I did a video a while back about the first steps in troubleshooting a vintage computer. In a nutshell think ‘PCR’ Power, Clock, Reset. Make sure that all power supply rails are functional, then check that you have a good clock signal and finally check for a properly working reset. Without these 3 basic things nothing else will work and you can get confusing results. For example a reset that does not work properly can cause everything from not booting at all (held in reset) to the system coming up various random states as things were not properly reset.
Jeff Birt From: M100 <[email protected]> On Behalf Of Brad Grier Sent: Monday, March 22, 2021 10:33 PM To: [email protected] Subject: [M100] In over my head? Or a Challenge!! Hi everyone, as the subject line says, am I in over my head (for someone with old basic electronics knowledge), or is this a worthy challenge? [TL;DR] System symptoms: Won't power the screen, BASIC doesn't really work, unusual voltages on LCD connector pins. What to do? And why?? A few months ago I received a M100 that wasn't really working. Initial symptom is no display. I was looking at this as a learning experience -- to see if I could do some simple fixes and get it going again, and dust off my ancient basic electronics knowledge. I only have a multimeter, so I knew this could be a challenge. Initial testing revealed that it did power up and will 'Beep' on command (blindly entering Basic and typing Beep<enter>). LCD does work -- I connected it to my NEC PC-8201a and had a functioning display (with a tiny line of dead pixels in zone 1). So I'm ruling out a bad LCD. The mainboard looks fine. No obvious scratches or leaking battery or caps. No obviously damaged components. No staining of any kind other than the standard-issue coating of flux (which I've read can turn conductive so I'm open to cleaning all that off too). Display-related transistors and diodes (according to the troubleshooting flowchart) check out. The caps look great too -- but I haven't desoldered each of them to test them out of circuit. I've read recommendations to recap anyway, but I'm not sure it'd be worth it if the other problems aren't related to bad caps. Voltages on the LCD Connector pins seemed weird when compared with my NEC PC8201a. Image here: <https://imgur.com/a/xfNIdF1> https://imgur.com/a/xfNIdF1 Related to caps? Something else? The LCD is getting these voltages (the cable is fine). So now I'm thinking there might be something with the logic. So I tried typing a simple basic program, blindly, but it's a short program so I'm pretty sure I got it in properly: 10 beep 20 goto 10 Nothing. No string of beeps. And after that, a simple beep<enter> won't work either. But, power cycle or reset, enter basic, type beep, it works. beep:beep:beep also works. Now I'm thinking partially bad RAM? Or RAM select logic? So, two issues (display and BASIC), or is this all a case of a bunch of invisibly bad caps and I should just bite the bullet, desolder a few and test them. Thoughts? Ideas? What am I missing? Is this thing destined for a parts computer or could it be a good challenge to heal it up? All advice appreciated :) --Brad -- -- Brad Grier
