I guess I'm supposed to see about 5 volts there even if I don't press the "On" button? I don't with either batteries in or power adapter.
No, as the Lynx is off there is no current flow and thus not voltage drop so I would expect you to see the raw in comming voltage (around 12V with power unit and 9V with batteries).
Ah: if I put 5v directly to capacitor C41 it powers on nicely. I assume that means the mosfet is faulty. I just ordered a new mosfet.
The MOSFET maybe faulty, however a drop is highly unlikely to damage the MOSFET, but it may crack a solder joint.
I would suggest applying power and measuring the voltage across the supply pins of U6 (pins 7 & 14), if you cannot measure the supply voltage there then one of your power rails is open circuit and you will have to trace it back from there to the battery connectors/power socket.
Edited by Stephen Moss, Mon Jan 14, 2019 2:57 AM.