exxos's Atari, Amiga & retro forum

I do think that you have a working system there, you just can't see it, other than on the oscilloscope, which isn't that useful. :-)

stephen_usher wrote: 04 Feb 2024 16:39 I do think that you have a working system there, you just can't see it, other than on the oscilloscope, which isn't that useful. :-)

Me too.

Shame it doesn't have any sort of X/Y output, then I could set my scope up to be my display :)

I wonder if you could use the information from the ZX81 signal conditioning to get the OSI composite video good enough: https://www.evolutional.co.uk/post/zx81 ... ning-mods/

stephen_usher wrote: 04 Feb 2024 22:58 I wonder if you could use the information from the ZX81 signal conditioning to get the OSI composite video good enough: https://www.evolutional.co.uk/post/zx81 ... ning-mods/

Hmm interesting 🤔

OK folks, this little fella arrived today:

It had a cold solder joint on the TV on switch, however I was able to fix it and get it all back together in about half an hour. Another win for my little rechargeable soldering iron.

It's a 5.5" black and white TV from Plustron, but the key thing is that it has a composite input. What you're seeing on screen right now is the output from the OSI :)

Zooming in I see this:


You may have to zoom in further on this web page, but when you do, you might just see characters in amongst all that mess!! Also with the monitor ROM in I can elicit a response by hitting break, which changes the image, so I think I have a machine that is at least working in some capacity.

I have tried tuning using R58, and I also tweaked the V-Hold on the TV , and the picture does change, though it still doesn't display properly as above. I'll take this as a small win though.

Apparently if I drop to BASIC and type SAVE then return it'll show output on pin 6 of the 6850, so I may try that tomorrow.

I'm guessing that the telly doesn't do 60Hz, just 50Hz.

I wonder if you can slow the OSI's clock down by 1/6th so it does 50Hz video. Hehe. Oh, of course it'll have the wrong number of lines then. Bother.

stephen_usher wrote: 07 Feb 2024 23:31 I'm guessing that the telly doesn't do 60Hz, just 50Hz.

I wonder if you can slow the OSI's clock down by 1/6th so it does 50Hz video. Hehe. Oh, of course it'll have the wrong number of lines then. Bother.

Yes, that's a possibility for sure, and something I'd not considered (Annoyingly). Best I can do is 90% of 4Mhz...

Whilst it may not do 60 Hz, it has helped substantially in that I now know it's working in some sort fashion, so that is helpful.

I don't think I tried connecting to my main LCD tv, as that does support 60Hz. I know it might not like the signal, but I have to try. I only previously tried it on my RetroTink, and to be fair I haven't tried again since fixing the clocks. What have I got to lose?

So Klyball, the designer of the reproduction PCB said this over at OSIWeb:

"I'm not a expert on the signal but it could be a pal/ntsc issue. I'm comparing the 2 schematics uk101 to the 600 it looks like there doing the timing for hs and vs a bit different."

Hmm, interesting. I presume then that the guy I got the board from has a display that can do either PAL or NSTC, now wondering how I'm going to get this thing to display at all.

I don't know a lot about the video signals, but I didn't realise that composite could be PAL or NTSC. Clearly I got that wrong having just read this: https://en.wikipedia.org/wiki/Composite ... rrelevant.

So I've managed to find an NTSC to PAL converter on Amazon so I've ordered one to see if it helps me out at all. It is switchable too, so will also do PAL to NTSC.

Should arrive Saturday, so lets keep fingers crossed!!!

Hopefully the new box does the trick. It'll have to have a frame buffer to read the frame from the source, store it and then write it out at a different frequency and number of lines (and potentially colour encoding).

With respect to "composite video", you have to think of it as just the TV signal before it gets modulated for transmission over RF, so it would be whatever the TVs in the region expect for their input. So not only will the number of frames per second change but the number of scan lines and the colour encoding (not an issue here).

By the way, if you have one of these devices it may be able to do the business: