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Cmherndon79

Atari 7800 only black, white and yellow in 7800 mode

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Hi there,

 

Got an NTSC 7800 in from someone that wanted it modded for composite video and audio output.  Board is CO25233-002 Rev A.  This is just the simple and cheap mod using the 2.2K, 3.3K resistors and the 2N3904 transistor, mounted on a PCB.  Done many of these on 2600s (maybe 150+ by now) and probably 10 7800s before.  When I got the unit in, I just booted up Frogger in 2600 mode over RF channel 3 and sound and video were working fine.  Foolishly I didn't check to see that the MARIA chip in 7800 mode was working before I did anything further.  I took the board out and replaced the 3 electrolytic capacitors from a kit from Console5 that I had on hand and replaced the audio polystyrene caps at C4 and C5 with so 820pF MLCC equivalents.  I removed the RF modulator from the board and I installed the composite mod.

 

I booted Frogger back up and the screen looked normal, colors were correct.  TIA audio was playing fine on the startup sound in Frogger.  Next, I put in Ballblazer since that had a POKEY chip in it, it is typically the first one I'll use.  When it switched into using the MARIA for the 7800 I got almost all black and white in the Atari rainbow, except for yellow.  When I watch Ballblazer in attract mode I see all black and white, but after the ball is hit, I see it turn yellow and after a second grey again.  No problems with POKEY audio.  Put in Food Fight and Centipede (7800) and same behavior. 

 

Started looking for problems on the board.  Noticed the rear leg of C57 was sitting in the mounting hole for the power connector, seemed unlikely this was the problem, but I connected it to where it was supposed to go and no change.  Checked all the component connections around MARIA and didn't find any problems, but did reflow a few things that looked like they could be marginal.  Checked for bad solder joints everywhere else and same thing, nothing apparent but reflowed a few points anywhere I thought things could be marginal.  Double checked the polarity of the electrolytics I installed and they were right.  Reflowed the solder on the 220uF cap sitting next to the MARIA just in case.  Did a search and found someone had B/W only here on a 7800, which was a bad solder connection at the slot, so I checked that, but didn't find anything wrong there.  Can't see any cut traces and checked continuity if I wasn't sure.

 

Just looking to see if anyone else has run into behavior that fits this description.  I've got a known working MARIA in a socket on another 7800, but I really don't want to desolder the one from this board, unless I'm fairly confident that is the problem.

 

Thanks!

 

 

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I did run into this with a 7800 that was sent to me for a UAV upgrade. The 7800 had one of the comp mods in it like you described but arrived with only a scrambled picture screen. I took out the simpler comp mod and restored the system back to factory (This one still had the RF modulator too as I recall). Anyway, I had to replace the small 74x logic chip on the left that sits just above maria first. That got me from a scrambled picture to one that was missing colors but at least the luma was working enough to see a picture. Finally traced it down to something in the maria itself. Couldn't see anything wrong with the board but once I found that out, I reinstalled the original in case if was a lose solder point..etc and same results. I think blues and purples were missing on this one. Anyway it was a bad maria in the end along with that 74x logic not providing a proper sync for the TV to latch onto. First and so far the only 7800 I've worked on that has done this and required the maria to be replaced.

 

And the missing colors only on 7800 games is kinda the giveaway here since the 2600 stuff is still all processed through the TIA. So..yeah...look at that maria and see if you have another on hand to swap into it to see if it makes a difference.

 

Also..if you haven't already make sure the color pot isn't to blame? Although that usually just gives a black and white or messed up colors but not just one color normally.

 

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29 minutes ago, -^CrossBow^- said:

I did run into this with a 7800 that was sent to me for a UAV upgrade. The 7800 had one of the comp mods in it like you described but arrived with only a scrambled picture screen. I took out the simpler comp mod and restored the system back to factory (This one still had the RF modulator too as I recall). Anyway, I had to replace the small 74x logic chip on the left that sits just above maria first. That got me from a scrambled picture to one that was missing colors but at least the luma was working enough to see a picture. Finally traced it down to something in the maria itself. Couldn't see anything wrong with the board but once I found that out, I reinstalled the original in case if was a lose solder point..etc and same results. I think blues and purples were missing on this one. Anyway it was a bad maria in the end along with that 74x logic not providing a proper sync for the TV to latch onto. First and so far the only 7800 I've worked on that has done this and required the maria to be replaced.

