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Graphics problems on Atari 400

atari 400

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#1 NickFolino OFFLINE  

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Posted Mon Apr 1, 2019 6:10 AM

I'm looking for assistance in repairing an Atari 400.  It has an intermittent issue with graphics.  Sometimes it starts up fine and everything looks normal.  When this happens the graphics and colors are fine until the computer is powered off.  Other times the colors are off on startup, for instance sometimes when playing Donkey Kong from a cartridge the graphics on the title screen are green.  Other times it starts with a white background.  The only way to resolve this is to power cycle.

 

Other times sprites are missing.  For instance when playing Pac-Man from a cartridge Pac-Man and the ghosts are missing.  You can still control the invisible Pac-Man by following the dots as they disappear.  You will never die from hitting a ghost.  Sometimes when playing Donkey Kong the Jumpman character is missing and just a solid vertical line runs down the screen at the position he should be in.  Again only power cycling will resolve these issues.

 

Any ideas what can cause this?  Could it be the ANTIC? 

 

Any help would be appreciated.

 

Nick

 



#2 _The Doctor__ ONLINE  

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Posted Mon Apr 1, 2019 6:47 AM

sounds like dirty contact connections.... de-oxit and a re-seating of all chips and cards would be in order. Once done, leave the 400 when running correctly colored turned on for a day... last place to go will be the color adjust and rf modulator... after alll of that is clean good and checked out... it will be time to delve into the video further and run down a suspect capacitor, pot, or chip... and maybe perhaps a cold solder joint as the problem appears intermittent.

 

I will make the assumption you have already verified voltages from the power supply section both during good color and bad color, as checking the power is the first step before all other trouble shooting.... Atari color is sensitive to power fluctuations/voltage.


Edited by _The Doctor__, Mon Apr 1, 2019 6:48 AM.


#3 NickFolino OFFLINE  

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Posted Mon Apr 1, 2019 7:03 AM

Thank you for the quick reply.  I have de-oxited the cartridge slot, but being that I've never worked on a 400 I figured I'd ask for help before tearing into it.  I'll tear it down, re-seat the boards and clean the contacts.  Look for weak solder while I'm in there.  It'll take me a few days to find the time, I'll let you know how it goes.

 

Anything odd I need to know about tearing this thing down?  Or is it pretty easy?

 

Nick



#4 Nezgar ONLINE  

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Posted Mon Apr 1, 2019 8:24 AM

Player Missile Graphics, which are what the Pac-Man character, and the ghosts are comprised of, are generated by the GTIA chip. I would target that as the first chip to extract, clean chip pins with isopropyl alcohol, (and/or a pencil eraser tip) reinsert in it's socket. Maybe you have another Atari 8-bit computer (any, or even 5200) to try swapping chips from.

 

On a 400, this chip should be CO14805, on the leftmost socket of the CPU board, which you'll have to take the machine apart to access...  But at that point also repeat the procedure for all the chips on the PCB card...

Attached Thumbnails

  • gtia400.jpg


#5 NickFolino OFFLINE  

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Posted Mon Apr 1, 2019 8:27 AM

Thanks, Nezgar!



#6 tf_hh OFFLINE  

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Posted Tue Apr 2, 2019 3:04 AM

I'm looking for assistance in repairing an Atari 400.  It has an intermittent issue with graphics.  Sometimes it starts up fine and everything looks normal.  When this happens the graphics and colors are fine until the computer is powered off.  Other times the colors are off on startup, for instance sometimes when playing Donkey Kong from a cartridge the graphics on the title screen are green.  Other times it starts with a white background.  The only way to resolve this is to power cycle.

 

Other times sprites are missing.  For instance when playing Pac-Man from a cartridge Pac-Man and the ghosts are missing.  You can still control the invisible Pac-Man by following the dots as they disappear.  You will never die from hitting a ghost.  Sometimes when playing Donkey Kong the Jumpman character is missing and just a solid vertical line runs down the screen at the position he should be in.  Again only power cycling will resolve these issues.

 

Any ideas what can cause this?  Could it be the ANTIC?

 

Maybe ANTIC or GTIA, but not very common. I would prefer first cleaning all contacts like some guys already wrote. Detach the CPU and the RAM board, clean the contacts of each board with a rubber. Take an unused toothbrush to clean the connectors where the boards sit on or use folded cardboard - move the cardboard up and down, but never sideways!

 

From my personal repair experience most times the RAM is the culprit for spurious issues like this. If you have another RAM cart (you can use the same like used in the Atari 800, but must remove the case from the RAM board if there´s any), check it first. Specially when the one you found in your 400 is modded with stacked RAMs to expand it to 32 or 48 KByte... most mods of the old times uses the same 16 KBit RAMs like Atari used on the 16 KByte RAM board... if you stack them in three layers, the whole thing gets sometimes very hot and will kill over years one or another of the RAM chips...



#7 NickFolino OFFLINE  

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Posted Sat Apr 6, 2019 9:47 AM

I took it apart and cleaned all of the contacts, removed all socketed chips and cleaned those as well.  Same problem.  I found this post where a similar problem with the vertical line in Donkey Kong was fixed with a new GTIA.  Guess I'll have to source one of those to try a swap.

 

http://blog.tynemout...am-upgrade.html

 

Nick



#8 NickFolino OFFLINE  

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Posted Sat Apr 13, 2019 10:32 AM

So I got a GTIA from ebay.  I arrived in a bubble mailer with the chip stuck in a piece of styrofoam.  Chip was no good, but it did help me prove that the GTIA was the fault.  The new chip worked fine, but had no color.  The seller is sending a replacement, hopefully not stuck in styrofoam.

 

Nick



#9 _The Doctor__ ONLINE  

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Posted Sat Apr 13, 2019 12:59 PM

a picture of the old chip would help make sure you have a gtia and not a ctia as well



#10 Nezgar ONLINE  

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Posted Sat Apr 13, 2019 2:03 PM

CTIA

  • CO12295 - NTSC

GTIA

  • CO14805 - NTSC
  • CO14889 - PAL
  • CO20120 - SECAM


#11 DrVenkman OFFLINE  

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Posted Sat Apr 13, 2019 2:19 PM

So I got a GTIA from ebay.  I arrived in a bubble mailer with the chip stuck in a piece of styrofoam.  Chip was no good, but it did help me prove that the GTIA was the fault.  The new chip worked fine, but had no color.  

 

Just out of curiosity, did you try the new chip and then cycle the color potentiometer a few times in both directions end to end? GTIA replacement will *always* require at least some adjustment to the color pot on an Atari. If the pot hasn't been adjusted in decades, it might have some corrosion or dust build up and need to be cycled to ensure good contact.



#12 NickFolino OFFLINE  

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Posted Sat Apr 13, 2019 8:00 PM

FYI - I did find the service manual.  It is a GTIA and it is bad.  Adjusting would get any color from it.



#13 Mclaneinc ONLINE  

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Posted Sun Apr 14, 2019 3:32 AM

Sounds a bit odd...Waiting to hear the outcome..



#14 NickFolino OFFLINE  

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Posted Mon Apr 15, 2019 10:21 AM

Another GTIA chip solved all issues.  It's working like new again.

 

Nick



#15 Mclaneinc ONLINE  

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Posted Mon Apr 15, 2019 10:31 AM

Great, that's the result we all wanted...


Edited by Mclaneinc, Mon Apr 15, 2019 10:31 AM.






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