ShredDec 10, 2022

This is my next part for this #RetroWeekend. It's a #ZXSpectrum issue 3 board. I only got the untested board, but no case or keyboard. It must have been a ZX Spectrum Plus, because there is a reset cable soldered to the board.

I removed the ULA and ROM from their sockets before I took the photo, to protect them from ESD damage. #Retro #RetroComputing #SinclairZXSpectrum

Show threadShredDec 10, 2022

I ran the diagnostics, and it showed that the D0 line is always LOW, so there must be a short somewhere on the data bus. Now I could try to find and fix the problem, but I have other plans: I'd like to build a new ZX Spectrum from old and new parts.

I checked the ULA in another Speccy, and it is working fine. 😀👍​​ The rest is replaceable.

The next step: I will strip all the coponents that I will need for the new board: all RAM chips, the LM1889N, and the coil.

Show threadShredDec 10, 2022
Oh great, at least one of the RAM chips is broken. I don't know if it was broken before, or if I killed it when I unsoldered it.
Show threadShred

I unsoldered all the important parts. The rest of the board is more or less for the trash now. It's a sad look, but the prospect that the recovered parts will be used in a brand new ZX Spectrum makes me happy again.

1 lower RAM chip, and 6 (!) upper RAM chips, are broken. This is bad. I have replacement parts here, but they don't grow on trees.

Okay, enough for today. Tomorrow I will assemble the new Speccy. 😀​

Dec 10, 2022 at 4:32pmWeb
Show threadShredDec 11, 2022
They say that Christmas time is that time of the year when you return home and fix your parents' computer problems. 😀​ Well, for today I planned to assemble my new ZX Spectrum, but my parents called and said their internet router is dead. So I have to postpone my Speccy plans for sometimes between Christmas and New Year.
Show threadShredDec 28, 2022
The #ZXSpectrum replica is still waiting to be completed. Start of December, I stripped all the rare parts from a broken ZX Spectrum board. Now I'm going to assemble a new Speccy, starting with the resistors, diodes, and capacitors. #RetroWeekend #RetroWorkshop #Sinclair
Show threadShredDec 28, 2022
There are a surprising lot of resistors and capacitors with different values. It took quite a while to locate the correct position for all the components. It's like painting by numbers. 😀​ Enough for today, I will solder in the rest of the parts tomorrow, and then (hopefully) have a brand new ZX Spectrum.
Show threadShredDec 29, 2022
Most of the other parts are soldered in now. I will give that board a bath in IPA before I solder in the remaining components and give it a first test.
Show threadShredDec 29, 2022
Everything is in its place now. I intentionally did not put the chips into the sockets yet. First I want to check if all voltages are correct.
Show threadShredDec 29, 2022
Oh no! The overcurrent protection of the power supply immediately kicks in. And indeed, there is continuity at the power connector. There must be a short circuit somewhere… But the solder side is looking good!
Show threadShredDec 29, 2022
Hmm… I removed the DC/DC converter, and the short circuit was gone. I checked the DC/DC converter, but it's fine. I soldered it back to the board, short circuit is gone. 🤔​ All the voltages are present now. So it's time to add the modulator, and put the ICs into their sockets.
Show threadShredDec 29, 2022
I did not transplant the original modulator, since I don't have a TV with antenna connector anymore. Instead I use an S-Video mod board. It should give a much better video quality. (S-Video project is here: https://github.com/redhawk668/ZX-Spectrum-S-Video)
GitHub - redhawk668/ZX-Spectrum-S-Video: S-Video Modification PCB for the ZX Spectrum

S-Video Modification PCB for the ZX Spectrum. Contribute to redhawk668/ZX-Spectrum-S-Video development by creating an account on GitHub.

GitHub
Show threadShredDec 29, 2022

Everything is in place and connected. Powering up, and…

Nope. This is not was I was hoping for.

Show threadShredDec 29, 2022
Well, at least the image quality is indeed better compared to composite. 😂​
Show threadShredDec 29, 2022
The diag module shows no pulses on the CPU control lines. I changed the CPU for a known good one, but it didn't change anything. So I started by checking the clock. There is a 3.5 MHz pulse at the clock output of ULA pin 32, but at the clock input of the CPU pin 6 there is a flat line. 🧐​
Show threadShredDec 29, 2022
The problem must be around here. The clock signal is present on the right side of C67 and R24. On the left side there is no signal anymore. My guess is that TR3 is defective, pulling the signal to ground.
Show threadShredDec 29, 2022
I removed TR3, and then the signal at the left side of R24 was fine (as expected). If I place the transistor back, the signal breaks down again. I used a BC548C as replacement for the ZTX313, maybe it's the wrong type? Well, unfortunately I have to stop for today.
Show threadShredJan 3, 2023

Good news: I got the MPS2369, which is a known good replacement for the ZTX313. 😀​

Bad news: I'm still getting no CPU clock. The problem must be somewhere else. 😕​

Show threadShredJan 7, 2023
At this #RetroWeekend, I planned to find and fix the issue with the CPU clock. I connected probes to pin 32 of the ULA (CPU clock output, cyan) and to the base of TR3 (yellow). I then used one of the new MPS2369 as TR3, turned on the #ZXSpectrum, and… well… it just worked! 😀​
Show threadShredJan 7, 2023
A check test with the BC548C shows the same issue again: ULA clock was there, but nothing at the base of the transistor. So the problem was definitely that I used the wrong replacement type for the original ZTX313 at TR3. Problem solved, my ZX Spectrum made from (mostly) new components is finally working!
Show threadShredJan 7, 2023
The diagnostics module says that the CPU is be bad. There were also 7 DRAM chips damaged. The donator board has really been in a bad shape. I'm lucky that at least the ULA and ROM is working. Okay, let's replace the CPU with a brand new Z84C00.
Show threadShredJan 7, 2023
All tests are passing now! 😀​ It's time to move this ZX Spectrum into its new house. (I also added a "before" photo of the old donator board.)
Show threadShredJan 7, 2023

I decided to use a clear replica case, so the beautiful board inside is visible (at least a part of it). So here it is, an almost brand new ZX Spectrum, made from new components whenever possible.

What's missing? A test run! Let's load a game.

Show threadShredJan 7, 2023
Starquake has always been one of my most favorite ZX Spectrum games. To load it, I connected the Speccy to the headphone output of my PC, and then used tzxplay to playback a TZX file. tzxplay is part of my tzxtools: https://github.com/shred/tzxtools
GitHub - shred/tzxtools: Tools for processing TZX files

Tools for processing TZX files. Contribute to shred/tzxtools development by creating an account on GitHub.

GitHub
Show threadShredJan 7, 2023
Okay, I'll now enjoy my new Speccy. 🙂​ If you now got appetite to build your own ZX Spectrum, I assembled a "bill of material" from the components I have used: https://shred.github.io/zxspectrum-bom/ . A blogpost will follow up later, too. Thanks for reading, and for keeping your fingers crossed about the CPU clock issue. 😉​
ZX Spectrum 48K Issue 3B: Bill of Material

Show thread48kRAMDec 29, 2022
@shred I think I've got that same mod and it's wonderful on my 48k
Show threadOlemDec 29, 2022
@shred Looks good! Good luck with the troubleshooting. Love a good retro remake.
Show threadAlibanDec 29, 2022
@shred it’s a pity to see this old boards striped, love the look of the traces and the touting. Still like your idea of bring it back in a new board.