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Investgating PAL 7800 picture issues (was 7800 RGB Musings)


juansolo

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Fantastic work, Juan!  Thanks to you and Cleggy for sharing this with the community.  It is appreciated!

 

Regarding tuning the color pot for 7800 mode on NTSC consoles, the Diagnostic Cart ROM can be leveraged...

 

Binary: Diagnostic Test Cartridge (19xx) (Atari) [!].bin With Header: Diagnostic Test Cartridge (19xx) (Atari) [!].a78 

 

When the ROM is started there is an ~8 seconds delay showing a blank screen while it runs the diagnostic.

 

It is available for purchase as a stand alone cart at Best Electronics, or the above files can be used with a flash cart accordingly.

 

Spoiler

Atari 7800 US Diagnostic Cartridge.  Will display a single 7800 Diagnostic 7800 boot up Screen.  Necessary to adjust the 7800 top color pot back to factory color settings.  You plug in the Atari 7800 Diagnostic Cartridge and turn on your Atari 7800 console and wait about 5 to 10 seconds for the 7800 Diagnostic screen to appear.   CB101195 $25.00

 

Ideally, this ROM should be leveraged starting with a console 'cold' (powered off for at least 20 minutes), and tuned within the first 30-90 seconds of the unit being powered on.  All that needs to be done is make the right bar solid.

 

 

 

 

 

Here is a simulation with the color swatches labelled, demonstrating the COOL, WARM, and HOT temperature differences of the system:

 

Atari 7800 Diagnostic - COLD - WARM - HOT.gif

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That'd be super useful, but I assume like the colour bars it's NTSC only. Which is a shame as this machine will be winging it's way back to Mimo shortly when we're done with it and we don't have a flash cart for the 7800 (which would be lovely, I've been watching the Concerto updates for ages hoping...).

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1 hour ago, juansolo said:

That'd be super useful, but I assume like the colour bars it's NTSC only.

The Diagnostic Cart is NTSC only.  However, the principle found with it for tuning the color pot...Matching the colors of $1x and $Fx, under a cold system, within 30-90 seconds of it being powered on...can be used with other color bar/palette ROMs for the 7800. 

 

For example, RevEng's Utility Cart can be utilized under NTSC or PAL, and has the "MATCH COLOR" at the bottom, making it super easy.  Plus there is the added benefit of a slew of tests that are included checking audio, memory, controllers, and more:

 

image.thumb.png.924c716eac90c9f2efda5da01e83114a.png

 

BINARY: 7800 Utility Cart (20161210).bin  WITH HEADER: 7800 Utility Cart (20161210).a78

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  • 3 weeks later...
8 hours ago, Icelvlan said:

Is there an RGB mod in the works?

Tim Worthington (does the NES, PAL N64 and 2600 RGB mods amongst others) has mentioned he's been working on one for pretty much years now. I'd love it and would be the first down for one (his 2600 RGB mod is magnificent), but given the nature of the 7800 I wouldn't hold out for it.

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2 hours ago, chicgamer said:

SainT just mentioned on Twitter that he's finishing up an RGB solution for the 7800:

 

"I think the RGB mod board is now complete. Last bit of polish then off to get some PCB's made."

 

Pretty exciting news!

:o

 

Well, it goes without saying I'm in!

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  • 2 weeks later...

A small amendment to the doc. We picked up an S-Video to RGB box to do some testing with which allowed us to compare the levels of the UAV to the All-In-One mod and found that the MARIA chroma levels were a bit too high. Adding a 1K6 resistor to the Maria Chroma line seems to bring it down to a much nicer level. If you've got an all-in-one mod it's a simple add, just put it on the line on the MARIA chroma in. I've added it to the layouts in the doc anyhow, so if you build the mod on the 2.1 instructions, it's got you covered. This doesn't effect the UAV add-on mods in any way.

 

AntiJackModDoc_v2_1.pdf

Edited by juansolo
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More level tweaking, this time to the TIA chroma, this time an in-line 820R resistor. Again, this only effects the all-in-one mods and not the UAV add on mods. Also it's not mandatory, it's just something we noticed now we can more accurately compare the UAV output to the all-in-one.

AntiJackModDoc_2_2.pdf

Edited by juansolo
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  • 3 months later...
On 11/13/2020 at 10:37 AM, juansolo said:

Also it's not mandatory, it's just something we noticed now we can more accurately compare the UAV output to the all-in-one.

Is any part of the AntiJack upgrade useful in helping picture quality on a SCART (Péritel) RGB Atari 7800? That model doesn't seem worth a UAV upgrade (downgrade?), but if something here may help with the RGB video board output that is interesting.

