First post, by AndrewBSSC4
Hi folks,
Hoping for some help. So still trying to get this Commodore PC10-III fixed. Having resolved the BIOS issues and changing out the bad RAM I'm now at an issue where I'm having intermittent RAM problems now.
- Sometimes the computer will detect all RAM fine and run fine. All memory tests will pass and running software on it will run with no issues. I've had a few times run for 9+ and even 12+ hours straight with no issues. Even with warm and cold boots mixed in. No issues. But this has been very rare.
- Sometimes during boot it will detect bad memory Sometimes it might be after 512KB, or 384KB, 256KB, or even less. The machine will let me continue and run with the detected memory. But often that won't be for last long as eventually it will either detect more bad memory during memory tests or freeze.
- Often when I get the weird bad memory error above I can reset the machine and often the memory test will be different. So I might reset from a 512 and get a 384. Reset again and be back at 512. Reset and get the full 640. Reset and get another change.
- And then sometimes it'll just get "stuck" with complete memory failure in the 16KB critical. At which point I change the BIOS to the Supersoft-Landmark BIOS and the memory problems are visible. And again the memory errors will fluctuate. Once test cycle might show 2 bad chips. Next might be 1. Next might be no bad chips and all tests are fine. Next might be all chips bad.
- Often when it enters the worst case scenario it never comes back regardless of resets. I often have to leave it for a few hours. I've notice most often it will start up and detect all ram and run fine for the longest first thing in the morning after it has sat all night. But often it might start having problems after running between half hour to 2 hours.
The memory chips have been tested each individually and in another system. I've tried 3 different brands. All react the same. And as mentioned the few times where it all checks out and runs - it is rock solid. So I don't believe it's the memory chips or my socketing work.
I checked power to the chips and all looks fine.
I've even tried freezing the chips and board to see if temperature would play a role. No effect.
It's driving me crazy because it's so unpredictable. Lookign at the schematic I'm leaning it towards being on the LS chips failing. From what I can see there's:
- One 74LS245
- 3 x 74LS158
- One 74LS04
- One 74LS00
I'm wondering if it might be the 74LS04 or 74LS00. Unfortunately I don't have a scope as part of my tool arsenal.
So I was wondering if anyone ever encountered this and might point me in the direction to debug and fix this. Am I going in the right direction in thinking it's an LS chip failing? Or should I be looking at something else? Any simple tests I can try with a multimeter and logic probe?
Any input would be appreciated.
Thanks.