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Potential problems with newly bought 800XL

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Today I had a delivery of an Atari 800XL from eBay. I got it for a really low price because it was described as "For parts or not working" and "No cables, so sold as spares". I plugged in the power supply I already had, which may have come from another 800XL, and it worked first time, booting up to the BASIC Ready prompt. I carried out various tests, including the self diagnostic (which seemed to pass on all counts), plugged in a game cartridge and ran it, as well as connecting my SIO2SD unit, booting from there and running various software from ATR files, including BASIC programs I'd written myself.

 

Some potential problems I noticed are as follows. The on/off rocker switch feels very stiff compared to my 65XE, so I have to press a lot harder to turn it on. The screen sometimes flashes red, but then quickly clears and boots up. The Atari/inverse video key doesn't work all the time, but that's obviously not as serious as the other two faults.

 

I see that someone has uploaded an 800XL service manual, but obviously there will be a lot in it and it may be too technical for me. I hope someone on here can advise me about the potential problems above.

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I'm trying to prevent it from going bang in the first place. After a few months, I've now got an A8 computer with a SELECT key that works, so I can play Star Raiders, and Realsports Football (in regulation time). It's also a computer which was nearly my first computer, but my 65XE wasn't.

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That flashing RED thing is a serious issue, which can be a few things:

 

problems with OS, problem with MMU, problem with CPU, problem with RAM.

 

It's very hard to analyze issue without having the atari on the desk....

Atari 800XL is a fabulous machine, I can understand you want to get it in excellent condition. Did you already check what is under the metal shield? Is everything on a IC socket?

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Thanks for your suggestions! I haven't opened the case and I've never checked under an Atari metal shield before. I'll try this in the near future.

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The system powers on, checks for memory size and then stores $00 into all the hardware chips. Until the GTIA gets set to $00, you will be in an undetermined state - could be red, could be green, whatever.

 

When you hear it go 'click' and the screen does not go totally blank, (about 1-2 seconds) then you can worry. Notice that the GTIA isn't connected to RESET.

 

Dump some WD-40 down the neck of the power switch - work it back and forth. You may have to replace the switch.

 

Bob

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Something's happened today which may shed more light on the problems. A few times, the Atari 800XL has powered up with a dark green screen instead of dark blue. Once it changed from green to the usual dark blue all by itself and another time when I pressed RESET it rebooted with the usual dark blue screen. I also got a weird effect while playing the game Mr. M, which is in the Homesoft ATR disk collection. This was that when the player character was going up the screen, it left a trail behind it, which was later erased by going down the screen along this trail. It even happened on the title screen before I pressed the fire button to start. What could cause this?

 

BTW, the inverse key now seems to be working OK.

 

I hope to open up the 800XL soon, take some pics and post them on here.

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The internal diagnostic memory checks seem to have passed every time. The results are the same as on my 65XE. One thing I don't understand is that this is represented by two lines of 16 squares, followed by a line of 8 squares. If a RAM chip became faulty, then how would I identify it? I read something about piggy backing RAM, but I'm not sure I understand this.

 

One change is that, instead of the screen just flashing red, it now stays red for much longer. I've stopped using the 800XL for the moment and will now open up the case, examine it, post and take pics in the hope that there's some sign what's wrong.

 

BTW, as for my electronics skills at the moment, I've just managed to work out how to take readings which tell me the values of resistors, because the colour codes are difficult to read. I'm trying to build a dimmer switch circuit on a breadboard. This means I couldn't do any more complicated repairs to the 800XL.

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Each square represents 1K.

On the 800XL, each of the 8 Ram chips provide 1 bit for every single Ram address in the system.

 

The standard Ram test isn't really helpful in identifying which chip/s might be faulty. Even then, a different Ram test would be required depending on machine and revision. e.g. early non-GS XEs had 1 bit chips but later ones and XEGS have 4 bit chips.

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If a RAM chip became faulty, then how would I identify it? I read something about piggy backing RAM, but I'm not sure I understand this.

 

if one or more of those 40 little squares show red it means you have a bad RAM chip, but as Rybags mentioned it won't tell you which one, so piggybacking a working RAM chip on top of each of the 8 chips inside the computer (one at a time of course) will help you identify which one(s) is(are) bad. You simply put the working chip on top of the suspect chip, align all the pins carefully and make sure each one touch the corresponding pins of the suspect chip and run the memory self test. If you see positive changes (i.e. less or no red squares) that means you located the (or one of the) bad chip(s). Repeat that with remaining 7 chips on the motherboard. If you have more than one good RAM module handy you could leave the first one piggybacked and proceed with the next one to continue your diagnosis.

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Each square represents 1K.

On the 800XL, each of the 8 Ram chips provide 1 bit for every single Ram address in the system.

 

The standard Ram test isn't really helpful in identifying which chip/s might be faulty. Even then, a different Ram test would be required depending on machine and revision. e.g. early non-GS XEs had 1 bit chips but later ones and XEGS have 4 bit chips.

You neglected to mention the first Atari to use the 4-bit chips, the 600XL with its pair of 4416s.

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