Investgating PAL 7800 picture issues (was 7800 RGB Musings)

[juansolo]

Yeah, I was kinda hoping it wasn't that as I've got some RAM but no RIOT or CPU... I'll socket everything and take it from there. Cheers dude.

 

EDIT: Cleggy informs me that we have a spare RIOT, so may as well start there. Fingers crossed.

Edited September 3, 2023 by juansolo

[juansolo]

Welll expected to spend today socketing up my 7800 to work through possibilities as to what had gone south. Popped out the Riot, dropped it into our socketed test 2600jr. RIOT deed. Interestingly running the 7800 without the RIOT and the pic was ok (obv other things stopped working). Cleggy probed the address lines and one of them was completely borked. It would have been a case of swapping every IC connected to it to find the prob, but thankfully the first thing we pulled was it. Handily we have a couple of good spare RIOTs so popped that in and we're off and rocking again.

 

So, feeling confident we went to my spares PAL 7800. This is one we bought as a spares/repair job that was just a complete nightmare. Someone had tried to mod it in the past and had just burned up everything... I was just using it as a chip donor to this point, but we decided to put our mod onto it because, well, we had one left. It didn't go quite as swimmingly, and it's bodges galore as a machine. But eventually it burst into life (Cleggy had to do some fettling after I left as one of the bodges was connected to the wrong luma and was causing problems). Now just have to swap a few chips back into it, write a BIOS chip (one of the many things that was wrong with it and it's finally prompted us to buy a burner) and I'll have another machine to drop into one of Bob's 3D printed boxes that we can use for testing. Nice!

 

There's a machine hopefully incoming that I'm going to document the full PAL install on. The mod seems to work best there anyhow. But that'll start to happen when it lands.

[+-^CrossBow^-]

20 minutes ago, juansolo said:

Welll expected to spend today socketing up my 7800 to work through possibilities as to what had gone south. Popped out the Riot, dropped it into our socketed test 2600jr. RIOT deed. Interestingly running the 7800 without the RIOT and the pic was ok (obv other things stopped working). Cleggy probed the address lines and one of them was completely borked. It would have been a case of swapping every IC connected to it to find the prob, but thankfully the first thing we pulled was it. Handily we have a couple of good spare RIOTs so popped that in and we're off and rocking again.

 

So, feeling confident we went to my spares PAL 7800. This is one we bought as a spares/repair job that was just a complete nightmare. Someone had tried to mod it in the past and had just burned up everything... I was just using it as a chip donor to this point, but we decided to put our mod onto it because, well, we had one left. It didn't go quite as swimmingly, and it's bodges galore as a machine. But eventually it burst into life (Cleggy had to do some fettling after I left as one of the bodges was connected to the wrong luma and was causing problems). Now just have to swap a few chips back into it, write a BIOS chip (one of the many things that was wrong with it and it's finally prompted us to buy a burner) and I'll have another machine to drop into one of Bob's 3D printed boxes that we can use for testing. Nice!

 

There's a machine hopefully incoming that I'm going to document the full PAL install on. The mod seems to work best there anyhow. But that'll start to happen when it lands.

I've heard others report about RIOTs being problematic on NTSC 7800s, but that hasn't been my experience and I've held a lot of NTSC 7800 consoles. But given the description of what was happening it seemed to be the culprit. 

 

For the record, I've only had one Maria that I know was bad that I had to swap out (Was missing some of the lum signals as the colors were just wrong). I've had to replace about a half dozen TIAs in 7800s, and 3 6502 CPUs at this point. I can't recall a RIOT being an issue in a 7800 I've serviced so for me, they appear to be pretty robust compared to the TIA and CPUs. Glad you got it going again!

[nobru]

Hello,
I'm new to the forum and a fan of ATARI 800,2600 and 7800.
The modifications on my Atari are:
Sophia on A800
Etim 2600RGB on A2600
and on my French 7800, I look at your excellent forum to choose a composite video modification or Svideo of better quality than the Atari RGB.
for juansolo:
I wanted to make your modification ANTIJACK 7800MOD (clock,CD4053).
But your last modification (7432, CD4050 and FMS6400) seems simpler and also effective for Svideo.
Can you share your last circuit diagrams to make it in proto mode on my 7800 (PAL/FRENCH)?
Thank you for all these test pages and console modifications.
attached 2 photos of my 7800.
I changed the RAMs (Hack french) by 2016 (6116).
I installed a bi-bios on 27256 (PAL or KILOPARSEC) switchable.

Sorry again for my French/English translation.

