I've worked all my life at troubleshooting ... computers, software bugs, radiocom interference problems ...
The first big lesson I learned is that what the client tells you is usually in good faith, but don't rely on it at all. Recheck everything. The OP tried to resolve the issue and I admire that. But, he's not a professional. It isn't like another tech at the shop stated that this list of items were checked, so the second tech shouldn't do it over again.
If I understand correctly, they actually worked 6 hours to find that it wasn't getting power! Glad it wasn't a complicated problem!