Investgating PAL 7800 picture issues (was 7800 RGB Musings)

[juansolo]

Have a schematic Should make a lot more sense than the veroboard.

[juansolo]

Took my main 7800 apart to make some measurements. There were no pics of it in it's final form because it's a little scruffy, but I thought I may as well while I'm in there.

 

 

I would love to see the schematic for the UAV, ours is obviously a very simple circuit, but I'd like to see what it's doing as it does do some things better so the above concoction is a best of both worlds. We even contemplated making a board that you had our mods on it plus then you could plug the UAV in, but the marginal improvement just isn't worth the cost of them together here (a UAV here is £35, plus our bits...). As it is, ours will do a better job than a UAV on it's own with a PAL machine.

[ckrtech]

@juansolo Would you mind summarizing your discoveries, benefits, and overall knowledge about the chroma component of the 7800 - your intentional switching of the TIA/Maria color, your relay circuit, and the benefits gained?

I read over the thread recently, but hindsight makes it easier to collect one's thoughts - provided you have the time for it, of course.

[juansolo]

We've got a couple of things going on with this at the moment (and also we're fiddling around with some a pair of Cassette Vision consoles and a Fairchild clone right now that are taking up some time). If it all comes to fruition, there will be a new updated doc with it all in. Might be a few weeks though to allow for testing and what have you. The long and short of it is (from the old doc):

 

There are two major problems with the PAL 7800 that result in it’s frankly god awful picture. The first is that the PAL 7800’s MARIA shares a clock with the TIA. This causes the majority of the diagonal dot crawl interference. Secondly the MARIA and TIA chroma signals are blended. So even though you’re only really using one at a time (either 2600 mode or 7800), they still have an effect on one another and getting a good picture from one comes at the expense of the other. The AJM addresses these issues by disconnecting the MARIA from the on-board clock, running it from it’s own stand alone clock. Also by adding an automatic switch that selects the active chroma signal and grounds the inactive one, outputting only one signal and eliminating any crosstalk.

 

There is a further issue that persists that results in colour running on some darker colours. It's most noticable on Pac-Man Collection's blue mazes and the Commando intro screen. It seems to be inherent in the MARIA and is effected by heat. Heating the MARIA briefly with a hair dryer can slow then stop the running, then it starts going the other way if you keep heating it. As it cools the process reverses. The degree to which this is apparent varies greatly from machine to machine. You see it with both our mod, and the UAV. My main machine is pretty mild for it, and on PMC, if I run it through an RGB encoder it pretty much disappears. But it's still noticable on Commando.

Edited May 5, 2021 by juansolo

[ckrtech]

2 hours ago, juansolo said:

It seems to be inherent in the MARIA and is effected by heat. Heating the MARIA briefly with a hair dryer can slow then stop the running, then it starts going the other way if you keep heating it. As it cools the process reverses. The degree to which this is apparent varies greatly from machine to machine.

Wow. That's wild.

 

I look forward to your updated document. I am wondering about "re-clocking" the TIA chroma on an NTSC Atari 7800 to see what effect it has on 2600 chroma output. The MARIA outputs the TIA CLK line to the TIA chip (by way of a 100 ohm resistor) on the NTSC console. Perhaps it is possible to clean-up color backgrounds among other things for 2600 games on an NTSC 7800.

[juansolo]

The NTSC 7800 is quite different and has far less issues (though jailbars on the 2600 side seem more prominent there). Where the PAL machine has a single clock and splits it between the TIA and MARIA. The NTSC machine sends the clock to the MARIA and the MARIA then sends it on to the TIA, a much better way of doing things. We didn't see the same kind of interference on the NTSC machine we had for a brief time, but it is far, far better than the PAL machine in terms of picture quality (I cannot emphasise enough that the PAL machine in standard form is a travesty). It still shares chroma so there are gains to be made there, but the situation is so much better that any improvement probably isn't worth the effort over just fitting a UAV, which is incredibly easy. The colour rolling thing, no idea if the NTSC MARIA has the same issue, we didn't have one long enough to really do any investigation there.

 

Again, if one turns up here at reasonable money, I'll buy one to play with. But sadly they're like rocking horse poo these days on this side of the pond. Importing one is also expensive now, where I can (and do, I'm watching one now that's looking suitably crusty) buy broken PAL 7800's for around £20, I'd be looking at way more than £100 for a US machine sadly.

