PinePhone Beta dead - no boot-related output on serial interface

[horalocal]

I have a PinePhone Beta Edition which doesn't show any signs of even starting to boot (no LEDs and no output on the serial port). It did work for a couple of weeks when it was new, and I'm not exactly sure what has changed. (To be honest, I've been focused on other projects, so it's been in a drawer for many months, but I wouldn't have expected that to cause anything more dramatic than the battery draining.)

I'm using the USB to serial/3.5mm adapter purchased from pine64 (CH340) and dip switch 6 is off. PuTTY or minicom will connect to /dev/usbTTY0 at 115200 8N1 but I get no activity in the terminal at all while attempting to start the phone.

The only signs of life from the phone are:

• When I connect it to PinePower's USB-C PD port, the display shows a voltage of 5.0V (but 0.0A drawn), so the USB power pins can complete a circuit in some form
  
• The green LED in the USB connector of the UART adapter cable stays on while the plug is connected to the phone's audio jack
  
• When I hold the phone's power button for about a second, the green LED blinks once, as if some data were briefly transmitted (but nothing appears in the terminal)
  

Things I've tried include:

• Booting with no SD card
  
• Rewriting the SD card with Jumpdrive, Jumpdrive charging, and various other things like the 17-OS image
  
• Booting with and without the battery in
  
• Fully charging the battery using the external charger (which gives a green LED after a couple hours of charging, so as best I can tell the battery is fine)
  
• Twiddling the other dip switches (in case one of the other components were somehow unhappy enough to inhibit boot?)
  

Am I right to think that the mainboard is probably fried if I can't even see U-Boot messages or some output related to boot starting through the serial console output? Is there anything else that might be worth trying, short of replacing the board?

Thanks in advance for any thoughts!