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Daedalus2097

800XL Scrambled / Brown video

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I recently retrieved an old 800XL from my parents' attic, and have found that it's not working. I can't remember if it was like this when I put it away 25-ish years ago, but I had two of them and the definitely both worked at one point. The fault is a brownish coloured screen as soon as the computer is turned on, usually with scrambled-looking output in light brown to the right of the screen, though occasionally it ends up as one or two solid vertical bars of the cream / light brown colour. The attached photos show the output. It's a Rev. C board with all socketed chips. I've tried removing and reseating them all, but no joy.

 

I have a good bit of experience troubleshooting and repairing Amigas, but I'm starting from scratch with this, and I can't seem to find any schematics of the 800XL anywhere, which doesn't help matters. So far I've gotten as far as checking that the CPU clocks, which are both running, and the halt pin, which has regular pulses. To me, this would suggest that it's not able to reset properly for some reason, but I thought I'd ask and see if anyone has suggestions on where to look.

 

Thanks!

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The OS isn't starting which suggests CPU, OS Rom or MMU.

 

If you have another like machine then swapping components to identify what's bad can help out.

Re the CPU - the SYNC pin can be a good test point as it should be active each time an instruction is fetched - unsure though if it stays active when the CPU locks up.

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The OS isn't starting which suggests CPU, OS Rom or MMU.

 

If you have another like machine then swapping components to identify what's bad can help out.

Re the CPU - the SYNC pin can be a good test point as it should be active each time an instruction is fetched - unsure though if it stays active when the CPU locks up.

Ah, that's good information, exactly the kind of thing I'm looking for, thanks! I do have another machine buried in my parents' attic, but from what I remember, its chips aren't socketed, which means a bit of a pain in the bum to swap parts. That is, if it still works too!

 

see attached! :)

Awesome, thanks! I don't suppose you know anything of the origin of those drawings? They look remarkably like they came from AmiCAD, a schematic CAD package I used on the Amiga years ago...

 

Here you go. If you're an experienced electronics troubleshooter, this is just what you need:

 

SAM'S COMPUTERFACTS 800XL

That's excellent as well, a quick scan shows lots of good information there. I'll give it a proper look later on.

 

Thanks guys, that's all very helpful, and should save me a lot of time! This (well, one of the two 800XLs) was my first computer, and would love to have it up and running.

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Okay, so some more investigations into this, and it seems that the CPU sometimes hangs, sometimes doesn't, though the display doesn't show any difference. There's generally a lot of bus activity going on, but the interesting thing I've found is that the data bus doesn't look right. Bits 4-7 look like normal logic levels, but are kinda noisy (this could be normal I guess - anyone know?), but bits 0-3 look like they're being pulled down by something. Removing the CPU and powering up shows that bits 4-7 sit high (to be expected with their pull-ups), but bits 0-3 sit low. Pull-ups are fine, swapping RAM chips around makes no difference, so I started pulling all the chips on the bus until I found that all 8 bits sit high after removing the GTIA. Sticking all other chips back in, and while nothing happens (CPU doesn't show activity, presumably missing some critical signal from the GTIA), all 8 bits are still sitting high.

 

Refitting the GTIA brings it back to where it was. However, if I gently bend out the D0-D3 pins and refit the GTIA, the display flickers a bit, but then stabilises into a slightly different pattern that looks like it might include a cursor. So it looks like the machine is trying to start now at least, even though the colours haven't changed... Still no READY prompt, but it feels like progress.

 

Now I just need to find my other machine, or another GTIA chip I can borrow to verify that this one is indeed the problem.

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many of the RAM PCB tracks are shared between all 8 ICs.

is it possible that a bad RAM chip or even the MMU are pulling those 4x lines low?

 

btw - dont know about he origin of the schematic but the maker kindly made a set for each of the A8 machines and theyre easy to process.

 

following on from dr venkman's upload, see the attached atari official one. The Sams ones are best overall but its nice to have both to cross-reference

 

 

800XL_FSM.pdf

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Thanks very much for that, I'd found the official Atari service manual but the Sams one is indeed much more useful, as are those schematics. Yeah, the address lines are common between the RAM chips, but one chip is used for each data bit so only one bit can be affected by each chip. I do feel like the GTIA is the problem (or at least one of the problems) - I've seen chips do that sort of bus loading before and causing all sorts of issues across a board.

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here's all the schematics while im at it:

ftp://ftp.pigwa.net/stuff/mirror/jsobola/atarisch/

 

but its an interesting site to generally wander around too :)

 

Just an FYI and a word to the wise - these "Sobola" schematics keep getting passed around but they are NOT safe to rely on as your only source of info. The 1200XL schematics, for instance, are just flat out wrong in several key areas of the support/memory address logic.

