Hi @colinbest
Try a full power refresh by removing the charger (if connected), disconnecting the battery from the motherboard (you don't have to remove it, just disconnect it) and then, fortunately for you, disconnecting the RTC coin cell battery from the motherboard (see image below). The battery is mounted on the underside of the motherboard but its cable is connected on the top side, so you don't have to remove the motherboard to disconnect it.
Next ''press and hold the power button operated for a full 30 seconds and release it.''
Reconnect the RTC coin cell battery, reconnect the main battery, connect the charger, switch on and check.
If still no good then here's the [document|41030|schematic] for the motherboard that may help. The power on voltage sequence chart is on p.5
[image|3546457]
(click on image)