ZX81 Unbuilt Kit - What changes to add while building

[Harry]

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

[1024MAK]

Hello Harry, welcome to our forum

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?

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

[Harry]

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

[bola_dor]

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