800XL Repair

[kb1ujs]

Hi all,

 

I recently bought an 800XL with some issues.  When the system is powered on, it immediately boots to the Memory Test.  Both ROMs fail the test, and the RAM test only completes two or three blocks before it fails with the screen attached.  The keyboard is unresponsive and I need to power cycle the machine to reboot.  I'm new to Atari 8-bits but I am confused how the ROMs fail the test yet the machine boots to the Memory Test screen.  I would assume based on my limited reading reseating the socketed chips (very few on this particular machine), testing voltages, and ordering up new RAM and RAM sockets would be where to start on getting this thing running again.  Am I missing anything else?

[TGB1718]

Might be worth checking your PSU first, make sure it's 5VDC +/- 5% on load, not sure as I've never had one, but I think the 

800XL can come with an ingot power supply, check if you have one of those, if so bin it 

Look here

 

 

Edited September 21, 2020 by TGB1718
Added Link

[cwilbar]

I haven't heard of sockets going bad on the Atari 8bits.  Though if you are changing a chip, obviously always put in a socket ?

 

Now I have heard of and had a few issues with some of the single wipe sockets used on arcade PCBs, though in my case they usually respond well to a cleaning as long as they are not corroded.

 

You could have bad RAM and ROM... especially if a dreaded ingot was used with the system at one time (and it wasn't producing the proper voltage still).  I think I only have one ingot, and it produces correct voltage, but I don't use it after everything I've read about them.

 

Do you have another XL you can swap parts with ?

 

Might want to check the MMU (which I believe should be socketed).  Not sure if it can cause what you are seeing, but it is used in addressing.

 

[kb1ujs]

None of the RAM is socketed, so if I replace it I'm going to put machine pin sockets in.

 

The computer came with a modern replacement switching power brick.  I am going to test it though as I don't know its provenance.  

 

This is the only Atari 8 bit I have, so swapping parts isn't an option.  I do have access to test equipment (oscilloscope, logic probe, multimeter) so I'm hoping to get a good enough idea of what's going on to get the right parts 

[mellis]

1 minute ago, kb1ujs said:

None of the RAM is socketed, so if I replace it I'm going to put machine pin sockets in.

 

The computer came with a modern replacement switching power brick.  I am going to test it though as I don't know its provenance.  

 

This is the only Atari 8 bit I have, so swapping parts isn't an option.  I do have access to test equipment (oscilloscope, logic probe, multimeter) so I'm hoping to get a good enough idea of what's going on to get the right parts 

I am currently troubleshooting an 800XL that went from working just fine to showing self-test garbage characters before my eyes.

 

I have socketed and replaced the RAM (it was time) and the two 74LS158 MUX ICs, but it turns out that wasn't it.  I've noticed that if I put a little downward pressure on the CPU, it tries to find a boot device and then hangs. At this point, I suspect either the CPU socket, the POKEY socket, or the POKEY chip.  Might also be the OS ROM.  

 

There is an 800XL schematic floating around.  It is helpful.

 

Good luck!  

[TGB1718]

This may help if you don't already have it

Atari_800XL_Sams_Computerfacts_Technical_Service.pdf

[+Nezgar]

3 hours ago, cwilbar said:

I haven't heard of sockets going bad on the Atari 8bits.

Oh really? These forums are full of accounts of issues with the cheap single-wipe sockets that Atari used. A little bit of corrosion will easily cause a bad contact with a pin. This is why first troubleshooting step is to reseat every socketed IC.  Second step would ideally be to swap each component into a known working system to quickly identify a faulty IC...

[+DrVenkman]

3 hours ago, cwilbar said:

I haven't heard of sockets going bad on the Atari 8bits.

My Ugly Duckling 1200XL would disagree. I ended up replacing 3 x 40-pin sockets in the course of my repair/restoration. This wipe on pin 19 had corroded through and broken clean off.

 

 

 

 

[kb1ujs]

I reseated the few chips on the board that are in sockets (POKEY, CPU, ANTIC, and GTIA) with no change.  Off to look for replacement RAM and ROMs.

[kb1ujs]

4 hours ago, TGB1718 said:

This may help if you don't already have it

Atari_800XL_Sams_Computerfacts_Technical_Service.pdf 29.89 MB · 5 downloads

Thank you for the links.  Lots of great info there.

[xrbrevin]

A ROM can be replaced with an EPROM if have a burner?

RAM chips can be piggybacked to identify the faulty culprit

Edited September 22, 2020 by xrbrevin
typo

[cwilbar]

guess I've gotten lucky on the Ataris I've worked on in regards to sockets.  I've seen sockets like those posted above on arcade boards.  Single wipe sockets are never ideal, but on Atari 8bits I've just been lucky enough that they have all been in good shape (as good as a single wip socket can get anyway ? ).

 

 

 

[+Nezgar]

18 hours ago, kb1ujs said:

I reseated the few chips on the board that are in sockets (POKEY, CPU, ANTIC, and GTIA) with no change.  Off to look for replacement RAM and ROMs.

If you're interested, PM me and I can mail you a replacement EPROM programmed with the stock XL OS. I can also send you a second EPROM programmed with Shoestring's RAM tester to further diagnose the system.

[kb1ujs]

Not meaning to necro this thread, but to help those in the future searching out answers to this question.

 

I removed U3 and programmed a GAL to replace it.  That made no difference in the behavior.

 

I then ordered a replacement set of RAM chips.  I removed the old ones, socketed the RAM locations, and installed the new RAM.  The machine now boots to the ready prompt.  

[hueyjones70]

Way to go.