The most common cause of this is the video cable. If the DOS Compatibility Card video cable is not securely fastened to monitor cable and DOS Compatibility Card, or you do not have the DOS Compatibility Card video cable connected properly, the screen dims, but you will not see the MS-DOS prompt.
You should check the cable connections, and then restart your Macintosh computer. If you do not restart your computer, the monitor will not sync properly. This is because when the PC Setup control panel loads it determines what monitor is connected. When it cannot determine what monitor is connected, the PC Setup control panel defaults to an Apple 16-in. Display setting.
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