When LCDs with these types of LCD driers first power up they turn on all the pixels. You will see this even in brand new ‘modern’ dot matrix LCD module (on the new LCDs the multiline modules tend to only make the first row black on power up.) Only after the computer sends a rest command to the LCD drivers they will clear the screen.
Since you have already confirmed PCR (Power Clock Reset) and the beep test does not work it would tend to indicate the computer is not booting up properly. There can be multiple reasons for this. 1. Bad ROM or broken lines to ROM. This can cause the computer to run random garbage code, but will sometimes result in it running a halt command which you can see as the processor will stop. 2. Bad RAM, specifically the #0 RAM module. This is where the stack gets set up and if the stack area of RAM is bad it will crash on the first bit of code that tries to pop anything off the stack (because it gets nonsense back). I have experienced both problems multiple times. The RAM issue is harder to find without a diagnostic ROM. I have a M100 test harness and the first things its diagnostic ROM does is test the RAM. If it finds a RAM module with a large enough working section for the stack it will continue testing. If not, it stops. The first M100 I worked on had a bad RAM module #0 which allowed the stack to be overwritten by writes to other addresses. On a cold boot / hard reset up the part of the initial start-up sequence is to copy some code from ROM to RAM (which is a self-modifying bit of code, thus needs to be in RAM.) The copying of the ROM into RAM overwrote the start and thus the return from the jump to that memory address went to never-never-land. I found it with an 8-challen logic analyzer by recording ROM accesses and following along with an annotated disassembly. It was a very painful and slow process. The diagnostic ROM/test harness makes this a doddle. Here is a video I did on the first M100: https://youtu.be/hbLWk7ir9sI Jeff Birt From: M100 <m100-boun...@lists.bitchin100.com> On Behalf Of Eric Sent: Wednesday, December 27, 2023 11:56 PM To: m100@lists.bitchin100.com Subject: [M100] Model 100 - LCD Shows Pixels Only 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) Troubleshooting Performed: New NiCad installed and charged (memory switch on). Board traces near battery corrosion have good continuity. All capacitors were replaced. Board traces near leaking capacitors have good continuity. Cold Start and "BEEP" attempted. Beep was not heard. DC/DC Converter Circuit meets specs per section V of Service Manual 26-3801/3802 Reset Signal meets specs per section V of Service Manual 26-3801/3802 Logic Circuit meets specs per section V of Service Manual 26-3801/3802 CPU, ROM, Ram all have activity on the scope. Clock signal looks acceptable. Eric
