from the manual.
"On anticipation of the coming of 4-channel stereo broadcasting, your Marantz 2270 is equipped with an output QUADRADIAL jack. the signal available at this jack is the unequalized, buffered output of the FM discriminator. It's level, frequency response characteristic and output impedance are ideal to drive any 4-channel adaptor. this jack can also be used as a simple white noise generator for frequency response check of loudspeakers or amplifiers with the 2270 in FM mode and tuned off from any FM signal."