Sinclair ZX80 / ZX81 / Z88 Forums

In terms of a replacement membrane, as you are in the E.U., maybe sintech. I can’t give you a web site address because my browser gets automatically redirected to their U.K. site. There are U.K. sellers, but then you have the hassle and expense of the item crossing the border (caused by the U.K. lea...

Hello and welcome to our forum Alex!



I hope you enjoy this forum and I look forward to hearing about what you are working on

Mark

Vsual check all address lines to/from the lower DRAM. If no problems are found, do continuity tests. Print out a copy of the schematic and use a coloured pen to mark each pin of each device as you test.

Then test for shorts.

If none of that finds the problem, try replacing IC3 or IC4.Yours

Mark

That photo in your last post, it looks to be failing the RAM check in the BASIC ROM. Hence the vertical red lines on a black paper (background).

The horizontal lines may indicate a problem with the address lines.

Did you get a diagnostic card or ROM?

Mark

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