Time to go back in time again - ZX80 replica build

[stephen_usher]

A "period correct" colour telly wouldn't work either as they use the backporch for brightness calibration too.

My Ferguson TX telly can't display the output from my ZX81 properly. You just about see something on the screen if you turn the brightness right up.

[rubber_jonnie]

A "period correct" colour telly wouldn't work either as they use the backporch for brightness calibration too.

My Ferguson TX telly can't display the output from my ZX81 properly. You just about see something on the screen if you turn the brightness right up.

I've ordered a kit that is supposedly for the ZX81 but since the ZX80 uses the same modulator it should work, it gives composite and also generates the back porch as well, though it can be removed if you wish.

I think the ZX80 is about the only thing that needs a genuine old school B&W TV at the moment!!!

I do have my little 5" B&W display and I just noted it has a 3.5mm jack for an external aerial, so I might be able to rig something up.

[rubber_jonnie]

OK, so chatting with folks over at Sinclair ZX World, it seems that R31 and R33 (Part of the video circuit) are not installed on UK ZX80's, so I have removed them. It didn't help but I at least know it's not got any extraneous parts installed.

I also rigged up composite like this: ZX80 composite just to be sure it's done right.

The video output is now at 15.8Khz horizontal refresh, as my crystal is a smidge fast though by less than 1%, but after doing digging should be OK as it's within 1% of the 'proper' 15.625Khz rate. What I get now is a solid black screen, but it shows as coming through as PAL. Pressing new and then newline causes the machine to reset as expected and the screen flickers.

Of course I don't yet have my PCB to add the back porch, and since the video signal shows it doesn't have one, I'll have to wait. It's been posted so hopefully it'll come soon.

[HigashiJun]

One more step...

[rubber_jonnie]

One more step...

Indeed, just very slow steps...

Hopefully the sockets will come soon too then I can use the genuine PSU I have rather than my bench PSU.

[rubber_jonnie]

Happily my power/mic/ear sockets and the composite mod including back porch arrived today. Sadly I was in London so couldn't do a thing, but hopefully I'll get this project done and dusted and all ready to bring along to CL4

[stephen_usher]

[mrbombermillzy]

Yes, it would be great to see 'in the flesh' at CL4.

[rubber_jonnie]

Yes, it would be great to see 'in the flesh' at CL4.

It would.

However the composite + back porch mod is installed and I still don't get a decent image on anything...

Screenshot_2025-09-25-11-38-29-92_92460851df6f172a4592fca41cc2d2e6.jpg (152.29 KiB) Viewed 110 times

So blue is the input, yellow is the output, supposedly with a back porch, but although PAL is detected, only my Dell display gave a very awful image that would barely show anything on screen. Bit of a head scratcher.

The machine is working in that I can still get some sort of image off the other pickup point and just about see keywords appearing when I press a key, and New does a reset still.

I tried swapping out the 555 in the composite mod, no real difference.

Looked at the ZX81 which has a great composite signal and I see this.

IMG20250925123441.jpg (215.44 KiB) Viewed 110 times

So quite a bit different. Yellow is the ZX81.

[stephen_usher]

Definitely no back porch there. The board is getting power isn’t it?