It is designed to amplify signals from transducers such as strain gauges, whose impedance varies based on, for example, mechanical strain. These transducers are usually connected in a bridge circuit.
The 3C66 drives the bridge with a 25 kHz "carrier" signal, which is modulated by the relative impedances of the arms of the bridge. The resulting signal is amplified and envelope-detected by the 3C66, producing a vertical deflection proportional to the the measured quantity (strain, for example).
The 3C66 provides built-in variable impedances for balancing the bridge. Because the carrier is 25 kHz, the maximum bandwidth of the measured quantity is 5kHz. This limitation is due to the settling time of the envelope detector. In addition to the vertical deflection signal, the 3C66 provides a front panel output of the measured signal.