my guess is there is too little gain to start a feedback loop; replacing the 1M resistor with 820K started the oscillator running.

garbled screen

Starting the clock didn’t fix everything; now, I get a garbled screen on boot. However, I now know the CPU and ROM is working, as the diagnostic code in the ROM chip runs without any issues. So the fault should be the RAM.

I suspected that it could be a solder bridge on the CPLD connecting to the RAM; I reinspected the board repeatedly to no avail.

At 4 am, I am exhausted; I took a break and took another look at the problem I am facing; the following strategy is to check all signals from the CPLD. I removed the CPU, RAM and ROM, and wrote a new VHDL code to send a 1Khz signal to all pins.

Slowly tracing the pins one by one, I found “WR” on the system bus shorted to the ground; it all makes sense now why the ROM would run, but the RAM is garbled; the ROM doesn’t use the “WR” bus. The culprit was traced to the area next to the write enable header; I placed the Regulator ground tab out of the line, shorting the WR bus through a break in the solder mask to the ground. I cut the PCB trace to disable the header pin as I do not want to remove and resolder the regulator.

The familiar copyright screen was a welcome sight after removing the short. I was really excited but tired too, time to catch some zzzzzs.

link to part 1

link to part 2

Building a ZX Spectrum Clone in 2021 – Sourcing for parts

Acknowledgements – ZX sizif 512K Project – all credits to Eugene Lozovoy.

sizif 512K PCB from JLCPCB

The first order is to get the PCB made; the Gerber files are readily available on Github; make sure you choose the latest version (rev.D1). I used JLCPCB to manufacture the PCB as I am familiar with them. The boards came in under a week.

Sourcing for components isn’t that easy; with the rising cost of chips, an EPM1270 cost SGD$70, the video encoder about SGD$25 on major online electronics distributors. To keep the cost low, almost everything I ordered was from China auction sites (Taobao); the problem with these sites is that there is a risk the chips are a dud, remarked or a total fake.

In total, based on the BOM, I ordered all the parts from 20 plus vendors; 3 or more pieces for ICs from different vendors if available.

A box full of components
No photo description available.
duds from China

  • CPU – Z84C0020PEC – these are all used at about $3 each, modern version for the Z80A, likely remarked from old Z80As. Out of 3 pieces 1 is a dud, the other 2 works fine.
  • RAM – AS6C4008-55PCN – used static RAM for about $4 each, ordered 5, 2 not working.
  • ROM – SST39F040 – used Flash ROM about $2 a piece, ordered 3 pieces all working.
  • Sound Chip – AY-3-8910 – These are hard to source in China, ordered 3 pieces all dead. Still cannot find a good source for it.
  • Video Encoder – AD724 – These are still in prodcution, got it as new chips, no issues with them.
  • ULA – EMP1270 – These are new parts, most sellers won’t ship without you commiting to a minimum order, need some back and forth wtih sellers to get it.

The rest of the parts are ok, except the 28Mhz crystal; it is sold without specifications that causes issues during the build.

Connectors and switches are a hit and miss; you buy them by looking at the picture of the item, which some might be misleading; most of them fit, but they gave me a short tactile switch instead of the long one.

The keyboard connectors are from Digikey; it would be tough to get them from China. It might be cheaper to get it from the regular UK retro parts vendors.

link to part 1

link to part 3

Building a ZX Spectrum Clone in 2021 – Begining.

ZX Spectrum – Recreated casing from ZX Renew.

The ZX Spectrum started me down the path as a maker in the 80’s, it was my first computer, and I had so much fun and learning with it. I always wanted to build a spectrum clone when I was young, but I don’t have the skills and resources.

Originally I wanted to make a retropie emulator but was quite turned off with the long loading time and navigation. So my idea was to have it much closer to the original experience with the machine when I was young.

While researching for a better Spectrum emulator, I stumbled on a RasPi bare metal emulator, the “ZXBaremulator” it boots at without going into the Linux screens. I have an old RasPi 1 Model B to hack with, and without thinking much, I ordered a recreated casing, keyboard membrane and faceplate from ZX Renew and waited patiently for it. I was disappointed when It came two weeks later in the mail; I did not order the rubber mat (I thought it was a mouse pad or something). So I placed another order for the mat (paid another £15 for shipping)

Complete set of the casing with RasPi.

Another two weeks passed, and I finally got the recreated case and started hacking the RasPi to fit the case.

Everything works fine except for one crucial part that doesn’t work for me; I cannot load tape the slow and regular way; that takes away the 80’s experience.

So I started looking for other solutions, and I decided to design and build a clone from scratch.

As part of my research, I ordered the book that everyone recommends for building a spectrum clone, “The ZX Spectrum ULA” by Chris Smith; it is a very well written and informative book. I spent many nights reading and pondering my approach to building a Spectrum clone.

While researching, I found several open-source ZX Spectrum projects, like the Harlequin 48K / 128K, ZX Uno, and many more. And of course, I lust over the Spectrum Next, hoping to support the 3rd issue in the future if there is one.
In the end, I decided not the reinvent the wheel and pick a project to work on; I chose the “sizif 512K” as it is the closest to what I wanted and more. I wanted a clone that uses as many real parts as possible, especially a real Z80. The only problem I have is there is little guidance in building this clone; I will have to spend some time figuring everything out and sourcing for parts.

link to part 2