I recently acquired an Apple IIGS. When turning the computer on, the screen fills with the "@" symbol for about half a second (which I believe is normal), then shows the following corrupted display:
Running the diagnostics gives error code 02E00100, which I believe points to a fault with the standard RAM. However, after replacing the four chips, I get the same reults:
The board has no visible damage or battery leakage. I have removed the battery, the system doesn't have one installed at the moment.
Am I barking up the wrong tree with regards to the fault being the standard RAM?
After a bit more testing:Adding a battery does not seem to affect the system
The 12V rail seems to be a bit low, it reads 11.6V. The other voltage rails are correct.
I am not sure if you already found this older thread or if it can be of any help, but here it is nevertheless: https://www.applefritter.com/content/apple-iigs-bad-onboard-ram
The bad address is in the slow RAM, but it is in the stack page as well, which may be significant. Do you have "Alternate Display Mode" turned on?
Errors in memory don't necessarily point to the physical memory chips. They may also be caused by bad traces, solder joints, or interface components. The pins of the Mega II chip are required to access the slow RAM, for example.
By the way, the address E0/0100 is in the main bank, so only the two chips UK3, UK4 are implicated. The other two chips comprise the alternate bank E1.
I've tried reflowing the solder on the Mega II, as well as different combinations of RAM chips, unfortunately there is no change. I poked around with a multimeter and oscilloscope, all the pins on the RAM are connected and have the expected signals on them during the diagnostic test.
The system won't boot to the control panel, so I can't check if Alternate Display Mode is on. However, I have removed the battery, so the system should be running with the factory defaults. I tried attaching a AA battery pack (~2.5V) with jumper wires, but it did not appear to make a difference.
My board looks to be a fairly early revision, there are a couple bodge wires on it that appear to be from the factory, and the chip locations are slightly different (the slow RAM is UM1-4). I have made sure the bodge wires are connected properly.