Mark Durenberger, former engineer at WCCO (now with the Minnesota Twins), recently donated a significant piece of broadcast history.

The audio mixer (at right) was designed and built by CBS, and through its controls passed all of WCCO Radio's programming for nearly four decades. This panel was the heart of WCCO's Master Control until the late 1970's.

Though this might seem a rather simple assembly for a full-service radio station, it was actually a "Master Mixer." Each knob could control the output of several other pre-mixed program sources, collectively including several dozen microphones, turntables, tape and disc recorders, the CBS Radio Network and dozens of "remote" telephone circuits that enabled WCCO to broadcast from throughout the country.

During typical "in-studio" operation, the bottom row of controls adjusted the main talent microphones and the top row included faders for "Remotes", the CBS radio network and the custom telephone system, and a "Transcription ('ET') Bus" (on which appeared all the tape recorders in the building as well as turntables in the control room).

This Studio 5 mixer was "passive" (all the electronics were in adjoining racks). The only maintenance necessary on this panel was to clean and lubricate the faders. Maintenance access was easy; you simply tilted down the panel on its convenient hinge (seen at the bottom of the picture).

In Master Control the "Studio 5" mixer was but one of 5 such sources that could be fed to any of 4 simultaneous outputs. For example, from 7PM to 8PM "Studio 5" might be feeding the WCCO transmitter, "Studio 2" might be feeding a program to CBS, while "Studio 3" might be feeding a regional network (Twins baseball for example). At the top of the hour, these feeds might have to be simultaneously rearranged into a different configuration. Since it would be difficult to push all those new buttons at the same time, a device called a "Pre-selector" could be programmed for the new feed arrangement by 'pre-assigning' each studio to a new destination...then at the exact top of the hour a single push-button would switch all sources to new destinations at the same time. 

Here it is in place with the other equipment. The "Pre-Selector" and the meters for the 4 output channels can be seen under the clock. In the rack to the right of the clock was the key bank that selected from among dozens of incoming telephone lines used for the "remote" broadcasts.

Mark said operating this equipment "was like piloting a space ship....."
                                                      snr

This just in from Scott Horner, a 1939 view of WCCO master control.