Sinclair ZX80 / ZX81 / Z88 Forums

Just tested the keyboard joints as suggested, KB1 (4) left most point as you look at the MB seems to be faulty, so suspect the ULA as others have suggested unless the ROM is fault (?) Unlikely to be a ROM fault. Mask ROMs normally fail in a fatal way if they fail at all (they are normally the most ...

An I right in thinking that this is the composite video board you have: link zx2020-zx-wespi.jpg There are some adjustments. As I don't have one of these, I am unable to advise on how to best adjust these without more information. A better, close up picture of the board would help. Mark Edit: crosse...

With respect to simulating the keyboard, you can also use the metal contacts in the PCB connectors. Unlike the ZX Spectrum where the entire video signal is generated by hardware (the ULA mainly), in the ZX81, most of the work is done by software, as in the Z80 executing code (if the Z80 is unable to...

Upload as a zip file

Mark

Okay

I look forward to your updates

Mark

Was this chip and / or board dead then?

Mark

As you can see, technology has moved on a bit since Sinclair's 16K RAM pack :lol: To help, I will require some good close up, in focus photos of both sides of each of the boards. The wire interconnections are a weak point, so check (visual) and electrically test (multimeter on the continuity range) ...

This board repair may take a lot of effort to repair then. As I would not rely on the workmanship of previous attempts at repair. The next suggested step is to check (visual inspection using a good light and a magnifying glass) and test (using a multimeter on the continuity range) the PCB tracks for...

It's not unusual. The unloaded output voltage will vary. It depends on the actual mains voltage, the temperature of the diodes (warm diodes conduct better) and on the (very, very small) leakage current flowing through the capacitor.

Mark

That's strange. I suspect multiple problems. Even if none of the 'lower' (4116 or equivalent) RAM is functional, at power up, if the Z80A is running, the ROM code should set the border to white. As this board has been worked on before, I don't think just swapping parts is going to get you very far. ...