It turns out that to make the internal monitor work, you need to plug in a special shunt connector that connects the raw video signals from one side of the board to the other side, which goes to the monitor. My son and I could have saved ourselves a lot of hassle and confusion if we had read the manual first...

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0:00 -- Introduction
0:29 -- Disassembly
1:25 -- Smoke test
2:08 -- Shunt revelation
2:47 -- Dodgy disk drive
3:00 -- Power supply tests
4:57 -- CPU clock signal
6:04 -- Video signals
8:25 -- Cleaning
8:55 -- What is this?