[juansolo]

That said...

 

I've just bought an NTSC machine on Ebay US... Cheap enough, though by the time it gets here it'll end up costing about £140 or so. Which pains me slightly, but I'll sell Rusty once it lands to cover some of that. It's the usual spares/repair machine at a somewhat inflated cost than I'm used to, but hopefully it'll be a runner.

Edited May 27, 2021 by juansolo

[stfouill]

Hi all !@juansolo you said taht maybe you will do a PCB for the modification, any news ?
I guess I'm not the only one to be interested

Envoyé de mon BLN-L21 en utilisant Tapatalk

[juansolo]

2 hours ago, stfouill said:

Hi all !@juansolo you said taht maybe you will do a PCB for the modification, any news ?
I guess I'm not the only one to be interested

Envoyé de mon BLN-L21 en utilisant Tapatalk
 

It's still on the cards, we've just been a little distracted. I'll update the thread when things happen.

[stfouill]

It's still on the cards, we've just been a little distracted. I'll update the thread when things happen.

Good news thanks !
The thread is huge now, I tried to read it quickly, I think the mod is compatible with French version (which is a PAL with an additionnal board if I understood correctly) am I right ?

Envoyé de mon BLN-L21 en utilisant Tapatalk

[juansolo]

6 hours ago, stfouill said:

Good news thanks !
The thread is huge now, I tried to read it quickly, I think the mod is compatible with French version (which is a PAL with an additionnal board if I understood correctly) am I right ?

Unfortunately not. The French machine is a little beyond saving I'm afraid unless you bypass the RGB encoder and output S-Video or composite video. The onboard encoder is very rudimentary and butchers the signals being put into it. We managed to make it slightly better, but it's hardly an improvement when the composite output is way, way better.

 

If you need a PAL machine and you've got a TV/monitor that can take S-Vid or composite, you're better picking up a non-RGB PAL 7800 and modding that. If you must run RGB... Run a modded non-RGB PAL 7800 into a Canon RGB100 (https://www.radiomuseum.org/r/canoninc_rgb_converter_rgb_100.html) external RGB encoder (which is what @marauder666 and I are doing). The results are very good in that scenario. They occasionally turn up on eBay. I think we paid about £40 each for ours.

 

If you're attached to that specific 7800 and want to bypass the RGB to output one of the other standards, that's totally possible. But you'd obviously be removing all the RGB circuitry and the associated bodges done to the 7800 to make it 'work'.

[stfouill]

6 hours ago, juansolo said:

Unfortunately not. The French machine is a little beyond saving I'm afraid unless you bypass the RGB encoder and output S-Video or composite video. The onboard encoder is very rudimentary and butchers the signals being put into it. We managed to make it slightly better, but it's hardly an improvement when the composite output is way, way better.

 

If you need a PAL machine and you've got a TV/monitor that can take S-Vid or composite, you're better picking up a non-RGB PAL 7800 and modding that. If you must run RGB... Run a modded non-RGB PAL 7800 into a Canon RGB100 (https://www.radiomuseum.org/r/canoninc_rgb_converter_rgb_100.html) external RGB encoder (which is what @marauder666 and I are doing). The results are very good in that scenario. They occasionally turn up on eBay. I think we paid about £40 each for ours.

 

If you're attached to that specific 7800 and want to bypass the RGB to output one of the other standards, that's totally possible. But you'd obviously be removing all the RGB circuitry and the associated bodges done to the 7800 to make it 'work'.

I don't know if I am attached to that specific 7800 but I want to give this grand'ma her chance to improve the video output - which is indeed totally crappy with the RGB encoder.

My TV is compatible with S-Video so I will try to bypass completely the RGB encoder as you proposed. I have to check beside the encoder what bodges are done to the motherboard and if they are easy to undone...

[juansolo]

There were a few bodge resistors on the back of the board on the one we looked at (that were in the wrong places!), you'd want to pull those and replace the parts they'd swapped with what should be there (I think they'd put caps in the place of the resistors on the top of the board). Otherwise it'd be unplugging the RGB mod and doing the AJM as normal.

 

We'd be quite happy to do it for you once we've got the boards sorted if you like. I'm still a little iffy when it comes to our mod given people's soldering experience. Most people are ok when it come to the UAV becuase you're just adding wires to existing parts. Ours you have to pull parts from the board and that's where most people come unstuck as de-soldering can be a bit of an art... Though on the plus side, you wouldn't have the modulator to deal with!