[juansolo]

Hi @nobru

 

We're in the process of documenting the current mod. We've done it to 3 PAL machines now and thats where it seems to be working best also. @Frax_Excelsior is sending us his machine, once that lands I'm going to use it to make a full installation guide as this mod isn't straight forward. When that's done, another board is going over to @karri and he's going to have a go at fitting it himself. When that's proven and I have a bit more confidence in how difficult the install is then I'll potentially have some more PCBs made up. It'll be a couple of weeks or so I'd expect. Watch this thread.

[juansolo]

Found an error... Will try again later.

Edited September 14, 2023 by juansolo

[juansolo]

As if it were neccessary to reiterate the variability of PAL 7800's. See if you can tell what's different about these two (which are the reason I pulled my last post):

Meet Cleggy's PAL 7800. This is a really early example and of notably better quality than any of the other ones we've had through our hands. It pretty much will run anything. It also was the first to receive the new mod.

This machine is mine, I got it for buttons (like about £18 delivered IIRC) because it was very broken. Someone had tried to mod it and had really made a bit of a pigs ear of it. Pads and traces long gone, odd bodges. I pretty much bought it as a parts machine for that money until Cleggy decided to see if he could resurrect it. It took a LOT of effort and it's really ugly up close (particularly on the back with lots of bodge wires), but it lives again. Which is great, but it threw a massive spanner in the works today...

 

As you can see, there's no oscillator fitted to this one. Because we found that if we actually dropped the signal of the colourburst from the shared clock where it goes into the MARIA, it pretty much negated the diagonal interference. Thinking we just made the mod even easier we of course did it to the other two... Which it doesn't, like not even close doesn't. Why this works on this one we just can't explain. The MARIA in it does run hot. Which we've found on the PAL machine can help slow the colour rolling, but that's a separate issue from the interference so it's not that.

 

But that led us down another path with regard to the signal coming out of the Oscillators on the other two machines that was WAY higher than what we were putting into mine... So we tried pulling that down, which actually made Cleggy's machine work on a RGB encoder than only my machine will work with. K, so maybe this is a good thing. So we apply that to the other two machines that need the oscillator and start testing again running through all the TVs we have at our disposal and the bloody intereference is back with the separate oscillator.

 

As you can tell, it's been a fun day. We've backed out the resistor on the oscillator equipped machines and they're back with no interference. Mine we simply can't explain. It's the first PAL machine we've ever encountered that we could dial out the interference with the on-board shared clock by swapping the 2K resistor on the voltage divider to a 1.5K. It makes no sense to us. But we're leaving it as is.

 

It does mean we've had to fettle the machine we were doing the mod on for @Frax_Excelsior and take some new pics for the instructions. Again we're back at a stage where all three machines seem spot on. We did have a moment as I left today where we thought Cleggy's had died, but turned out a pin had broken on the S-Video lead. Hopefully that's not stuck inside something.

 

Anyhow, doc to follow.

Edited September 14, 2023 by juansolo

[juansolo]

Attempt #2 - Instructions for installing this thing, mainly for @karri. At some point this will probably end up being a PDF, but it's easier to just post it here for now. This is @Frax_Excelsior's machine done barring a load of testing.

 

AJM3 Mod for PAL 7800

DISCLAIMER

Let’s just get this out of the way; the PAL 7800 isn’t very nice to work on. It has tiny solder pads that’ll lift if you as much as look at one funny. The solder they often used wasn’t very good and they weren’t all very nicely put together. As such you will likely rip up some pads in the process of doing this mod. If you are uncomfortable with de-soldering and potentially repairing de-soldering gone wrong, DO NOT ATTEMPT THIS MOD. This mod isn’t for people who are not confident around electronics and repairing them.

 

ABOUT THE MOD

The AJM3 is, as the name might suggest, the third iteration of the Anti-Jack Mod. A mod specifically for the Atari 7800 to replace the RF output with something better. Like the mods before it, it isn’t perfect. We’re fighting a losing battle against some of the poor decisions made when the PAL 7800 came into being and as such, do not expect this thing to magically fix everything. It also is realistically an S-Video only mod right now. No idea why the composite out is behaving like it is and to be honest as this mod was never meant to go on general release, we’ve not really looked into fixing it as we use S and it looks way better regardless. Maybe in the future there might be a 3.1 version that does, for now just use S-Video.

 

INSTALLATION

Unbolt your 7800, extract it’s guts then remove the shield. This machine has also had a service so has got new caps, buttons, regulator and a 2.1mm DC jack. You should be left with something that looks like this:

There are two trimmers on the main PCB that set the MARIA (7800) chroma (R42) and the TIA (2600) chroma (R43). On a PAL machine they’re usually just hot-snotted. You’re going to have to adjust these so spray some IPA on the snot and it should come off easily with a poke

The audio mod is simple. Lift the left hand pins of R32 and R33, twist them together, then run them through a 10u electrolytic capacitor to an audio out jack. I’ve used a non-polarised cap here because that’s what I had. Using a regular one the positive side of the cap connects to the resistors. How you do your sockets is entirely up to you so I’m not going to go into that here.

