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hueyjones70

800XL with several bad ICs

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I just started troubleshooting a nonworking 800XL and started with memory chips. All of the memory chips were bad, the MMU was bad, and the BASIC and ROM were also bad. Before I replace a bunch of chips I would like to know if there is a likely cause for all of those chips being bad. I would hate to fry another set because I didn't know enough to check for a likely cause.

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A (very!) bad PSU can do this, as can a serious power surge. A common cause of such surges, aside from a bad PSU, might a very close-by lightning strike inducing a current in the line. 

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I ended up replacing the ICs mentioned earlier plus the GTIA, ANTIC, and that strange IC that in in a 16 pin socket but only has 4 pins on each side.  Now everything is working.

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I have another 800XL motherboard that I have tested every IC on the board and they are all good but the computer boots to a blank blue screen. If I boot with a Star Raiders cart it sometimes makes some static like noise and then the screen goes completely black.  I plan to eventually start checking voltages on pins but that is a job for a bad weather day or something.

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The strange IC with half the expected # of legs would be the delay line.  Used on earlier machines (before Freddie) for the Ras to Cas memory select delay.

 

A blank blue screen then locks up is at least some function which means the CPU is probably good and the OS, Antic and GTIA are probably OK too.

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21 hours ago, hueyjones70 said:

I have another 800XL motherboard that I have tested every IC on the board and they are all good but the computer boots to a blank blue screen. If I boot with a Star Raiders cart it sometimes makes some static like noise and then the screen goes completely black.  I plan to eventually start checking voltages on pins but that is a job for a bad weather day or something. 

 

Sounds like a POKEY issue to me. Do you hear the farting sound? Maybe the socket is bad, since you said you tested the chip...??  Be sure you are using a known good power supply.

 

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Yesterday I was troubleshooting an 800XL MB that gives nothing but a black screen. While I was checking memory chips, my test board suddenly went the way of the bad MB and gave nothing but a black screen even with its original supposedly good chip set.  Today when I turned it on I still got the black screen but it was making a noise like a screeching sound.  It will boot Star Raiders but nothing else and it still makes the screeching sound mixed with the sound that Star Raiders usually makes.  I tried swapping our the ROM and BASIC but I haven't had time to recheck all of the ICs.  What besides the ICs would be a good thing to check.

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1st of all, the power supply!  It could be killing your machines.

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If it's not the PSU as @Stephen suggests, take a close look at your sockets. The Atari originals are usually single-wipe. These are definitely not the best, they do get dirty, the do corrode and they can certainly even break (personal experience).

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I will check the sockets. I am using a white brick power supply that gets checked just about every time I work on a computer; if not directly, then indirectly because one of my checks is to verify voltages through the connector, the switch, and caps near the switch.

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This has been a great day for 800XL repairs. One MB had a bad delay chip, the one that fits in a 14 pin socket but only has 8 pins. Another had bad socket for the POKEY, and a third had one bad memory chip.

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Awesome - isn't it great keeping these old machines alive (or even better, reviving them from the dead)!

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Going to add to this thread, because it is a same type issue.

 

I have a good HK socketed 800xl that I use to test others. I am trying to fix a couple of 800xls for friends one is a Taiwan solded board and the other seems like a very early HK revision C  as you can see by the picture the ram sockets were dont this way to accommodate the factory mod.

 

So both the HK rev C and the revD Taiwan I have black screen. The HK rec C was Super hot ram.. and when I put them 1 by 1 in the HK tester.. it failed to boot.. but 8 bad ram??? Notice the HK tester does not have the resistors on the ram.

 

The Taiwan board ram is cool, I have swapped IC chips.. no change.. any suggestions??

 

James

revDTaiwan.jpg

revcHK.jpg

HKtester.jpg

revcHKbk.jpg

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Top PCB is MT RAM, I can see them all being bad certainly.  OR, a power surge / wrong power supply could have fried them and other chips in the machine.

 

Bottom PCB, there's a pin with zero solder on it (top row, rightmost pin, under the E.T).

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agreed, MT RAM is rarely functional. if it is, its on borrowed time...

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Also try deoxit and clean the chips legs and sockets. Fully inspect the Taiwan board for cold joints or burned components. Then a thorough continuity test is in order. Maybe start with the power into the board.

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So the MT ram.. is soldered on.. I guess the only way to determine if its good or not it to remove it. and put sockets in. I will see if the owner wants to do that.

 

The ram that is in the HK board.. this is the one with the factory "mod" of the resistor and the bottom pic with the drill holes breaking the original trace.. I will check that for missing or cold, etc..  

 

Both machines do trigger a signal to the TV.. though black screen.. I use IPA .. do people find this Deoxit better??

 

 

Thanks

James

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8 hours ago, Bikerbob said:

I guess the only way to determine if its good or not it to remove it. and put sockets in. I will see if the owner wants to do that.

Do you have an oscilloscope or at least a logic probe? If so, you can check the data and address bus lines on the chips to see if they’re responding properly or not. That won’t necessarily be definitive but it might help narrow things down.

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oscilloscope is really big on my list DrVenkman. I have run into so many times where I could use it. now that I am doing more repairs for people.

 

James

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