600XL with bad CPU

[flashjazzcat]

I thought I'd share this one as me and Beamer320i have been driven close to insanity by his 600XL, which I upgraded to 64K some weeks ago. It's been back and forward in the post more times than I can recall: it would work fine at my house, then keep crashing when it got to Dom's. First time around, I'd missed a dry joint on the cart slot. I fixed that and sent it back. It was still crashing, though, with a simple four line BASIC program. The tests I was doing on the machine - while seemingly comprehensive - seemed to be skirting around the problems: SpartaDOS X and The Last Word were running fine. So today I decided to get serious with it, and I was finally able to reproduce Dominic's problems by using the exact same test programs. The BASIC program would either lock the system, or crash giving illogical value errors. Sometimes the screen would corrupt while the program was running. I tried:

 

• Reflowing the solder on the OS ROM, BASIC, MMU and RAM sockets.
• Removing and reseating all ICs.
• Replacing the OS ROM.
• Replacing the BASIC ROM.

Still nothing worked. As a last ditch attempt I pulled a socketed Mexican 6502 from a dead 130XE and stuck that in. The computer seems to work fine now: I can no longer replicate the crashes and errors in the BASIC test program.

 

How strange for the processor to be the culprit? I guess these old chips go bad from time to time. The faulty CPU is marked C10794/322 Thailand on the back.

Edited August 3, 2010 by flashjazzcat

[jvas]

I thought I'd share this one as me and Beamer320i have been driven close to insanity by his 600XL, which I upgraded to 64K some weeks ago. It's been back and forward in the post more times than I can recall: it would work fine at my house, then keep crashing when it got to Dom's. First time around, I'd missed a dry joint on the cart slot. I fixed that and sent it back. It was still crashing, though, with a simple four line BASIC program. The tests I was doing on the machine - while seemingly comprehensive - seemed to be skirting around the problems: SpartaDOS X and The Last Word were running fine. So today I decided to get serious with it, and I was finally able to reproduce Dominic's problems by using the exact same test programs. The BASIC program would either lock the system, or crash giving illogical value errors. Sometimes the screen would corrupt while the program was running. I tried:

 

• Reflowing the solder on the OS ROM, BASIC, MMU and RAM sockets.
• Removing and reseating all ICs.
• Replacing the OS ROM.
• Replacing the BASIC ROM.

Still nothing worked. As a last ditch attempt I pulled a socketed Mexican 6502 from a dead 130XE and stuck that in. The computer seems to work fine now: I can no longer replicate the crashes and errors in the BASIC test program.

 

How strange for the processor to be the culprit? I guess these old chips go bad from time to time. The faulty CPU is marked C10794/322 Thailand on the back.

 

It would worth a try to put the old CPU back (if the computer is still near you). Maybe only a bad contact ...

[flashjazzcat]

It would worth a try to put the old CPU back (if the computer is still near you). Maybe only a bad contact ...

One might think so, but I tested the old one and the problem returned immediately. Next challenge is to figure out why RAM 320XL unit is preventing the computer from booting. Cleaned PBI contacts, now warming up the iron again...

[dmlloyd]

It would worth a try to put the old CPU back (if the computer is still near you). Maybe only a bad contact ...

One might think so, but I tested the old one and the problem returned immediately. Next challenge is to figure out why RAM 320XL unit is preventing the computer from booting. Cleaned PBI contacts, now warming up the iron again...

 

If the problem is only BASIC, maybe the ROM chip is putting some kind of undue load on phi2 or something?

[spookt]

Interesingly I resurected my 600XL tonight and after an hour with the logic probe it turned out to be a bad CPU too. The CPU was the only custom chip I didn't change the other night. I dug out my "donor" 800XL motherboard and pulled the (thankfully) socketed CPU and swapped it into the 600XL and hey presto, working machine.

 

Now if only I could get my RAM 320XL working on the 600XL

[Back2skooldaze]

Does the atari 800xl cpu Atari C014806-35 CPU IC chip as used in the 800 XL and XE

6502C / Sally / C014806 / Replacement CPU for Atari 800 XL

Work in a 600XL ??? I think i have a bad cpu in my 600XL and have seen a new old stock cpu but its advertised for the 800XL ?? So i was wondering if the 800xl and 600xl where compatible?

[+Bikerbob]

Wow, good thing we all enjoy this as a hobby, because that is a lot of time spent on a $100 computer.

 

Flash - you said you tested the "bad" cpu - I assume you mean in another machine? not just re-seating and re-testing in the original?

 

600xl and 800xl were of the same vintage (series) as far as I know the cpu should be interchangeable.

 

James.

[flashjazzcat]

I can't remember much from six years ago, but if I encountered the same problem now I wouldn't even have posted about it. Swapping major ICs to stabilise Ataris is pretty much par for the course.

[+mytek]

Perhaps Transistor Aging is the reason for some A8 CPU's (as well as other LSI chips) failing down the road, when they presumably worked correctly when the computer was first built.

 

- Michael

[flashjazzcat]

In my experience (which isn't the experience of an expert in electronics, but just that of someone who repaired a lot of Ataris), the problems arise the moment the machine is taken out of spec by upgrades. I have a large stock of SALLYs, ANTICs and GTIAs here in my parts box and out of every one of them which still has all of its legs, only one (a PAL ANTIC) is something I would pronounce faulty. The rest all work fine once you happen to find the magic combination of 74LS08, CPU, etc, which works with whatever upgrade has been fitted or plugged into the machine. Six years ago I was pretty green at all that stuff, but now I just accept it as part of the A8's marginal bus timing. As for transistor ageing: I don't know enough to have an opinion, but if it is a factor, then it's certainly going to make things worse.

[+tf_hh]

Hi Jon,

 

is this faulty CPU a Synertek one? In the last 20 years I´ve repairing Atari computers dead or faulty CPUs are the second place in my personal "failure list" - 1st place was bad memory.

 

In my "defects" drawer over 10 Synertek CPUs have their last habitat. Mostly out of the 600XL models. My suggestion for this accumulation of faults is a.) the manufacturer itself and b.) the less free space in a 600XL.

 

Specially in the 600XL the CPU can reach 5-8 degrees Celsius more than in all other Atari computers. Second, the Synertek ones always getting up to 10 degrees Celsius warmer than Rockwell or NEC CPUs. Only the rar MOS one beat them. So it´s possible, than just one transistor got a defect - and maybe this one fails only in a combination forced by the BASIC. Nothing is impossible

 

Jurgen

[ricortes]

I've only had one cpu give me problems and that was a Rockwell Made in Mexico 6502C. Heat problems like Jurgen mentions. What happened was the machine would boot and run for a minute or two while the processor was reaching its 'I give up' over temperature then things would start glitching. Once it hit that temperature, fast cycling the power would not do anything positive with immediate problems. Letting it cool down would let it run for those few precious seconds again. I mention this just as a debugging aid.