 

And the missing colors only on 7800 games is kinda the giveaway here since the 2600 stuff is still all processed through the TIA. So..yeah...look at that maria and see if you have another on hand to swap into it to see if it makes a difference.

 

Also..if you haven't already make sure the color pot isn't to blame? Although that usually just gives a black and white or messed up colors but not just one color normally.

 

Thanks! I'll give the MARIA a shot. I did check the color pot and reflowed it as well just in case, so fairly certain that isn't the issue.

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K, so desoldered the MARIA from the board I'm working on.  Removed the known working MARIA from the socket of the other working 7800 I've got here.  Installed the desoldered MARIA and we're getting color which appears normal as I can tell.  Thinking it must be something else.  At least it isn't an Atari chip that isn't made any longer, so that's a relief.  

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Put the previously socketed MARIA into the 7800 lacking color, other than yellow and the problem follows the board.  Sure is an odd defect.  If chroma were not being mixed at all it would make more sense, but I'm getting yellow, and it isn't the combination as far as I know of Luma/Chroma into composite, at the high level, or else TIA graphics would have the same/similar issues, I'd think at least.  Might just have to set this one aside and think on it...  Thanks @-^CrossBow^- for the suggestion, as it gave me the confirmation I needed to think I wasn't crazy and it might be the MARIA.  Also, appreciate the video you did on the UAV for the 7800.  Had someone request S-Video on a 7800 and used your video to help me along that project.  It was fantastic and got everything wired up without any issues at all. 

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Okay, if it's not the MARIA, it's gotta be one of the discrete components between MARIA and the points where you pick up signals for your video mod. Time to break out the schematics. But as a clue, the only 7800 game you tested with is Ballblazer, correct? Well, that playfield is predominately shades of green with a blue sky. So if you're seeing yellow and black, then you're missing the blue component of the output. 

 

Do you have any other 7800 carts to try before replacing more components on the board? Might help limit your work.

 

EDIT: Per the schematic, MARIA's LUM signals all get passed through U3 (I think that's the 74LS32 chip). That's probably the problem child.

 

EDIT EDIT: $0.61 each at Mouser. Cheap fix, aside from minimum order quantities and/or shipping charges. 

 

https://www.mouser.com/ProductDetail/Texas-Instruments/SN74LS32N?qs=q2XTDbzbm6DA9Mnew5GiLA%3D%3D

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Thanks @DrVenkman.  Sounds logical.  Don't have any 74S32s on hand, mainly just logic chips for the C64, which the 64 doesn't use that model.  I usually place an order with Mouser about every 10-14 days or so, but may need to really take some inventory, since I just got my last order in yesterday.

 

I had tried Food Fight and Centipede (7800) earlier and the behavior is the same there.  Thanks for checking the schematics, as I don't have the 7800's readily available as I do the 2600.  I appreciate it!

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@DrVenkman is that the 74x logic that I was thinking of? I've had to replace that before at least 2 other times but not for color issues. As I said in my reply I had one of those go back before and it just resulted in a scrambled picture but it was doing that on both 2600 and 7800 games in my case.

 

I guess I need to break out my test 7800 and hook up the maria colorburst to my scope so a comparison might be possible?

 

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6 hours ago, -^CrossBow^- said:

@DrVenkman is that the 74x logic that I was thinking of? I've had to replace that before at least 2 other times but not for color issues. As I said in my reply I had one of those go back before and it just resulted in a scrambled picture but it was doing that on both 2600 and 7800 games in my case.

 

I guess I need to break out my test 7800 and hook up the maria colorburst to my scope so a comparison might be possible?

 

Yeah I think it must be. It’s a Quad Or gate that passes either the MARIA  *or* the TIA LUM and SYNC signals on to the resistor ladder and then out to the RF modulator in an unmodified machine. If any of the four internal OR gates fails, it will stop passing signals properly for either TIA or MARIA.

 

Here’s the relevant chunk of the schematic:  

 

 

A0BFD0F1-8018-470A-B777-7F85CD950E7B.jpeg

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Thanks @DrVenkman.  Ordered a few 74LS32s from Unicorn Electronics.  Needed some 4164-150 DRAM as well, so they covered both needs.  Will update once the part comes in.  Thanks again.

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