Time to go back in time again - ZX80 replica build

[rubber_jonnie]

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

My thoughts exactly. I will double check for power, I'm pretty sure it is.

For now though I gotta do a work call

[rubber_jonnie]

@stephen_usher Yes the board has power. I'm considering hooking it up to my ZX81 to see if it works via that. Only problem is my ZX81 does have a backporch from the ULA.

My ULA is a replacement ZLA81 so may have the option to turn it off.

[rubber_jonnie]

OK, so new image:

IMG20250925154058.jpg (247.99 KiB) Viewed 161 times

Yellow is the ZX81, Blue is the ZX80. The ZX81 has a rather nice back porch. Looking at the ZX80 it kind of looks almost like the back porch is being added in the wrong place, but there are no adjustments I can make.

What I am considering next is stealing the comp mod from my ZX81 complete in the modulator can to see if it will work with the ZX80. Since the ULA I have in the ZX81 has back porch generation, I may just need to swap the devices over as what's in the ZX81 also provides back porch and is known to be working.

More to follow...

[rubber_jonnie]

So I swapped the ZX81 comp mod into the ZX80, still nothing, though I didn't adjust the pots as I had it perfect for the ZX81. Perhaps I should have.

Next up I tried the ZX80 mod board in the ZX81 and it didn't work...

Also, with back porch turned off on the vLA81, i wasn't getting a proper back porch on the mod board from the ZX80, and in fact it looks like the back porch is actually too high based on my previous images. It is there, it just isn't correct.

This is driving me bonkers now!

[rubber_jonnie]

Just to rule out the clocks, I borrowed the resonator from the ZX81 and used it to replace the crystal I'd used in the ZX80.

No change at all and the crystal worked just fine in the ZX81...

I forgot to mention in my last post that the ZX80 composite board did work in the ZX81, but I had to disable the back porch and enable on the ULA.

Image quality was fine though there were some jailbars. The other comp mod originally from the ZX81 had no jailbars.

All swapped back now.

[stephen_usher]

Hmm... sounds like that new board's 555 circuit isn't working. The capacitors and resistors are in the right places so as to last the correct length of time aren't they? I must admit that working out the time for 555 timer switching always feels like magic.

[rubber_jonnie]

Hmm... sounds like that new board's 555 circuit isn't working. The capacitors and resistors are in the right places so as to last the correct length of time aren't they? I must admit that working out the time for 555 timer switching always feels like magic.

Yeah, I'm going to have to agree. I reckon the timing is something that requires some sort of sacrifice to the gods of silicon...

Having swapped the 555 I know that made no difference and on both the ZX80 and ZX81 (with back porch turned off on the vLA81) I get the same result as below:

Screenshot 2025-09-26 at 09.23.04.png (244.28 KiB) Viewed 115 times

The green lines are where the sync pulse is and the red lines/arrow are supposed to be the back porch, Only it's way too high, it should look like this taken from the ZX81's working as normal:

IMG_20250926_100327.jpg (161.06 KiB) Viewed 115 times

Again, the green lines are where the sync pulse is and the red lines/arrow are the back porch. So the mod board I have is not working as expected since it does the same thing in both the ZX81 and the ZX80.

The composite signal from the board does work in the ZX81 if I pull the 555 (This disables back porch generation), however I do wonder if there is another fundamental issue since the the ZX80 still doesn't work with the mod board from the ZX81.

I'm going to look at the schematic and may order some alternate IC's for the video generation circuit, though I'll probably test in the TL866 before I do so.

Perhaps I'd better contact the seller...

[stephen_usher]

It's almost as if the mod board has the output from the backporch generation inverted, so it's pulling the signal up rather than down.

[rubber_jonnie]

So doing some reading over at ByteDelight, it suggested that with the ZX80, you might even need a transistor mod even with a composite board.

So I tried and added a transistor mod that takes the output of the mod board and feeds it to the output socket:

IMG20250926120521.jpg (299.32 KiB) Viewed 99 times

It gave me this image:

IMG20250926120539.jpg (430.61 KiB) Viewed 99 times

And though it is barely visible, there is activity on screen when keypresses are made. This is very reminiscent of the bad ULA I had in my ZX81. it also highlighted this:

IMG20250926120454.jpg (208.25 KiB) Viewed 99 times

It's made the back porch that's in the wrong place stand out even more. I've contacted the seller about the issues and am waiting to hear back.

[rubber_jonnie]

It's almost as if the mod board has the output from the backporch generation inverted, so it's pulling the signal up rather than down.

Definitely looks like it.