Sinclair ZX80 / ZX81 / Z88 Forums

Hi!

ZX81 #2 arrived today; a series 3 with all the fixed bits. As i have nothing that can decode an RF signal, my first job was to do the transistor composite mod. Then I gave it a go...

• First, i tried it with my tiny car monitor, which has a composite input. I just got a white screen with no cursor.
• Then i tried it with a scan doubler, which had a horrible picture, but showed the cursor.
• Then i realised it should work with my rgbtohdmi, but that was just a white screen

So, i think the cursor is there when there's just a white screen, but it's fallen off the bottom. is that likely?

Is there anything i can try to adjust on the zx81, or shall i try fiddling with settings on the rgbtohdmi?

To use the RGB to HDMI, you need to note a couple of things which you might already be aware of, but I didn't see it in your post...

1) The RGBtoHDMI needs an analogue input board.
2) The input to the RGBtoHDMI is taken from the composite output before it goes into the UHF module and not the output of the UHF module and needs a 220 ohm resistor.

This might give you a hand...
https://msknight.com/bbc/zx81-video.html

EDIT - Your ULA looks to be the type that does produce the back porch, so that's one thing you shouldn't need to worry about.

I've already checked the compatibility list, and set the CPLD to Analogue YUV, and done a composite mod, but i did the one with a 2N3904 and a 100R. I've never heard of doing it with just a 220R before. I had the rgbtohdmi set to zx81 composite.

I guess i could try the 220R version, but I'm not keen on removing my handiwork. I'll see if i can tweak the settings first.

The cursor is not likely to be off-screen, more likely is that the video contrast (voltage) is wrong. Hence the K cursor may be there, just not visible.

If the picture size was incorrect, I would expect a rolling image, due to incorrect sync.

Keep in mind that the transistor type composite modification is not perfect. It may not work for all displays.

Mark

aah. it's never simple is it?

in that case, i'll switch the mod for the 220R resistor and keep my fingers crossed.

if that fails, I've ordered parts to build a ZX8-CCB that are still in the post.

edit: The 220R mod worked! Extra handy, as I already have lumacode on one of my Speccys, my MSX, and two C64s. I'm assuming this mod won't work with a ULA that doesn't generate a back porch, but hopefully the ZX8-CCB will work with the composite option selected.

Now onto the 16K RAM mod..

edit 2:
woo!

The ULA you have does generate the back porch signal.

You might want to use a 1K Hi-res game to see if the ULA is capable of WRX high res before you do the RAM upgrade.

I put a little about that here - https://msknight.com/bbc/zx81-ram.html

I went slightly bonkers trying to solve what I thought was a RAM timing issue, when in fact the ULA wasn't up to WRX. Out of six zeddys two ULA's couldn't do it.

Moggy wrote: ↑Sat Apr 06, 2024 7:18 pm The ULA you have does generate the back porch signal.

This one does, but my other ZX81 is less fortunate, which is why I also bought parts for a mod that adds a back porch. I keep calling it a ZX8-CCB, but it's not - it's a design from zigg.net

Thanks for the extra advice, msknight. I hadn't considered hi rez games yet - I'll give one a go and see what happens!

I've now done the zigg composite mod on my porch-less ZX, and it works! I really wasn't expecting it to, especially as I had no 56R resistors, and used a 47+10 instead, and my last veroboard circuit smoked a resistor, somehow. I think that might've been a duff resistor, but it shook my confidence.

Gutting the modulator was a pain, and took ages, but it did expose a few rotten-looking components. It looked like someone had left some berries in it that had gone mouldy. The mod only *just* squeezed in.

and, it works with the rgbtohdmi set to zx81 composite, and and the keyboard still works, which is ace, coz it's grey and not white like the repro version. next, I'm gonna load some stuff and see what this hi res stuff is about.

Hi res won't work with the original 16k ram pack, by the way. The mod for that is here...
https://msknight.com/bbc/zx81-rampack.html