plexxer

Thanks to everyone for your advice so far! Unfortunately, life has intervened and I have to shelve this project for the time being. I was able to manipulate the schematic in the technical repair manual into something that makes a little more sense to me. Hopefully someone will find it useful.

I have a 1200XL in my collection with an unknown history, but that has apparently had a hard life. Along with the nefarious physical damage, it has spent some time in a high humidity environment. The PF shield is pretty rusted, but the motherboard seems to be in decent shape. I bought a power supply for it and connected it to a Commodore 1702, but I am getting nothing but a black screen. I currently have just the motherboard on my bench and am doing some diagnostics, but not exactly sure what I am seeing, and was hoping for any recommendations. The LEDs weren't working, so I replaced them and the power LED lights up when the unit powers on. I tested the LM340s and both are putting out 5v on the output pins. I removed and reseated all of the socketed ICs. There is corrosion on some pins, but I cleaned the ones that I could see with a fiberglass pen and all the ones I tested did show continuity from the pin to the solder point on the reverse side of the motherboard. One of the pins on U16 (SN74LS114N) broke when I removed it so I replaced that IC with one from Ebay. I removed the two EPROMs and dumped them with an EPROM dumper I built and both seem good. When I use my BitScope logic probes, on boot up, I only see activity on D1,D3,D5,D6 and D7. Also, the only activity on the Address lines are that A8,A9 and A10 are high the entire time. The CPU reset line is also high. The Phase 0 clock seems fine. I also get signals from Pin 21 (COL) and Pin 25 (CSYNC) on the GTIA. They last for a few seconds. If I crank up the brightness on the monitor, the screen looks dark initially and then gets brighter. It is still blank the entire time. This corresponds with the signals on Pin 21 & Pin 25. I am a bit stumped as to why I am not getting any activity on D0,D2 and D4 nor on any other address lines. Any suggestions are welcome! IMG_1542.mp4