Hello Jeff, I took a look around and I see a number of motherboards for sale on eBay but because they also house the storage you run the risk of buying one that is locked behind someone else's Google Account. They seem to be going for about $120 - $165 USD before shipping. It looks like even if you hard factory reset if the Google Protection was set-up on the device it would still need the log-in credentials. [br]
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What's wrong with the motherboard? Replacing the whole thing seems like it's not the best option but maybe there is an easier fix. If you're not sure what's exactly wrong then try explaining what problems it is having and maybe we can try and figure out what's broken and it could be an easier and safer fix than the whole board!