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erk, thanks for the tip. i havent needed to mess much with my 1200xl, hopefully i wont have to!

always worth cross-referencing tho

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Okay, so I was having another look at this today and couldn't find any decent sources for the suspect GTIA chip in my machine (C014889-01). In fact, the only place that looks promising is the wonderfully Geocities-looking ball of internet nostalgia that is http://www.best-electronics-ca.com. Is this the place to go for these things? For simplicity, I prefer to order within Europe whenever possible, but it looks like that's not an option.

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Okay, so I was having another look at this today and couldn't find any decent sources for the suspect GTIA chip in my machine (C014889-01). In fact, the only place that looks promising is the wonderfully Geocities-looking ball of internet nostalgia that is http://www.best-electronics-ca.com. Is this the place to go for these things? For simplicity, I prefer to order within Europe whenever possible, but it looks like that's not an option.

PM AtariAge user flashjazzcat. He's in the UK, should be able to help.

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Cheers guys, not to worry. So Best Electronics would be where you'd order from anyway? I might give it a shot... Maybe a group order could be an idea if anyone else needs anything from them?

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I'm out of spares, so would simply end up ordering one in anyway.

 

I have some GTIA PAL spares... shipping from Germany maybe cheaper than from U.S.?

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Indeed, and there appears to be a minimum order of $50 from the US site too. I'm interested - how much for one chip shipped to the UK?

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Refitting the GTIA brings it back to where it was. However, if I gently bend out the D0-D3 pins and refit the GTIA, the display flickers a bit, but then stabilises into a slightly different pattern that looks like it might include a cursor. So it looks like the machine is trying to start now at least, even though the colours haven't changed... Still no READY prompt, but it feels like progress.

 

Now I just need to find my other machine, or another GTIA chip I can borrow to verify that this one is indeed the problem.

 

1st: I´ve sent a new GTIA to Daedalus2097 :-)

 

2nd: Maybe it´s only my sense, but I watch the significant rise of defective Atari custom chips over the last 2 years. After repairing several hundred systems in the last +20 years, most times the usual suspects of this list were the culprits...

 

1. Memory chips (specially Micron)

2. CPU - only Sally... I didn´t have any single defect 6502B until today! (Standard MOS CPU used in the 400/800, Happy 1050 enhancements...)

3. Delay-Line / MMU / OS-ROM / Basic-ROM

4. One of the 74 series TTL chips

 

Defect ANTIC, GTIA or Freddie were extremely rare. Defect POKEY sometimes, but mostly they can be used for stereo further. Defect PIA only when somebody tinkered with the joystickports and put overvoltages on it etc.

 

But since 2...3 years I have several defect custom chips. Order of fault frequency (most = first):

 

1. GTIA

2. ANTIC

3. POKEY

4. Freddie

 

About 15 defect GTIA are lying in my "defects" drawer. Most of them aren´t really dead, but they make problems like stated here. One or more defect databits (most times causes wrong colors, wrong PMG effects, but system runs "normal"). Defects around PMG are the 2nd most issues (collision detections error, wrong colors only in PMG or no PMG at all) or simply no Fast_PHI0 output for ANTIC.

 

Of course I´m talking about PAL chips only here in PAL-land, but for me it looks like that some aging issues came up.

 

Any other comments from repair guys?

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Yep, thanks very much, and in a couple of days I'll let you know whether that sorts the issue or if something else is going on as well.

 

Interesting stuff... I wonder if it's an age issue with the chips themselves, or a secondary effect of something else. For example, as the capacitors in the PSU or main board become less effective over the years, they will be allowing a higher ripple in the supply, and won't be filtering the logic-induced noise out. Both of these conditions could be putting extra stresses on the I/O stages of the chips, which could be accelerating their demise.

Edited by Daedalus2097

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Well, I received a new GTIA chip from tf_hh today. Stuck it in and...

 

It works :) Proper logic levels on the bus and a READY prompt straight away!

 

Thanks very much!

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I have a defective PAL GTIA which only produces B&W, otherwise all else works fine. It was NOS I got from BEST, and for the first few days it worked fine, but then lost it's color. I tried it on several other systems with the same result, and on the first system that showed the problem changing out the GTIA solved it. So even when this stuff has been sitting unused for years, it can still fail after power-up or extended running beyond. Nothing's guaranteed :) .

 

BTW, that's the only Atari chip I've ever had go bad on me.

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