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1 minute ago, MrZarniwoop said:

Is any part of the AntiJack upgrade useful in helping picture quality on a SCART (Péritel) RGB Atari 7800? That model doesn't seem worth a UAV upgrade (downgrade?), but if something here may help with the RGB video board output that is interesting.

We've never had one to play with so we just don't know. Given how different the PAL machine is to the NTSC, we've no idea what they did in the French 7800. The signal is Luma + Chroma so it must be converted to RGB at some point. Whether it has the same problems as the PAL machine for the bit in the middle we just don't know. We tried to grab a French machine a while back for buttons but sadly it was pulled from sale. :(

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20 minutes ago, juansolo said:

Given how different the PAL machine is to the NTSC, we've no idea what they did in the French 7800. The signal is Luma + Chroma so it must be converted to RGB at some point. Whether it has the same problems as the PAL machine for the bit in the middle we just don't know. We tried to grab a French machine a while back for buttons but sadly it was pulled from sale. :(

My impression by reading other descriptions and poking around the inside is that it's a normal PAL 7800 with the RF modulator ripped out and a RGB Encoder daughterboard (Atari part #C301645-001, using Sony V7021 video decoder per Curt Vendel) in its place. The video quality is good, but UAV S-Video on an NTSC 7800 is better.

 

Some good pics on this site and this AtariAge post.

 

They do pop up on eBay now and then.

Edited by MrZarniwoop
added link to Sony V7021 specs on Internet Archive
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Yep, that's a PAL machine alright so it'll have the shared clock problem creating the vast majority of the interference.

 

You could potentially get rid of most of the problem with a clock (4.433619Mhz CMOS oscillator) with a 100nF cap across the power pins, pulling 5v and ground from whereever on the board then pulling R74 from the 7800 and running the clock out to the upper most pad.

 

Pretty easy to do and might make a big difference.

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  • 3 weeks later...

More PAL 7800 oddness that I didn't think warranted a new thread so I thought I'd tack it on the end here.

 

I keep buying PAL 7800's that are broken/spares or repair jobs. I just see any 7800 that's less than twenty quid as a potential ressurection job (that's oddly satisfying) or otherwise, spare parts. So grab them when they appear. As it was I got another recently (that must've had a reversed polarity PSU plugged into it at some point as the large electrolytic on the power in had gone pop), and it's proven something we noticed from an earlier one: The later PAL 7800's are not very nicely made...

 

By that I mean they use dreadful solder. It predates unleaded but it still requires an uncomfortable amount of heat to melt and it's generally horrible stuff to deal with. What seems to exacerbate this is that the PAL machine, certainly the later ones do not really have much in the way of pads on the PCB, relying almost completely on the through hole. So horrible solder + weedy pads = quite a few ripped off pads. Which is somewhat frustrating.

 

Add to this the latest problem... Cleggy has been having problems with his 7800. We've already determined that PAL 7800's are a bit variable to say the least. Mine is a bit of an anomaly in that it appears to play almost anything, including 2600 homebrew. Most struggle with that and Cleggy's 7800 took that one step further by being somewhat compatible with the Dragonfly, but with induceable crashes. Now it plays all 7800 carts and most commercial 2600 carts no problem, but the problem consistently followed the 7800, so I decided to pick up another to experiment.

 

At this point he was about to throw in the towel as the DF didn't work at all with the new one, until we found that the cup for the cart not only had sprues still in there, it was mis-sized and the cart wasn't getting fully home. I had to dremel out the locating pin holes by a fairly dramatic amount (probably took 1mm out of the inside of both sides) then I had to go around the outside to take more to take the girth. of just a normal cart. That came to light when even with normal carts it was stupid tight.

 

Finally the good news is that after fairly significant dremelling, a lot of swearing at the soldering, and the usual video mods, Cleggy now has a machine that appears to behave itself. But wow, what an absolute basket-case of a machine! I'd think it was a one-off had we not had one that was not quite this bad beforehand. But it looks like a bit of a trend with the later PAL 7800s.

 

If you can @marauder666, see if you can get a decent pic down the slot with a flash to show what I had to take out of it (PM it to me). It's a bit of a shocker just how unbelievably tight it was.

 

In other news, we've got @MrZarniwoop's French 'RGB' 7800 on the way. This should be really interesting and I'll certainly update the thread on that one. The clock mod alone might be all that's required. But we shall see!

Edited by juansolo
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Hard to see, but the flash of the molding was still inside the plastic cup. Holes and prongs dremeled to easily accept the prongs on the Dragonfly.  

The build quality on these later PAL 7800s is pretty bad.  I was shocked to see even the plastic moulding of the cup and slot aren't as good as earlier ones.

