zx81 clone - help needed

[bifteki]

Hi Forum

I am trying to assemble a zx81 clone.

So the computer works.. that is the good news... but it has issues.

The 1st issue seams to be ground-related. I need to restart the computer like 10-20 for it to initialize, and when I touch it or shake it, it malfunctions.

This behavior is exactly similar on the rest 4 pieces that I have; I had to manufacture 5 pcbs (the minimum).

The 2nd issue is that the 16K rom is read as 8K. I am pretty sure the lines are correct and the DIP, which changes from 8k to 16k, works fine, but still there is an explainable problem there.

[jrlefort]

which clone are you building ?

The first problem sounds like a capacitor value that is not correct somewhere.

The second depends on the build, on my ZX81+38 v1.9 by Mahjongg the ROM is even a 32KB, but it uses only the first 8KB that contains the original ROM of the ZX81.

[1024MAK]

On all ZX81 / TS1000 machines, the ROM is only 8K bytes in size but due to partial address decoding, it actually appears in the Z80 address range four times.

Some ‘clones’ or compatibles modify the address decoding to reduce this, allowing RAM in the areas where the ROM ‘echos’ were.

Mark