On Thu, Dec 28, 2023 at 1:13 AM Eric <eric_1...@yahoo.com> wrote: > Hi, > > I'm working to troubleshoot a non-functional Model 100 that only displays > pixels when powered up. Seems these computers occasionally have RAM issues > but I'm not sure how to identify faulty RAM that has not been removed. The > RAM modules are soldered in place and I would like to troubleshoot the > board to avoid unnecessary removal of parts. If anyone has other > troubleshooting steps, I'm happy to try them out. I have access to a logic > analyzer (Saleae software), oscilloscope, and DMM. > > Issue: When powered up, LCD shows pixels only (black screen w/adjustable > contrast) >
Hi Eric, Just to clarify, the LCD is entirely "black" right? All pixels are on? I think this is an issue with the LCD board, and here's why: On power-up, the LCD will very briefly display random data where the state of each pixel is random based on contents of the controller IC RAM. Shortly after this, the "boot" sequence will "clear" the screen which causes all pixels to turn on (black) and then off (white). The main CPU will actually wait for this to happen before proceeding with the power-on sequence. So, I suspect that the command to complete the "reset" is not completing. The failure of the "blind BEEP" test is consistent with this. The CPU can run code, but is hanging during the power-on sequence. I would carefully inspect the LCD PCB for corrosion and/or damaged traces. Possibly re-flow the controller chips if you feel comfortable working with those. If you happen to have another working LCD board, you can swap them and make sure the problem follows the LCD board. Good luck! -Josh
