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Help needed with strangely modified Atari 7800

Atari 7800

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#1 Macross_VF1 OFFLINE  

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Posted Sun Dec 9, 2018 1:55 PM

I fairly recently got my hands on a 7800 intended for the scrap heap. The power supply plug was in somewhat bad shape and the power jack was broken off. When I opened it up I discovered that the soldering points for the power jack was almost completely ruined, as you can see below. However with some careful consideration I managed to find a work around, fitted a new power plug and jack, and got it to power on.

However I got nothing but a black screen and some humming/buzzing. Also, I found it strange that the plug wasn't a standard RF jack, but rather the same type found for connecting a monitor to an Atari ST computer. Curious that I am, I decided to open it up further and well, as you can see in the second picture, someone has been a busy little bugger. It has certainly been heavily modified during its lifetime. Still, this brings me no closer to a solution. Anyone know what might be wrong with the system or is it a lost cause?


Z15R14n.jpg

9tfUa4f.jpg



#2 Mitch OFFLINE  

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Posted Sun Dec 9, 2018 2:46 PM

That is a pal RGB 7800.

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#3 Macross_VF1 OFFLINE  

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Posted Sun Dec 9, 2018 3:38 PM

So... it's like this from the factory then? But it's looks so shoddy! Still, I need help in getting it to work again.


Edited by Macross_VF1, Sun Dec 9, 2018 3:39 PM.


#4 Mitch OFFLINE  

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Posted Sun Dec 9, 2018 8:22 PM

Do you have the 13 pin din to scart cable?

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#5 Shawn OFFLINE  

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Posted Sun Dec 9, 2018 8:25 PM

So... it's like this from the factory then? But it's looks so shoddy! Still, I need help in getting it to work again.

 

Yep, those are legit Atari pcbs. I agree the work is pretty rough. Never thought I'd see Atari using hot glue on a retail product to secure the power jack like that!



#6 slx OFFLINE  

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Posted Sun Dec 9, 2018 8:57 PM

Its a French unit, they had mandatory SCART. It should work with an ST to SCART cable (or ST to RGB), maybe also with an SC1224, but those are rare, too. IIRC correctly the ST cables have a resistor on one line which the 7800 doesnt need for optimum picture quality.

The power plug might be a DIY fix and you might want to change it for a more common plug when you have to replace it anyway.

Id be interested to know whether your unit shows color for 7800 carts. Mine doesnt.

#7 Macross_VF1 OFFLINE  

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Posted Mon Dec 10, 2018 2:57 AM

Its a French unit, they had mandatory SCART. It should work with an ST to SCART cable (or ST to RGB), maybe also with an SC1224, but those are rare, too. IIRC correctly the ST cables have a resistor on one line which the 7800 doesnt need for optimum picture quality.

The power plug might be a DIY fix and you might want to change it for a more common plug when you have to replace it anyway.

Id be interested to know whether your unit shows color for 7800 carts. Mine doesnt.

 

Interesting, I had no idea that it was French. Sacre bleu and all that.  :)  Now it does 'work' with an Atari ST scart cable as I have one of those lying around, but as I write above, while it powers on (and no sign of the magical blue smoke) I only get a black screen and some background noise. What might be the problem?



#8 Mitch OFFLINE  

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Posted Mon Dec 10, 2018 2:48 PM

Are you trying it with a cart or the builtin Asteroids game?

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#9 Macross_VF1 OFFLINE  

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Posted Mon Dec 10, 2018 4:42 PM

Both, though to be fair only 2600 carts. I've never actually owned a 7800 cart.



#10 Mitch OFFLINE  

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Posted Mon Dec 10, 2018 8:43 PM

Do you have much soldering experience?

 

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#11 Macross_VF1 OFFLINE  

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Posted Tue Dec 11, 2018 8:44 AM

I am decent with a soldering iron so yeah, I know what I'm doing. All I need to know is what to do, what to solder or what to test.



