Presume that's just a poor 5V connection to the board.
The LED is switched by transistor Q2. Resistor R107 switches the Q2 transistor "on" at power-up. Once code begins to execute properly after reset, it tells the 6821 at U11 to switch transistor Q2 (and subsequently the LED) "off".
RE: reset, the schematics list all the voltages in the valid power section.
Knowing you, you should be able to work it out
Below was some measurements I took a year or two ago for someone else. I can't 100% remember but I think the red resistance readings were taken with the black meter lead on ground and red meter lead at the point. The black resistance readings are with the meter leads swapped around.