When do I decide that my SNES VRAM chips are nonfunctioning?

Hi all, I'm repairing a few SNES consoles and one is giving me trouble. It starts fine, plays sound, and "things" move in the background, only the graphics are garbled, with horizontal and vertical strips of green. From reading these forums it seems that the VRAM might be at fault, specifically the address pins VAA/VAB0-14.

What I've done to try and fix it:
Trace all connections from U4/5 to U2-PPU1. All look fine.
Reflow solder on U2,3,4,5,6 (should I touch up the pins with extra solder?)

So far, the graphics have not changed. So, are my VRAM chips just blown? My next step is to trace PPU1 to PPU2. Would checking the S-WRAM chip be useful?

I am working with a circuit diagram I got from wiki.superfamicom.org

Thanks!

 

Seems several people have those Working SNES with Garbled graphics. Here I rather get those with the Black screen of death or games quickly crashing. I suspect wram in my case. I have at least 4 of these consoles.

Troubleshooting by checking all the traces can be tedious. I gather some testing could be done with a scope too. However what I would like to know is actually some sort of statistics about those issues.

Someone who would have repaired a lot of snes in an official nintendo repair center would probably be able to tell you things such as ( Games crashing immediately or not running at all was bad Wram 75% of the time )
I think such stats would be very useful. As much as I like these consoles and the idea to save them, checking all the traces one by one is something I hate doing.

Also, I think having a burn in test cart might help.