Here’s where you pull a load of components. Not everything we’ve pulled here needs to come out (like the polystyrene caps for example), but it’s all stuff that either needs to come out for the mod, has a slim chance of causing some interference or is simply annoying and in the way.

 

Pull: R25 R29 C56 C57 C66 R61 R62 R66 Q8 L6 C53 R35 and R74

Install luma bodge wire from left hand pad of R62 to top right pad of T1. Running any of chroma and luma lines near each other or making them excessively long will cause a degraded picture. We run this along the back across the shield to keep it out of the way, and it allows us to pick up that particular luma line from the bottom pad of R73 now.

 

Replace R66 with a 12K resistor. This pulls the sync signal down to a more sensible level.

 

Again ignore how I’ve wired the jacks, do these however you please. Also there’s a cut missing from the pic that I did later with regard to that (the red mark).

Install off board MARIA colourburst clock. I’ll make a nice board for this later. As it is it’s the end hacked off a previous failed iteration of this mod. We’re pulling power and ground from the legs of C50, the clock goes to the top pad of R74.

Here is the mod board itself with all it’s wires cut to length. I highly recommend fitting it where we have with some sticky back pads. It’ll give the shortest cable runs possible and it’s dead space there anyhow. As it was so wee, I used acronyms for the pads which are as follows:

LMO - Luma Out - Goes to your S-Video Jack
CVO - Composite Video Out - Unused in this mod as it doesn’t work right. At some point we’ll figure out why…
CRO - Chroma Out - Goes to your S-Video Jack
GND - Ground - The one at the top. Put there for convenience if you need one for your jacks. We’re getting it from the board in this install so it’s unused.
LMI - Luma In - Goes to bottom pad of R73 (through the bodge wire that ultimately ends up on the left pad of R62)
TCI - TIA Chroma In - Goes to the right pad of R35
MLO - MARIA Luma Out - Goes to left pad of R61
MLI - MARIA Luma In - Goes to bottom pad of R29
GND - Ground - Goes to left pad of R68 or bottom pad of modulator
5V - Power - Goes to second from bottom pad of modulator
MCI - MARIA Chroma In - Goes to the left pad of R25

All done, it should look something like this:

SET UP

You will have to set the trimmers to get the colour right, the top one is for the MARIA (7800) chroma (R42) and the bottom is for the TIA (2600) chroma (R43). Now this will be a bit of a thing if you haven’t done it before. On the upside the colours don’t drift like they do on an NTSC machine so you don’t have to worry about that. But on the downside you can’t use the utility that exists to set up the 7800 side as that’s geared toward NTSC. Also if you’re trying to do this on a panel, go slow as they can take a while to lock onto changes in the signal. Don’t try and do this through a Koryuu if you can help it as that thing auto compensates for the incoming image.

 

So this is how we do it, we’re so used to how certain things look we can set up a PAL machines colours with BallBlazer and Combat. Please bear in mind that photographing CRT’s is hard and that our cameras don’t do colours well, so I’ll try and describe it and give examples:

Left CRT / Right LCD

 

For BallBlazer start a game with two human players, get up to the opponents spinner and hit pause. The top spinner should be a reddy brown (mostly red) and the background blue. These pics are fairly accurate. If you go to the side of the pitch, out of bounds is a burgundy colour. On boot, the logo should be on a dull brown background and you should see even gradients in the lettering.

Left CRT / Right LCD

 

For Combat change the game to number two and hit start. The background is a deep green, the left player a pastel blue and the right a reasonably strong pink. The play field surround should be a sandy yellow (not too intense). In this pic the pink is washed out and the surround isn’t quite yellow enough. That’s the camera being a complete arse and refusing to deal with reds properly.

 

That’ll get them close anyhow, tweak to taste and to the TV you’re using also. Commando 7800 is another good test as it’s got a lot of dark greens on the boot screen. The PAL 7800 hates those and it’s where you’ll most see the colour rolling that the PAL MARIA does (nothing we can do about it). But the opening screen should be dark green and the flesh tones browny. The game itself, the background is green and brown. If it looks wrong, it probably is. Slight tweakage might be required.

 

FINAL NOTES

The quality of your S-Video cable matters. Try to get one with shielded co-axial wiring inside.

 

Compatibility of old mods with modern sets is completely random. We seem to have this playing well with most things. But we still occasionally find a set that it won’t play nice with. That’s just the way of the world now it seems.

