After swapping, like, dozens of RAMs and not getting anywhere new and exciting, I’m wondering if anything else is amiss with my system. I look at the CPU board, then the I/O board. It’s a quick glance and I almost miss it.. but I just catch a glimpse of a potential problem — one of my 74100s on the board has a pin bent out. Pulling it and straightening the pin, I reinsert into the socket, and then reinstall the card. For kicks I hit the power button, at this point not really expecting anything. But then this happens:
OMG
Not quite all there, but there all the same: the ‘famed’ Initialize message. Still a lot of screen garbage but man, am I excited to see that prompt. For me, it’s like finding the lost city of Atlantis! I know now that there’s a good chance my EPROM is okay and that the basics are working. At this stage I could well try loading a tape, but I’d like to clean up the garbage on the screen first, which I’m now certain is just more bad video RAMs.
Since I don’t have a working machine that uses 2102s, I’m forced to resort to removing an IC, subbing in an IC from my spare board, and then powering up to check the result. If something changes, I set aside the IC I pulled and carry on. If nothing, I pull the next IC and insert the apparently working IC from the last socket into its place. This is tedious and probably hard on the power supply and capacitors, but, slowly but surely by process of elimination, the screen gets better.
https://www.youtube.com/watch?v=58uFALQf2is
And then at last, with one more RAM chip swap..
I’ve hit paydirt! Yahoo!! And all I can do is look at that beautiful prompt in astonishment. Against all odds, I have raised this old girl from the dead! It’s almost like traveling in time!
It’s tempting now to try to load something, but I realize the video board might actually make a really useful diagnostic tool. Bad RAM chips will immediately distort the video in some way, so this is an opportunity to start swapping in all the chips I have knowing I have 8 known good, and start weeding out the bad. It’ll take hours to do them all, 3-4 RAMs at a time gambling most will be okay, but it’ll be worth it in the long run — that will give me confidence the machine is in fact running right and capable of loading something. Stay tuned!