Investgating PAL 7800 picture issues (was 7800 RGB Musings)

[juansolo]

There's also a distinct possibility that the UAV 7800 wouldn't need the buffer, so making the mods really easy... Guess that means I'll be restoring 2600 backward compatibility to mine then!

[+-^CrossBow^-]

2 hours ago, juansolo said:

EDIT: Of course we've just found an easier way of doing it by looking at the Longhorn mod...

Two issues I see with using the LHE..

- The LHE suffered from a flaw in that the chroma from the 2600 would still leak through and cause diagonal interference noise patterns when playing 7800 games. This required a separate switch to be wired off the TIA signal line to physically cut the TIA chroma when playing a 7800 game. 

- The LHE availability isn't there. Unless someone else is making them, I though ElectronicSentimentalities was the only source and I believe he has closed up shop again as none of my emails to them asking about stuff being available have been answered and there are several who have reporting ordering stuff from the website, but never getting a confirm of the order and never receiving anything.

 

There is also the issue that the values used on the LHE were not consistent across the 7800 variants. So some people had great results with their LHE mods while some like myself had image issues with the blues bleeding badly and other issues. I remember with my LHE that the detail on the ground on 7800 Dark Chambers wasn't visible unless I had both the composite and s-video plugged in at the same time?! Just strange stuff like that.

 

So I don't know if the LHE performed better on PAL machines, but I think the Magic Knight/Saundby circuit is a simpler design and works better even with the tweaks it still required. I need to go back to the Magic Knight and work with it more on my test unit because in the process of trying to get better combined chroma and luma to give a good composite, I ended up getting a great composite image that in some instance worked better than the s-video out did originally. It was due to changing the values of the resistors but I can't recall off hand which ones I did to make it happen.

 

[marauder666]

The 7800 Juansolo posted showed previously is mine.

 

The only thing from the LHE on this is the automatic CMOS switching on the 2 chroma signals, but used a CD4066, so maybe that has more isolation as its 4 single pole switches, rather than the double pole in the LHE. Needed to generate an inverted signal to toggle the 2600 chroma line.  Didnt even know the LHE did the switching, until I looked at schematic after I added this to mine today.

 

With the chroma disconnected from the motherboard at the pin, theres still that diagonal interference on a PAL 7800, its got 2 sources of it.

Theres another oscillator circuit for the PAL colourburst, which cannot be total isolated from the PAL clock input pin on the MARIA.  Theres a output on the TIA that seems to keep it running.

I totally disconnected this oscillator circuit from the MARIA, and its now running its own tin can PAL crystal oscillator.  The interference is gone.

 

The BAT85 diode on the Magic Knight board really helps the luma balance between the 7800 and 2600 side. 

Heres a close up of Ballblazer, looks fantastic.

 

 

[juansolo]

 

1 hour ago, -^CrossBow^- said:

The LHE suffered from a flaw in that the chroma from the 2600 would still leak through and cause diagonal interference noise patterns when playing 7800 games. This required a separate switch to be wired off the TIA signal line to physically cut the TIA chroma when playing a 7800 game.

The diagonal interference on the PAL systems seems to come from the shared colourburst circuit and running the 7800 side from it's own oscillator pretty much eliminates that.

[juansolo]

Mine is now done. All the interference is gone, it even looks good on a panel.

 

 

It takes two changes, the source of the diagonal dot crawl is the shared colour burst clock circuit. That we solve by pulling R74 (to the left of the MARIA), which disconnects the MARIA from the on-board circuit. We then use a 4.433619Mhz CMOS oscillator to drive the MARIA. Connect a 100nf capacitor across the power pins and connect it to the board. Run the clock out to the top pad of R74. That’s it. Simple as that. Use a socket so you’re not soldering directly to the clock.

 

 

The other source of problems is the blending of the two chroma signals. To get around that we add a little circuit on vero (I know, it’s not pretty…) that auto selects the active chroma signal and only outputs that one. That just needs the two chroma outs feeding to it, power/ground and the trigger signal is pulled from pin 8 of U7 (74LS02 in the bottom right).

 

 

Use all the attached pics as reference.

 

You also don’t need the buffering on the chroma on the UAV, it helps on the Magic Knight board however and it’s also worth swapping the diode as mentioned in the previous posts.

 

 

 

[emmanuelf]

?

[juansolo]

R74 is of course to the right of the MARIA...

[Frank.Bullitt]

Wow I have to say this is exactly what I've been looking for on my REV C board.

 

I also have the UAV mod in mine, are you saying that you now have a clear s video picture with both 7800 and 2600?

 

I'm having issues with certain 2600 games with mine. Does your REV C have that extra compatibility circuit for 2600 Dark Chambers?

 

Well done on making this mod, if you start selling pre assembled kits, I'll certainly be buying one lol

Edited July 11, 2020 by Frank.Bullitt

[juansolo]

21 minutes ago, Frank.Bullitt said:

Wow I have to say this is exactly what I've been looking for on my REV C board.

 

I also have the UAV mod in mine, are you saying that you now have a clear s video picture with both 7800 and 2600?

