My old NES started having a graphical issue, it seemed like it was drawing sprites but no backgrounds (solid green in SMB as seen in the pic, but different colors with other games).

I was hopeful that this indicated a failure of the VRAM, since (as I understand it) it stores all of the information about how to draw the background tiles (image references, attributes, palettes). I found some replacement chips, removed the VRAM and installed a new one (in a socket, as seen on the bottom of the second pic). Unfortunately nothing changed.

On a whim I replaced all of the electrolytic capacitors, which I'd been planning to do anyway, but this also made no difference.

I saved it as a last resort since they aren't readily available, but I finally replaced the PPU (seen replaced with a socket in the second pic). Success!