Edited September 15, 2023 by juansolo

[+karri]

I just started to do the mod. The 1st surprice is two 10k pull-down resistors on the 74LS174 chip on legs 3 and 4. So two of the flip-flops have a 10k pull-down on the data inputs.

 

[juansolo]

I can honestly say I've never seen that bodge before. Looks like it might be factory. No idea why, @marauder666 any ideas?

[+-^CrossBow^-]

1 hour ago, juansolo said:

I can honestly say I've never seen that bodge before. Looks like it might be factory. No idea why, @marauder666 any ideas?

That is a mid 1990 made system so perhaps this was needed as a timing adjustment? 

 

The 7800 is never a boring console to work on and really seems to have been a patchwork console from the beginning needing bodges like this on case by case basis. 

 

 

[marauder666]

I've seen them on the bottom before on PAL consoles, never on the top.

[juansolo]

17 minutes ago, marauder666 said:

I've seen them on the bottom before on PAL consoles, never on the top.

Indeed he mentioned that the basket case machine I have might have them on the back. This thing is so bodged... I bought it as broken and the previous owner had tried to mod it doing no end of damage requiring multiple bodging.

[+karri]

I have no wires on the top or the bottom of the board. My grand daughter came over for tonight so there was a few hours break.

 

In order to avoid ruining anything I used the solder iron only on the RF module and the L6 coil. All other components were removed by cutting the wires with plyers.

[juansolo]

The last machine I did was actually really good and I didn't lift a single pad. The one above however is just an utter nightmare. Just look at one funny and they fall off. I suspect that's why the fella had so much trouble modding it. All the wires on the back we added... Only the yellow one pertains to the mod, the others are all fixes. I wouldn't be comfortable ever selling this machine because it's so heavily bodged, even though it does actually work now. Doesn't have a case either, it's in one of Bob's 3D printed boxes.

Edited October 4, 2023 by juansolo

[+karri]

Yay! Finally I got the mod done.

 

I still have not decided how to install the S-Video connector and the 3.5mm audio socket. Perhaps tomorrow I know. 3D printing a panel might be the best way and glueing it to the PCB.

[juansolo]

An open frame phono jack will drop in where the modulator was with no modifications, that's what we use for audio. It requries no cutting or drilling. S-video I'm currently drilling the PCBs to fit as I don't personally like putting them through the case as it makes taking the machine apart a real pain in the butt.

 

All working ok?

Edited October 5, 2023 by juansolo

[+karri]

1 hour ago, juansolo said:

An open frame phono jack will drop in where the modulator was with no modifications, that's what we use for audio. It requries no cutting or drilling. S-video I'm currently drilling the PCBs to fit as I don't personally like putting them through the case as it makes taking the machine apart a real pain in the butt.

 

All working ok?

I have not powered it up yet as I am still waiting for my cables to arrive. Plus my grand daughter eats up most of my time. I am planning to make a 3D print to hold the connectors. Perhaps in black ABC so the result would fit in exactly with the cover.

[marauder666]

16 minutes ago, karri said:

I have not powered it up yet as I am still waiting for my cables to arrive. Plus my grand daughter eats up most of my time. I am planning to make a 3D print to hold the connectors. Perhaps in black ABC so the result would fit in exactly with the cover.

Test the video before you mount the connectors.

[+karri]

3 minutes ago, marauder666 said:

Test the video before you mount the connectors.

Ok. I should get a bit time off as the kid will have her nap time soon...

[marauder666]

Thats the first 7800 I've seen with Goldstar logic ICs.

Jack must have got a really good deal.

[juansolo]

The croma trimmers will definitely need adjusting. So it might not look right first plug in. Also might effect your TV's ability to lock S-Video depending on the set. If you've got a S-Video CRT it'll be easier to set it up on that than anything else.

[+karri]

First test using an old Philips TV and SCART connector did not find a signal. The good thing is that there was no blue smoke :). I do have a scope that hopefully still works. I dig it up later tonight.

[marauder666]

First, check the new board has power.

 

Check the luma input on the new board, and then the luma output.  It'll be higher level than you expect on output, but its right when connected.

Ignore chroma for now.

 

You could try plugging the luma output into a composite input, should have an image.  It will probably have weird colour as TV tries to decode the non-existant chroma.

 

[juansolo]

4 hours ago, karri said:

First test using an old Philips TV and SCART connector did not find a signal. The good thing is that there was no blue smoke :). I do have a scope that hopefully still works. I dig it up later tonight.

Is it RGB SCART? If it is, it won't have the S-Video bits in it. They're either or, which is why on Sony sets they usually have two sockets and one is marked with three dots (RGB) and the other isn't (S-Video)

Edited October 5, 2023 by juansolo