Sinclair ZX80 / ZX81 / Z88 Forums

I've
had some original 1K zx81 on which hires doesn't work, while on other zx81 1K hi res works.
There is an explanation?
thanks

It helps when you tell which game doesn't run, as well what ROM is in your ZX81 and how much RAM do you have.

Each game has its own hires displaydriver so more info is needed.

juzzo72 wrote: ↑Fri Sep 15, 2023 5:21 pm I've
had some original 1K zx81 on which hires doesn't work, while on other zx81 1K hi res works.
There is an explanation?
thanks

If you mean only different machines, yes this is known and the reason why the were not sold on a tape.
Some ZX81's have problems, most known is only a cosmetic display issue.

On this zx81 video Is not hires

There Is a mod that i can do?

Can you please post a photo of the screen image when it’s running the hires game/demo.

Mark

See attach

Have a read of the thread around this post

You could try adding a capacitor..

But my two zx81 are original with 1k, not 16.
On One i have connect capacitors between RAM CS and groun, 33pico farad and after 66 Pico farad, and nothing Is changed.

Unfortunately, I think it is more likely that the RAM fitted to that board may be too slow. Those chips are rated at 450ns. On most boards I have seen, they use 300ns or faster parts.

The topic that Mike links to is about modern RAM that switches off it’s outputs too quickly.

Here’s a copy of the relevant datasheet for the Toshiba TMM314APL (2114) SRAM: Mark