Sinclair ZX80 / ZX81 / Z88 Forums

About 40 years ago I ordered a ZX81 Kit but it came in assembled.
So now after 40 years I finally decided to get one in kit form.

The Kit has an Issue 1board with a ULA that has the back porch (210E).
Since it's easier to just make the changes as I'm building it instead of
making them later I figured I'd see what the suggested changes are.

I'm considering making the following changes:
Replacing the original Z80 with a newer Z80 CMOS CPU
Replacing the original 7805 Voltage regulator with a DC/DC Converter
Converting the RF output to Composite output
Replacing the original Memory with an internal 16/32K memory upgrade.
I have an EPROM burner so I could change out the EPROM to a 27C64

I also have a Timex/Sinclair 1000 that I picked up as a test bed of sorts
to test things out on before installing them in the Kit ZX81.

Any other changes that would be wise to install?

Thoughts?

Thank you

Hello Harry, welcome to our forum

Harry wrote: ↑Sun Feb 28, 2021 8:04 pm About 40 years ago I ordered a ZX81 Kit but it came in assembled.

Wow! Did you have to pay the assembled price or the kit price?

Harry wrote: ↑Sun Feb 28, 2021 8:04 pm I'm considering making the following changes:
Replacing the original Z80 with a newer Z80 CMOS CPU
Replacing the original 7805 Voltage regulator with a DC/DC Converter
Converting the RF output to Composite output
Replacing the original Memory with an internal 16/32K memory upgrade.
I have an EPROM burner so I could change out the EPROM to a 27C64

I’m a bit curious for your reasons for going for a CMOS Z80. I have no problem with it. Just wondering why.
Are you intending to burn Zsolt’s ZX81X2.ROM to EPROM?
Agree with everything else.

The only other things I can think of adding are:

• Wilf’s Why Wait modification (link),
• Using a larger capacity EPROM and a switch so you can select which ROM to use,
• Adding a reset switch button.

Mark

On my original one (40 years ago) I had paid the kit price.

As far as the CMOS Z80, they draw less power so should run cooler plus they are only about 5 dollars US

I'll just add WRX High Res compatibility to the combo..
And perhaps a joystick port..