[juansolo]

Took a while, but this has finally landed. There's always a moment why you buy a spares/repair machine where you're trying to figure out the amount of work it'll take to get it going, this is good news (it's not in colour because it's not an NTSC compatible panel). It has potential to clean up nice.

 

Mainly this'll become my NTSC 2600/7800 machine, but also a test bed for video mods. I haven't decided what I'm going to do yet... Our mod will go on, but there's a temptation to UAV it first to get a baseline, then mod the UAV, then fit our mod... Maybe, UAVs are pricey here now, so I might just skip it and go straight to our mod. If so, I might end up holding on until we have a PCB. We'll see there. The other thing is whether or not to Asteroids BIOS mod it or not. It'll get all the usual stuff as part of the restore anyhow; new buttons, cap & reg swap, and what have you.

 

 

 

EDIT: Bugger! Well there's one repair needed.

 

Edited June 7, 2021 by juansolo
Me crack was showin'

[juansolo]

Well I'm guessing this didn't come from one of the dryer states...

 

 

Crunchy...

 

 

Stickers upon stickers. Two serial number stickers with a Made in China stuck over the Made in Taiwan on top of that!

 

 

I'm already loving this one. It's a worthy replacement for the basket case that Rusty was!

Edited June 7, 2021 by juansolo

[MattelAquarius]

41 minutes ago, juansolo said:

Stickers upon stickers. Two serial number stickers with a Made in China stuck over the Made in Taiwan on top of that!

 

This is oddly fascinating to me. 

 

Was the original case made in Taiwan where it received the first Ser# sticker, but then not completed before being shipped to China for final assembly?

 

What, exactly, was printed in Hong Kong?  Was it the second Ser# sticker or the tiny "Made in China" overlay sticker?  If it was the latter, did trade rules really require the sticker to be extended just to say it was printed in Hong Kong, or was that a matter of Hong Kong pride?  Were those tiny overlay stickers shipped from Hong Kong to mainland China for these specific cases that were originally made in Taiwan, but shipped to China?  Why couldn't those tiny overlay stickers be printed in China, to save the hassle and need for extending the length long enough to tell us where it was printed?

 

My circuits are overloading worse than Mudd's women! 

 

[juansolo]

We've had worse. Indeed modulator aside, it's quite clean under the hood.

 

 

Do you think I should document the restoration of this in another thread? Just wondering if people would be interested, and I'm wary I'm littering this one a little (even though it's mine :D) with marginally unrelated content (it will get back to the video mods shortly!). I'm just excited to have a new 7800 to play with...

Edited June 7, 2021 by juansolo

[stfouill]

On 6/6/2021 at 8:05 AM, juansolo said:

There were a few bodge resistors on the back of the board on the one we looked at (that were in the wrong places!), you'd want to pull those and replace the parts they'd swapped with what should be there (I think they'd put caps in the place of the resistors on the top of the board). Otherwise it'd be unplugging the RGB mod and doing the AJM as normal.

 

We'd be quite happy to do it for you once we've got the boards sorted if you like. I'm still a little iffy when it comes to our mod given people's soldering experience. Most people are ok when it come to the UAV becuase you're just adding wires to existing parts. Ours you have to pull parts from the board and that's where most people come unstuck as de-soldering can be a bit of an art... Though on the plus side, you wouldn't have the modulator to deal with!

I'll check the bodge and cross check with PAL schematic !

Thanks for the proposition, I'll try myself I think, I already have soldered Hi-Def NES, SOPHIA and such kits which require some solder/desolder skills also ?

(or let's say I fear postal services more than my solder skills haha ?)

[juansolo]

Science has been happening!

 

The heat related colour rolling thing that goes on with the PAL MARIA we decided to quantify by sticking a temperature probe to it, and taking a load of measurements. Essentially what we had observed beforehand was that the colour rolling could be stopped by heating it up with a hairdryer to a certain temperature, going beyond this temp caused the colour to start rolling the other way. For no other reason we decided to find out what those temps were and if we could get the MARIA to stay at a temp that stopped the rolling...

