Apple II Plus system board without 16K Language card boots correctly.
Ctrl-Reset give the flashing prompt.
Even typing HOME returns an out of memory error.
Installing known good 16K Language card computer boot normally
but when typing HOME the screen goes to gibberish.
I know memory issues with these old critters can drive you into the nut house.
Any idea on where to start?
I would start by obtaining a good set of Qty=8 4116 RAM chips and start building rows beginning with the row closest to the keyboard.
You can get good sets of 8 for around $15 on eBay. Use the RAM test utility from Apple or you can perform a range fill statement (ex: 1000:FF 1001<1000.3FFFM) from the command prompt and verify statement as a quick Memory check if you're not using a disk drive. I don't remember which RAM IC is for which bit but you can actually narrow down the IC that way if a memory bit never changes. Often, even that doesn't work because you really need 8 good RAM (one good row) to start with.