Replica of a Soviet ZX Spectrum clone

[cj7hawk]

What software did you design those PCBs in?

You have me thinking about what a minimalistic Spectrum/CPM machine might look like... ( Without using ULAs ) - Maybe something Pandora-like.

Thanks for posting the journey - When you anticipate having a working model?

Also, why the lack of expansion port? I've noticed that isn't all that uncommon for some Spectrum clones?

 

[GlueLogic]

What software did you design those PCBs in?

I used KiCad 8 for the first version, but now I'm using KiCad 9. I can definitely recommend it, it makes the design process way easier than I thought it'd be when I first got into PCB design.

It also lets you add custom fonts and logos to boards to make them look more interesting.

Thanks for posting the journey - When you anticipate having a working model?

I'll order the PCBs and components once I get paid at the end of this month. My JLCPCB orders usually take around 1-2 weeks to arrive.

I'm hopeful that the new version will work first time without any modifications; I've tested the new features in Logisim, so I'm fairly confident they'll function properly.

Also, why the lack of expansion port? I've noticed that isn't all that uncommon for some Spectrum clones?

Due to the hardware differences in the Leningrad design, I have doubts that accessories like the DivMMC would work properly with it. With some further improvements to I/O addressing, it might be possible to get an SD card interface working with it, but it'd probably have to be designed specifically for this computer.

I'll also admit that I'm still fairly new to hardware design - my first really ambitious hardware project was a simple 6502 computer I built based on the Ben Eater design about six months ago.

 

[GlueLogic]

All the parts are finally here - time to put it together and see if it works!

For those of you that are interested, I've also been working on a replica of the Specialist (a homemade Soviet computer from 1987) alongside members of the zx-pk.ru forums.

 

[GlueLogic]

It works! I'm testing it here with the keyboard of the first Leningrad - the keyboard PCB for the new one is among the pile of parts on the right.

 

[GlueLogic]

At last, the files are ready for download!

I have made one last small change. In Revision 1, the black level reference pulse was 24 pixels long; it turns out this is too short for it to be recognized by most TVs, so the black level was influenced by the border color.

In the new Revision 2, the black level pulse is now 32 pixels long. I've tested this fix on my Revision 1 system with a bodge wire, and it works perfectly now.

Keyboard PCB:
alex-j-lowry.github.io/download/lngrad-m/leningrad-m-kbd-allfiles.zip

System PCB (Revision 2):
alex-j-lowry.github.io/download/lngrad-m/leningrad-m-r2-allfiles.zip

System PCB (Revision 1 - provided for those who are interested to see the changes, but I don't recommend having this one produced):
alex-j-lowry.github.io/download/lngrad-m/leningrad-m-r1-allfiles.zip

Soon, I will write more documentation on the finished project on my website.

 

[GlueLogic]

I've also made a PAL color encoder that connects to the main board based on the NIMF-4, a PAL encoder sold in Russia in the 90's.

You can download all the files for it here. It should be suitable for getting a PAL composite output from any system that outputs TTL-level RGB video signals and a separate sync signal. (so long as it has TV-compatible signal timing).

alex-j-lowry.github.io/download/nimf-4/nimf-4-r1-allfiles.zip

Some notes for those who want to build one of these:

It turns out Piher doesn't make 1.5kOhm potentiometers in the required form factor, but it works fine with 1kOhm potentiometers.

I also couldn't find a 5-25pF variable capacitor, but I found a 6.5-30pF one also works.

 

[RUBEN]

Hello,

I'm thinking of making a clone of the ZX Spectrum, as minimalist as possible while respecting its functionality.

My idea is to make a clone of the ZX Spectrum 48k Issue 1, but without the ULA or replacing it with TTL chips, but with minimal functions or with its original functions of reading and writing to cassette tapes and composite or VHF video.

It shouldn't be too big and it shouldn't have more than 30-40 ICs.

The problem is that I don't know where to start designing the motherboard.

Could someone help me design it or tell me if there's anything that could help me design it?

Thanks.

 

[RUBEN]

And with 8-bit ram like 62256

 

[RUBEN]

A

 

[RUBEN]

And do another versión that works with the controller for the spectrum -2a cassette

 

[GlueLogic]

Hello,

I'm thinking of making a clone of the ZX Spectrum, as minimalist as possible while respecting its functionality.

My idea is to make a clone of the ZX Spectrum 48k Issue 1, but without the ULA or replacing it with TTL chips, but with minimal functions or with its original functions of reading and writing to cassette tapes and composite or VHF video.

It shouldn't be too big and it shouldn't have more than 30-40 ICs.

The problem is that I don't know where to start designing the motherboard.

Could someone help me design it or tell me if there's anything that could help me design it?

Thanks.

For schematic/PCB design software, most people (including me) use KiCad. It's more intuitive than I assumed it would be when I first got into PCB design.

When it comes to understanding how the Spectrum's circuitry works and how to modify it, I'd recommend checking out some ex-USSR Spectrum sites, like zx-pk.ru, speccy.info and Tarjan's list of ZX Spectrum clone hardware. You'll find schematics for dozens of variants of TTL-based Spectrum clones produced in the 80's and 90's.

Good luck on your project!

 

[cjs]

I recommend KiCad as well. I would not call it intuitive, but at least it's open source, so you know that you and others are going to be able to read and edit the files a year or two down the road, as opposed getting pulled into one of those, "We've made this a yearly subscription product so pay up if you ever want access to your data again" things (*cough* *Eagle* *cough*).

I really do recommend doing some very simple, very tiny projects first. KiCad was always a barrier for me until I did super-simple stuff such as an adapter to convert 2364 ROM pinout to 2764 EPROM pinout, which, having only two components, set the level of challenge low enough that I could actually handle it.

For the Spectrum clone, people have already done this before (I think even non-ULA versions), so it might be worth looking at what they've done. There's absolutely no harm in looking at others' work and essentially duplicating it, as even if it may not be totally useful to others, it helps get you to the point where you can do original stuff useful to others. (Which may well be a modification of your initial copied design!)

 

[clueless_engineer (Brett)]

What software did you design those PCBs in?

You have me thinking about what a minimalistic Spectrum/CPM machine might look like... ( Without using ULAs ) - Maybe something Pandora-like.

Thanks for posting the journey - When you anticipate having a working model?

Also, why the lack of expansion port? I've noticed that isn't all that uncommon for some Spectrum clones?

I don't think I've seen a smaller Speccy than this one from the early 90s, thanks to the magic of the Soviet-era БМК at its heart. Just add some RAM, the CPU, the ROM, a hex inverter (for tape) and a couple of other bits of logic!