I'm stuck. I'm chasing my tail on a Mario Bros, and I wanted to step back and ask for some help. I have a pulsing Read signal, but Write stays high. All of my previous experience points me to RAM and program ROMs on the CPU board, but I believe I have ruled those out, and rung out all circuit traces associated with them.
Symptoms: Rug pattern, (bootup garbage on screen)
stuff I have verified working:
- z80 clock signal good
- ROMs on CPU board have all been verified.
- replaced both CPU and DMA sockets
- I've swapped CPU and DMA chips with known good
- socketed and swapped 6116 RAM with known good
- Z80 /Read signal is pulsing
- replaced ECL, and 2148 RAM sections on video board
- CPU reset signal stays high
- all z80 address and data lines are all pulsing as expected.
I'd assume if my CPU board has a good CPU, DMA, clock signal, ROMs and CPU RAM that I would normally see the /Write pin pulsing. Am I mistaken?
Does anyone have any advice?
FYI, full schematics are on pages 37,38 of this link:
https://archive.org/details/mariobrospak/page/n37/mode/2up