OOPS... There is no secondary board on the MV1C. It's the tiny all-in-one board.
Dirty carts, dirty slot, or damaged/scraped traces are the most common cause. Other causes: bad traces, surface mount chips that need resoldering, or bad surface mount chips.
What is happening is that the CPU can't read the program ROMs to determine if a cartridge is installed. When this happens it goes straight to hardware diagnostics and you see the crosshatch screen.