Possible reasons: the whole building visual extension no image, check the door machine video output V+.G. to a level video amplifier, input terminal is divided into positive and negative answer, video amplifier is connected power supply, pay attention to; input and output port can not take the wrong;
Processing method: the video amplifier connected to the power supply, the correct answer to the input and output video line.
Encounter such problems, first consider the VB+ line (check the host and the extension of the line is correct, the line is in line with the standard).