#12 Mitch OFFLINE  

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Posted Tue Dec 11, 2018 4:01 PM

I would start with checking voltages. Make sure the voltage regulator is putting out a steady 5 volts. If you a spare tia chip you could try swapping that out next.

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#13 Macross_VF1 OFFLINE  

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Posted Wed Dec 12, 2018 3:57 PM

Voltage from the PSU is about 13.5V but under load it drops to about 10.5V which is rather typical of this type of old style PSU. The voltage regulator then does its' job and converts it to around 5V as intended so that's certainly not the problem. I've checked the chips and not one of them seem to become abnormally hot when the power is on. Is there some kind of OS in this machine or is everything directly hardwired? 



#14 Mitch OFFLINE  

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Posted Wed Dec 12, 2018 4:14 PM

There is a limited os chip. I don't recall offhand where it is on that board. I would have to pull one apart and check it.

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#15 Lynxpro OFFLINE  

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Posted Thu Dec 13, 2018 1:12 PM

Would it be difficult to clone that SCART board?  Is it an Atari Inc or Atari Corp design?  And does it export real RGB?



#16 Macross_VF1 OFFLINE  

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Posted Thu Dec 13, 2018 3:03 PM

It does say Atari on the extra boards so it does seem like a legit design. As for real RGB, well I can't tell, though I'm certain there are ways to find out.



#17 Mitch OFFLINE  

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Posted Thu Dec 13, 2018 10:52 PM

It takes composite video off of the 7800 Maria and TIA chips and splits it out to RGB. So not really a true RGB output.

 

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#18 fluxit OFFLINE  

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Posted Fri Dec 14, 2018 3:19 AM

Is the result typically better than just outputting composite or s-video over SCART?



#19 SmittyB ONLINE  

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Posted Fri Dec 14, 2018 4:36 AM

It should certainly be better than the standard RF but my guess is that a modern video modification will output a cleaner signal given there's a lot of extra processing going on to turn it into an RGB signal here coupled with Atari's world famous quality control.

#20 Lynxpro OFFLINE  

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Posted Sat Dec 15, 2018 10:57 AM

It should certainly be better than the standard RF but my guess is that a modern video modification will output a cleaner signal given there's a lot of extra processing going on to turn it into an RGB signal here coupled with Atari's world famous quality control.

 

Hey now, Atari Inc's quality control was far different than the later Atari Corp's quality control. This SCART board could've been developed by Atari Inc. And I know one of the last things Dan Kramer worked on before he left the Tramielian Occupation was finishing up the SECAM versions of the Atari XL computers. And he left before 1985...

 

If this is a decent board, I'm surprised nobody has cloned it. I'd take SCART over Composite or SVideo. Then again, a lot of 7800 mod boards come from the States and SCART wasn't popular here plus not too many enthusiasts here own French 7800s...



#21 Macross_VF1 OFFLINE  

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Posted Sat Dec 15, 2018 2:16 PM

So from what you guys are discussing, this console might actually have value to others, if for no other reason than learning how to reverse engineer the parts? Neat! I suppose if I can't get it to work, I can consider donating the unit to someone with the know-how to clone the extra boards.

That said, if at all possible I would like to get it operational. Since I do seem to get the appropriate power, but it doesn't boot up any games, should I consider replacing some of the chips, possibly the ones that hold the OS?



#22 Mitch OFFLINE  

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Posted Sat Dec 15, 2018 4:44 PM

The OS chip is an easy way to start. You can burn the ROM to an EPROM and it is a straight swap. If there is room for a socket under the daughter boards then you may considering putting one in to make it easier to swap in the future.
The OS chip should be labeled C300558. You can burn it to a 27C128 EPROM.
If you look at this pic: http://www.atari7800...ns/C300633.jpg you can see it in the middle of the bottom row of chips. This is the same board that you have minus the daughter boards.
ROM file is here: http://www.atari7800.../pal/a7800p.zip
 
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#23 Macross_VF1 OFFLINE  

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Posted Sun Dec 16, 2018 5:12 PM

I'll try it though it will have to be some time after Christmas. Anyway, thanks for the help.







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