 

The French one will be s-video or composite into an internal converter, I'm 99% certain.  The converter won't be as good as the Canon we have.

 

Had the Canon apart yesterday and added a sync stripper circuit to the output, so it works through the Extron switcher which doesnt work with sync over composite video (SCART standard).  It's now CSYNC output.

 

Interestingly, it's all Sony parts inside, some dated 1991, in a 1996ish Canon product.  Bet Sony sold them the PCBs from an earlier product they made.  Its very well made.  Was hoping to find a jumper on the board labelled NTSC/PAL, but no luck.

 

P1050706.JPG

Edited by marauder666
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3 hours ago, juansolo said:

In other news, we've got @MrZarniwoop's French 'RGB' 7800 on the way. This should be really interesting and I'll certainly update the thread on that one. The clock mod alone might be all that's required. But we shall see!

I was asked to look at that 7800 first but suggest to @MrZarniwoop that he contact you since I've not worked on any PAL units and am not as familiar with them. Additionally I only have one display in my house that can display PAL and it is my small PVM that is composite and s-video only. In other words, I felt @juansolo would be the best person for the job in this case.

 

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  • 2 weeks later...

We've had @GDT2019's 7800 in to give it a service and swap out the comp mod for a AJM+UAV. There's been some interesting things to note:

 

- This one had lovely nice and easy to deal with solder, unlike the last few we've had.

- Despite the last 4 or so 7800's all being the same board revision, they all behave a little differently. In this case this one behaves like Rusty (my original 7800) when it comes to the Dragonfly cart. In that original carts seem ok in it, but some images on the DF crash. BallBlazer being the noteable one as the original cart runs just fine. Just noticed it was the same with Food Fight today. There's just something odd about this machine. Both the ones with these identical symptoms have a pair of bodge resistors underneath. They don't seem related, but mine (the one that works fine and plays most 2600 homebrew, but has a shitty 2600 picture) doesn't have them, and nor does Cleggy's super horrible solder one (the other one that works fine but doesn't like 2600 homebrew, though has a great 2600 picture).

- This was another one with a poorly formed cartridge guide that needed some love with the Dremmel to get the DF cart to sit nice in there. It was also uncomfortably tight with with some original carts.

 

Anyhow, a hard to track down fault aside (a fet connected to the TIA had a burned up pad on it that was causing the TIA output to be very low and therefore the 2600 picture was super shitty, solved with another bodge wire...), this one now has a revised version of the AJM+UAV mod on it. We've just simplified it, so I thought it warranted another post. Much easier to make as rather than the IC, it's now running a 5v non-latching relay for the chroma switching.

AJMv2-UAV.thumb.jpg.cbdbc35f3005e2ee8f113362f6b8cda5.jpg

The audio I've not put on the board to keep it as small as possible. It's super easy to add in as you see fit regardless. Otherwise, the usual work done here: re-cap, replace buttons, swap reg, replace power jack, tidy up and what have you.

 

GDT-7800-1.thumb.jpg.d7c96d676392aa52171e82166f550279.jpg

 

Pics of the screen, they actually look better in person. You still get the colour running on certain dark colours, that just seems inherent on the PAL 7800. It's still massively better than stock.

 

GDT-7800-2.thumb.jpg.5a4c67e78a080a72777d1d68a22628e1.jpg

 

GDT-7800-3.thumb.jpg.d135e3cf0b4c723225544b0f17493977.jpg

Edited by juansolo
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31 minutes ago, juansolo said:

Pics of the screen, they actually look better in person. You still get the colour running on certain dark colours, that just seems inherent on the PAL 7800. It's still massively better than stock.

 

GDT-7800-2.thumb.jpg.5a4c67e78a080a72777d1d68a22628e1.jpg

 

 

The 'shadow's happen on the NTSC unit as well. Pitfall and other games is lots of contrast between the colors seem to show it more.

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4 minutes ago, -^CrossBow^- said:

The 'shadow's happen on the NTSC unit as well. Pitfall and other games is lots of contrast between the colors seem to show it more.

Yeah, it's particularly visible on a panel also. For what it's worth, it has a bloody lovely 2600 picture that one. Given that before we tracked down the fault it had the worst, it's flipped the other way now and is probably the best of the lot. The 7800 is as the [PAL] 7800 usually is; some stuff looks amazing, some stuff you still get the colour running on. The only way around that it seems is to output the S-Video signal through an S-Video to RGB transcoder (which is what I do) that almost eliminates it. It's also why we're really interested in the French machine and whether or not we can get it looking good. Because it's essentially doing the same thing, but internally.

Edited by juansolo
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