800XL Yellow Screen

[swissarmytin]

Recently picked up an 800XL in really nice shape, however I am completely stuck at a yellow screen upon powering on. While I was at a local vintage computer usergroup, a member with a 'Chip Tester Pro' pulled and tested most of the ICs, all coming back as fine on the tester. Still a yellow screen. After bringing it home and downloading Atari's field service manual for the 800XL, it pointed to a faulty U28 IC, a TI SN74lS375N latch. Since I have a spare parts unit, I yoinked the IC out of it and swapped it in. Still a yellow screen. Assuming the chip works, maybe it was a bad socket? Single wipes are known to suck, so I replaced it with a dual-wipe. While soldering, I checked for any bad solder joints or obviously failed components, finding none. Tested it again, still yellow screen. Can't get it to boot from a cartridge or into the self-test (the keyboard is of the mylar variety though so take this with a grain of salt).

 

Anyone have a clue what's wrong? I've never had the greatest luck with the XL line so it could be something obvious or simple I'm overlooking. The PSU I'm currently using is the massive beige one that matches the case XL case design, probing it reads a steady 5v so I don't think that's the issue. The one used at the usergroup was a modern one iirc, brought by our club's Atari specialist. Video is composite out, same cable I use for my 800 which works fine. 

 

If anyone can help, I'd be greatly appreciative. 

 

Thanks!

[reifsnyderb]

1 hour ago, swissarmytin said:

Recently picked up an 800XL in really nice shape, however I am completely stuck at a yellow screen upon powering on. While I was at a local vintage computer usergroup, a member with a 'Chip Tester Pro' pulled and tested most of the ICs, all coming back as fine on the tester. Still a yellow screen. After bringing it home and downloading Atari's field service manual for the 800XL, it pointed to a faulty U28 IC, a TI SN74lS375N latch. Since I have a spare parts unit, I yoinked the IC out of it and swapped it in. Still a yellow screen. Assuming the chip works, maybe it was a bad socket? Single wipes are known to suck, so I replaced it with a dual-wipe. While soldering, I checked for any bad solder joints or obviously failed components, finding none. Tested it again, still yellow screen. Can't get it to boot from a cartridge or into the self-test (the keyboard is of the mylar variety though so take this with a grain of salt).

 

Anyone have a clue what's wrong? I've never had the greatest luck with the XL line so it could be something obvious or simple I'm overlooking. The PSU I'm currently using is the massive beige one that matches the case XL case design, probing it reads a steady 5v so I don't think that's the issue. The one used at the usergroup was a modern one iirc, brought by our club's Atari specialist. Video is composite out, same cable I use for my 800 which works fine. 

 

If anyone can help, I'd be greatly appreciative. 

 

Thanks!

 

If you are sure it points to U28, I'd check out U18 and U19 next as they send signals to U28.

 

Edit to add:  I'd check all traces between U18, U19, and U28 as well.  Make sure those chips are getting power, too.

 

 

 

Edited June 19, 2022 by reifsnyderb

[Beeblebrox]

@swissarmytin  I had a look around regarding this on the 800XL for some other insight, just in case. Had a look at Sam's computer facts for the 800XL, etc.

 

Interestingly there is an old post here on AA regarding an Atari 400 with a yellow screen where replacing a bad Rom IC fixed it. Since you have nothing to lose, after following @reifsnyderb advice above it might be worth having a look at the Rom? 

 

?

[GoodByteXL]

As it was not mentioned: Did you check the setting of CADJ ?

[swissarmytin]

3 hours ago, Beeblebrox said:

@swissarmytin  I had a look around regarding this on the 800XL for some other insight, just in case. Had a look at Sam's computer facts for the 800XL, etc.

 

Interestingly there is an old post here on AA regarding an Atari 400 with a yellow screen where replacing a bad Rom IC fixed it. Since you have nothing to lose, after following @reifsnyderb advice above it might be worth having a look at the Rom? 

 

?

Well seems we have a winner here. I tried everything else suggested first, only to continue to face the yellow screen. Even started swapping random chips from the parts machine out of frustration to try and get any change (the parts machine's GTIA spat out a brown screen...maybe a hint to what's wrong with that unit) until I remembered the ROMs. Lo and behold, swapping them out and the machine fired up to the Basic prompt! Ran the self-test and everything came back fine, even the keyboard which is a shocker. I remembered the two ROMs passed the Chip Tester just fine, but remembering this motherboard used crummy sockets, I inspected the pins and yep, the OS rom had a bit of corrosion. Wouldn't surprise me if cleaning the pins would've solved the entire issue, but for now the XL is doing just fine with the replacements. 

 

 

Thanks for the help guys, I really owe ya one

[Beeblebrox]

@swissarmytin  great news. ? 800XLs in my experience are very well built and robust. The Hong Kong ones are great fully socketed, but sometimes those old style single and double wipe sockets can be trouble. Enjoy. 

[Mclaneinc]

oops..

Edited June 19, 2022 by Mclaneinc

[+DrVenkman]

10 hours ago, swissarmytin said:

I remembered the two ROMs passed the Chip Tester just fine,

What sort of "Chip Tester" can test Atari-custom mask ROMs? 

[TGB1718]

8 minutes ago, DrVenkman said:

What sort of "Chip Tester" can test Atari-custom mask ROMs? 

Was thinking exactly the same

 

[+DrVenkman]

1 minute ago, TGB1718 said:

Was thinking exactly the same

 

It's possible they were stuck into a some kind of EEPROM burner and dumped, then checksums compared, but that's not terribly likely. And of course, there's no standalone chip tester that can check SALLY, ANTIC, GTIA, POKEY or PIA, let alone the Delay Line (which wouldn't be socketed anyway). 

[swissarmytin]

19 minutes ago, DrVenkman said:

It's possible they were stuck into a some kind of EEPROM burner and dumped, then checksums compared, but that's not terribly likely. And of course, there's no standalone chip tester that can check SALLY, ANTIC, GTIA, POKEY or PIA, let alone the Delay Line (which wouldn't be socketed anyway). 

The guy had this doohickey: https://store.backbit.io/product/chip-tester/

It claims to be able to test ANTIC, POKEY, and the GTIA, among other things. I could totally be wrong about him using this particular device with the ROMs as my attention was split ten different ways that day, but I remember the checksums being compared and verified in some way or another. 

[TGB1718]

1 hour ago, DrVenkman said:

It's possible they were stuck into a some kind of EEPROM burner and dumped, then checksums compared, but that's not terribly likely. And of course, there's no standalone chip tester that can check SALLY, ANTIC, GTIA, POKEY or PIA, let alone the Delay Line (which wouldn't be socketed anyway). 

It does say it can test them all, in theory it could test the delay line while in-circuit.

Many moons ago I worked in a test department where we tested all components passive/active/digital while in-circuit

we could also test ROM's and EPROMS by having the programmed data stored and compare with the chip being tested.

 

Maybe this test has that functionality built, it seems to claim it can do it.

[Beeblebrox]

Just a quick one. If it's an 800XL Hong Kong socketed PCB then the delay line IC is socketed also. All of the HK socketed PAL 800XL's I have had over the last year were at factory BITD:

 

Edited June 19, 2022 by Beeblebrox

[swissarmytin]

I stand corrected, the OS ROM is super dead. Cleaned it up real good and it instantly returned to yellow screens. The original Basic ROM is perfectly fine though. More dead parts for the parts machine, more good parts for the working machine.