Picture is clear and excellent for both 7800 and 2600.

 

21 minutes ago, Frank.Bullitt said:

I'm having issues with certain 2600 games with mine. Does your REV C have that extra compatibility circuit for 2600 Dark Chambers?

The compatibility mod for Dark Chambers is snipping a cap, there's another thread about that below.  It doesn't work for Space Rocks or Star Castle sadly and I don't have Dark Chambers to test.

 

21 minutes ago, Frank.Bullitt said:

Well done on making this mod, if you start selling pre assembled kits, I'll certainly be buying one lol

We weren't planning on it. Where abouts are you? If you're nearby we've got a couple of clocks and 4066's left. Could mod yours for you. Either pop in or send it over... EDIT: Cleggy has just mentioned that we could make up the vero and the adapter and do an impromptu bag of bits for you of you want to DIY...

Edited July 11, 2020 by juansolo

[Frank.Bullitt]

Quote

33 minutes ago, juansolo said:

We weren't planning on it. Where abouts are you? If you're nearby we've got a couple of clocks and 4066's left. Could mod yours for you. Either pop in or send it over...

 

Any chance you could send me a kit to solder in myself?

I'd happily accept a PayPal invoice based on what you feel is fair.

 

33 minutes ago, juansolo said:

Picture is clear and excellent for both 7800 and 2600.

There is a thread about adding 820 ohm resistors to improve the colour on both the TIA and MARIA chips, have you tried this in combination with your mod?

 

I don't own a 2600, but I've been messing around with a 7800 since lockdown, got it running on my 4K TV through a Koryuu and an OSSC. Improving the picture quality further would be awesome.

33 minutes ago, juansolo said:

The compatibility mod for Dark Chambers is snipping a cap, there's another thread about that below.  It doesn't work for Space Rocks or Star Castle sadly and I don't have Dark Chambers to test

I'm so glad you linked me to that thread, everything I could find was on the NTSC boards haha. I'm going to install an on off switch for maximum compatibility.

[juansolo]

15 minutes ago, Frank.Bullitt said:

Any chance you could send me a kit to solder in myself?

I'd happily accept a PayPal invoice based on what you feel is fair.

Yep, we can do that. I'll be over Cleggys mid week likely, we'll knock something together.

 

15 minutes ago, Frank.Bullitt said:

There is a thread about adding 820 ohm resistors to improve the colour on both the TIA and MARIA chips, have you tried this in combination with your mod?

That's not required. I expect that's trying to address the chroma blending issues. This mod completely eliminates that.

 

15 minutes ago, Frank.Bullitt said:

I don't own a 2600, but I've been messing around with a 7800 since lockdown, got it running on my 4K TV through a Koryuu and an OSSC. Improving the picture quality further would be awesome.

We've only run it on a small 24" panel. But the improvement is vast. We both run on CRTs normally and it goes from being borderline intolerable to being really, really bloody good.

 

15 minutes ago, Frank.Bullitt said:

I'm so glad you linked me to that thread, everything I could find was on the NTSC boards haha. I'm going to install an on off switch for maximum compatibility.

We're not convinced it'll work on the PAL machine. Unsolder C42 (so you can put it back if needs be) and give it a go with Dark Chambers. It'd be good to know if it works either way. If you find out, please update that thread as it'd be useful info to have.

Edited July 11, 2020 by juansolo

[Frank.Bullitt]

11 hours ago, juansolo said:

We're not convinced it'll work on the PAL machine. Unsolder C42 (so you can put it back if needs be) and give it a go with Dark Chambers. It'd be good to know if it works either way. If you find out, please update that thread as it'd be useful info to have

It definitely improved playability of Haunted House for me.

 

I'll look to install a switch to make the compatibility circuit optional.

 

I wish the screen wouldn't go blank when it refreshes, perhaps I need to try a new TIA chip. Or maybe that's just common place with playing on an LCD.

 

Either way, installing this switch along with this improvement for the UAV will give me something to do with all this extra time I've had lately haha.

[juansolo]

12 hours ago, Frank.Bullitt said:

I'll look to install a switch to make the compatibility circuit optional.

Before you start drilling holes in your case, give us a few days to try something. As I say, we're not convinced that works, but there's potentially something else that will. I just need to get over to Cleggys and take mine apart (again) to try it.

[juansolo]

Anyone else out there with a PAL 7800 fitted with a UAV that wants to try the picture mod and isn't afraid of a little soldering let me know. We've got enough bits to make two kits. One for Frank and one for someone else.

[juansolo]

k treat these layouts as unverified... It's been a while since I've done some so beware! Also I'm not entirely convinced about putting the clock on there so I've done layouts with and without. The only one we've built is the top one, so I'm fairly confident about that. Probably going to build the bottom one to test and drop it into Cleggy's 7800 this week.

Edited July 12, 2020 by juansolo
updated veros slightly

[juansolo]

Frank, you might want to hold off a little while on the kit as we're still experimenting... This is a bit messy on the install side at the moment as it's a bit of a proof of concept, so there's lots of little bits all over the place. But the results are pretty nice. Easily up with the UAV in mine.

