Corrupted Graphics Are Getting Worse

[Soyale]

I recently dug my Atari ST out of storage and have been enjoying reliving my teenage years of tinkering and gaming.  It's been a joy.  Unfortunately a problem seems to be developing with the graphics and I'm worried the ST is on it's way out.  The main symptom is color palette shifting i.e. graphics fine but in the wrong colors.  There's a picture of the issue below.  The palette shifting isn't always the whole screen, for example in Dungeon Master the other night only one of the characters rectangles at the top was shifting and it was flickering between normal and shifted.  There's another symptom which is graphics being overwritten, e.g. if the word ATARI was supposed to be displayed , the letters would all appear in one position, or maybe overlapping a little bit.  This issue is more common in TOS / GEM applications.  It also seems to happen in menu discs.  It's a bit random what shows up where and when.  E.g. last night Defender of the Crown was fine, but then I followed up with Dungeon Master which was corrupt.  A couple of nights previously Dungeon Master was fine for an extended playthrough.  I've had a game have problems with palette shifting while starting up, and then be normal when the game starts.

 

The machine is a 520 STFM with a 1M memory upgrade, I don't remember who made the upgrade.  The video is running out of the monitor port through an OSSC into a modern TV.  I've used both VGA and SCART cables to see if it's a cable problem.  I even tried hot-plugging one for the other when the issue showed up to test if it's the cable.  It doesn't seem to happen in High Res but to be fair I haven't tested recently.  My perception is that the problems show up more in low res than medium, but I don't have any hard data to that effect. 

 

Any suggests for some basic debugging steps, and / or what the issue might be based on your experience?

 

 

 

 

 

[ParanoidLittleMan]

It needs opening of machine and perform usual steps: reseating chips in sockets, best to do some cleaning of contacts carefully too.

Because of age PSU electrolyte capacitors are most likely weak, so need replacement to new ones. Material is cheap, and can do it with little soldering experience.

If above helps not will be harder to find culprit - may be Glue, Shifter, MMU ...

 

[Soyale]

Thanks.  Forgive my chronic ignorance here, I'm assuming reseating chips = taking socketed chips out and replacing?  My memory upgrade is a rectangular board connected to one of the square sockets, any tips on taking that out without bending pins etc?  Are there any physical signs that capacitors are on the way out? e.g appearances, smells etc.

 

In my session last night there were no problems until I was done, then switched the machine off, remembered I wanted to do one more thing, switched back on and hey presto, white screen.  I'm starting to think that time spent playing has something to do with it which implies it's something caused by heat build up.  Thoughts?

[ParanoidLittleMan]

Socketed chips need to be carefully removed, cleaning contacts (pins) on socket and chip too, then insert them back.  Problems appear mostly at those square ones with small pins and distance between. To remove them you can use very tiny screwdriver and put it in at holes at edges. Then push out side down slowly . Best to do it at 2 opposite sides, and only when it went up couple mm to remove totally.

 

Usually there are no physical signs of weakened capacitors. Maybe smaller weight - but you don't have original weights. Things are that it is faster and cheaper to replace them than getting meter capable to measure their capacity. And after 30++ years it's pretty much sure that their lost some capacity. And overheating can be result of bad capacitor too.

 

[Soyale]

Awesome. I'm looking to see if there's someone local who can do the recapping, and if not I'll be taking on the job myself.

[Soyale]

As an update, I reseated the socketed chips and replaced the PSU with an EXXOS one (it seemed easier than recapping).  The chips were VERY stuck and took some levering to get them out and all sorts of grinding.

 

As far as I can tell the issue has gone away.  I was a little haphazard about cleaning the pins (because I forgot your instruction but cleaned some to help get them out).  I wasn't disciplined about testing in between reseating the chips and adding the PSU, so I can't be sure which one fixed the issue but I'd bet it was the reseating.