 

The MARIA cold / at room temperature was 70F / 21C

 

It takes approximately 15 minutes for the temperature to stabilise (under load) at 115F (+45F) / 46C (+25C)

 

The 'Happy Temp', achieved with a hairdryer,  where the rolling stops on this particular machine is 134F (+19F) / 57C (+11C)

 

None of these are horrific temperatures. Indeed the MARIA was still the hottest component on the board, that includes the regulator.

 

So we thought we'd try some ways to raise the temperature of the MARIA to see of we could get it to it's happy temp. Now obviously don't go doing this as getting ICs hot on purpose is generally a bad thing. So with a bit of foam as cladding, we could raise the temp by 11F where it once again stabilised. No matter what we tried beyond that we couldn't get it to come up the remaining 8F, concluding that it would require active heating at this point and the whole thing was getting a bit silly.

 

Still thought it might be interesting

Edited August 28, 2021 by juansolo

[+MrZarniwoop]

The French PAL 7800 with OEM RGB encoder is now “home” in Vilnius where I’m visiting. I totally see why you guys wanted to bypass the RGB, but I couldn’t do it to such a rare 7800. Still fun to play, and it does look a bit better than before you tweaked the PAL signal in front of the RGB.

 

Merry Christmas everyone.

[+-^CrossBow^-]

39 minutes ago, MrZarniwoop said:

The French PAL 7800 with OEM RGB encoder is now “home” in Vilnius where I’m visiting. I totally see why you guys wanted to bypass the RGB, but I couldn’t do it to such a rare 7800. Still fun to play, and it does look a bit better than before you tweaked the PAL signal in front of the RGB.

 

Merry Christmas everyone.

Why is the picture stretched? I'm not talking about the LCD aspect ratio.. I mean the entire picture? Look at the 7800 as an example. Like a wide angle mode was used?

 

And Vilnius as in Lithuania?

 

 

[+MrZarniwoop]

iPhone 0.5x wide angle lens because it was so close. TV mode is vertical stretch to try and fill the screen without too much distortion. Kids. ?

 

Vilnius, Lithuania for the holidays. New home for the French 7800.

[juansolo]

It pains me what that encoder does to the signal that's fed into it. All we could really do was eliminate some of the usual PAL interference you get from the colourburst and the chroma crosstalk. So it's better than it was, but it still hurts... It is what it is.

 

For an update on our own machines in this thread. Cleggy sourced himself an NTSC 7800 and so did I (There's another thread for what we did with those). Cleggy sold his PAL machine, Rusty went, and I'm now trying to decide what to do with mine as because it's super upgraded and running into an external RGB encoder, it's actually pretty much as good as it gets in PAL land. But since I got the NTSC 7800, that think just kills it and it did so with relatively little effort (it wasn't without it's quirks). Turns out I only had 5 PAL only homebrews so I've stuck those on ebay (my few commercial PAL games can go whenever, they're worth very little). If I can reclaim some cost from them and replace them, then that's my last reason to keep my PAL box. Which is a shame as it's one of the good ones*.

 

Sadly, because of the variability of the PAL 7800 and it's staggering flaws, the only conclusion can be; if you want a good 7800 experience on real hardware, import a NTSC machine, slap a UAV and chroma shifter in it, and you're pretty much done.

 

*I've not even mentioned the weird thing that a great many of them do with a DF cart... Getting one that doesn't have that quirk seems to be completely random as we currently have 3 seemingly identical late PAL 7800's in, and one of them is a magic one that works. It also seems to correlate that these are the same ones that play a chunk of 2600 homebrew that won't on the others. It would be interesting to see if the new BIOS actually fixes those issues. But we may never find out if we clear out all these consoles before it lands.

 

Oh and I've just picked up a Schrödinger 5200 also. Got that apart and bodged some power on, plugged it into the only TV that gets some sort of picture out of an NTSC signal from RF to see if it worked as the seller genuinely had no idea. It turns out the 5200 in the box was indeed alive. So that's nice. Just going to drop a Sophia RGB into that because my NTSC 7800 is occupying the S-Vid input and I've a single RGB free on my Extron. Though Cleggy picked a 5200 up also and we fettled our own mod for his with actually bloody lovely results... Again, I should probably start a thread but our mod aside, it's not anything that hasn't been done before.

Edited December 23, 2021 by juansolo

[+MrZarniwoop]

Atari 7800 Christmas Eve in Europe

 

[+Muddyfunster]

Love it!