 

 

 

The above is essentially an test of an all-in-one layout in it's individual parts currently. I've done a layout, but it's not tested. But for someone comfortable with vero, it could be a sweet fully DIY solution for the PAL 7800.

Edited July 13, 2020 by juansolo

[RevEng]

You guys have done an impressive job of tackling this problem. I'm an NTSC guy, but I'm glad for what you've done for the PAL side of the community, and it's been a fun read too. ?

[Frank.Bullitt]

3 hours ago, juansolo said:

Frank, you might want to hold off a little while on the kit as we're still experimenting... This is a bit messy on the install side at the moment as it's a bit of a proof of concept, so there's lots of little bits all over the place. But the results are pretty nice. Easily up with the UAV in mine.

This looks super impressive, well done.

 

I can wait on the kit if it still needs some work, pretty excited to try it out with my Rikki and Vikki cart.

[juansolo]

1 hour ago, Frank.Bullitt said:

This looks super impressive, well done.

 

I can wait on the kit if it still needs some work, pretty excited to try it out with my Rikki and Vikki cart.

Cheers, there's fine tuning occuring. It's already messing with my layouts... Bah!

 1 hour ago, RevEng said:

You guys have done an impressive job of tackling this problem. I'm an NTSC guy, but I'm glad for what you've done for the PAL side of the community, and it's been a fun read too. ?

Cheers. The PAL machine is really quite different to the NTSC one, as we've now discovered, and has a whole different set of issues to address. We've still got something to try when I get over to Cleggy's mid-week regarding the 2600 homebrew compatibility also. But it's also fun

Edited July 13, 2020 by juansolo

[+mksmith]

Thanks guys for helping to make some improvements for us PAL users ?  I have a UAV in one of mine so might investigate this great work once the dust settles.

 

 

[juansolo]

What I've done to mine is a huge improvement (the seperate clock and little switcher board) with the UAV. Which is the essence of the fixes. If you want to keep the UAV, that'll not change as it's doing the simplest mod of the lot. What we're working on at the moment is an all-in-one PAL layout. Basically if we can get that as good as the setup with the UAV (currently the least hassle and best output of the options), then I'll probably drop that into mine and put my UAV into my 800XL.

 

Cleggy hit a snag with that yesterday that doesn't make any sense. So we'll be having a play with that at some point when I can get over there with mine again to try some things. This pandemic doesn't half get in the way of things! I'll continue to update the thread as we carry on with this.

[davyK]

Just found this thread and it has picqued my interest. I've had a PAL 7800 for a long time and was always disappointed with the RF picture. However I have recently acquired a replacement CRT for the one that died and the picture is noticeably better so the urge has been sated a bit but I'd still like to consider an upgrade.

 

I don't have that many games and I'm considering paying for a modified NTSC machine instead and getting a few of my favourites for a nice curated collection.

 

@juansolo - you mentioned PAL60 games - were you referring to 7800 games there? Or are you just talking about PAL60 ROMS of 2600 games?

 

I've often wondered if PAL games are compromised in the usual way re speed for example..

 

 

[juansolo]

RF hides a lot of what's wrong with the 7800's picture. It's really only when you try to get a good picture out of it you find out just how horrible it is. The PAL machine particularly so. 50 vs 60hz is a thing, so a PAL60 game won't seem slower than an NTSC one, but a PAL50 one will. The NTSC 7800 has a different set of issues to a PAL one, they seem to have a better 7800 picture but worse 2600 one.

 

Interesting day today. Found the source of the weirdness (one of the panels scaling oddly, on another panel it looked fine). Tried some things on mine regarding the homebrew incompatibilty with Space Rocks and Star Castle. Pretty much confirming that the NTSC fix won't work on a PAL machine. On Cleggy's we tried eliminating the buffer on the chroma and using a nice video output IC. That didn't play as nicely as we expected so ended up backing out there.

 

I think the conclusion we've come to today is that if you want the absolute best picture we can find out of it, the solution in post #55 is the ticket. It's relatively easy and requires no setup. We're still going to try the stand alone board, which we now think we've got as good as we can get it. We'll be making that on Saturday and dropping that into Cleggy's machine to test it. It's not quite as good as the UAV solution, but it ain't far off and will be an inexpensive DIY alternative.

[RevEng]

5 hours ago, davyK said:

@juansolo - you mentioned PAL60 games - were you referring to 7800 games there? Or are you just talking about PAL60 ROMS of 2600 games?

There's no PAL60 games in 7800 mode, just 2600 mode. In 7800 mode Maria does all of the sync signalling, and you don't have an option to override this as a programmer.

 

Games can be programmed to compensate (e.g. the Salvo homebrew increases enemy and player speeds if PAL is detected) but otherwise NTSC games run slower on PAL decks. Some of our PAL folks also have spare NTSC decks, for this exact reason.

[juansolo]

Mini-update today, Cleggy has been fine-tuning his dogs breakfast install which will ultimately become the standalone board. Results are pretty good. I've also bought another PAL 7800, because we realised we didn't have a 'stock' point of reference. Our two 7800 have been so fiddled around with now we've lost track of where we started...