Unlike most monitors, the Electrohome black & white vector needs an external heater voltage....the chassis doesn't generate it itself. Atari used the transformer in the power brick to supply this voltage. Approximately 6 volts AC exits the transformer via the 15-pin amp connector.....then straight up the the neck socket. This voltage also supplies the juice to the coin door lights. Are they fading, also?
Look at both sides of the 15-pin connector on the transformer assembly...look at the pins for any obviously issues (oxidation, backing out of the housing, etc.).