I missed your first post about this board or I would have spoken up earlier, but I have some
experience with troubleshooting glitching M72 hardware and can offer some advice.
Here's the board breakdown:
- The bottom board draws the two background layers and has its own background EPROM chips onboard.
- The middle board has the CPU for executing game logic, performs foreground sprite rendering, and generates music & SFX, and has the palette RAM and DAC for final video output. It doesn't have any EPROMs onboard.
- The top board just has the EPROMs with the assets for the middle board.
The diagnostic VRAM check at startup only checks the middle board, so you should focus your troubleshooting there, not the bottom board.
Have you pulled the EPROMs on the top board and verified them against MAME? I would recommend doing that first - a bad EPROM could trigger diagnostic alerts.
After the EPROMs have been verified, take a look at both the D43256C RAM chips in the upper-right corner of the center board (IC 70 and IC71), as well as the TTL chips around it. I'm pretty sure those two RAM chips are V-RAM A and V-RAM B. The TTL logic chips (the smaller chips that start with 74 for their part number) handle memory mapping.
Check for physical damage first - rust on the legs, or damaged traces on the underside of the PCB. You may also want to desolder the RAM chips and test them out-of-circuit - most EPROM programmers have a SRAM testing function.
Schematics for R-Type are available here:
http://wiki.pldarchive.co.uk/images/5/5e/Irem_M72_schematics.pdf
You'll want to look at the pages for the M72-A-C, since that's the middle board. I believe Page 17 of the schematics is the VRAM specifically - it shows two 256k SRAM chips, so I bet those two SRAM chips are V-RAM A and V-RAM B. Check the ICs indicated on that page of the schematic.
If you want to probe the RAM chips, datasheets for the SRAM chips are here:
https://pdf1.alldatasheet.com/datasheet-pdf/view/6974/NEC/D43256BGU.html
You'll want to probe the input and output lines on the RAM chips for stuck lines (reading only High or Low constantly) while the bootup diagnosis runs. If you find a stuck line on the RAM, trace the line and see if it connects to a TTL chip that manages the line. If it does, pull up the datasheet for the TTL chip and probe whether it's getting activity on its Input lines. It's common for a TTL chip to fail such that its Input lines get activated properly while one or more of the Output lines fail and get stuck. Like the SRAM, TTL chips can be desoldered from the board and tested out-of-circuit on an EPROM programmer.
I know that logic probing some sections of the middle board while the game is running can be difficult, but I was able to do it by disconnecting the bottom board, flipping the top two boards upside down, connecting the power leads between the top board and bottom board with some wire, and probing the middle board from underneath. One saving grace is that IC 70 and IC 71 aren't covered up by the ROM board, so you should be able to probe those chips at least as-is.
Hopefully this gives you enough info - happy hunting!