This Photo shows Master Control at NBC in San Francisco, used from 1927 to 1942. At the extreme left is the morse code operator's position, where the NBC Pacific Division communicated with New York, Chicago and other cities in the network.
In the center is a panel containing the studio amplifiers, bridging equipment, power and testing equipment. Also in this location is a panel with the incoming broadcast lines and audio lines from other points in San Francisco. On the far wall are generator controls with a cable cabinet. To the right of that is the supervisor's desk, and the DC switchboard is at far right.
Below are two more views of the master control room in action.