This calibration system was originally developed for production lines and field service technicians.  By the mid-1960s it was offered to customers as well.  It contains three basic instruments used to calibrate AC and DC vacuum tube voltmeters and oscilloscope amplifiers.  The three instruments are (from top to bottom) the 200SR Oscillator, the 739AR Frequency Response Test Set, and the 738AR Voltmeter Calibrator.

This solid-state instrument is the primary frequency reference in my shop.  It receives the 60 kHz signal of National Institute of Standards and Technology station WWVB in Ft. Collins, CO.  The signal is compared with the standard being calibrated and offset is displayed on the meter.  A built-in chart recorder allows comparison over hours or days to determine the aging rate of a timebase.  A comparison period of 12 hours yields a calibration accuracy of about 1 part in 10¹⁰.

This specialized member of the HP 200 series of oscillators provides the low-frequency signals for calibration.  It covers 5 Hz to 600 kHz in five ranges.  Because it was developed for use in the calibration system the output voltage range is limited to 1 V to 3 V into 50 Ω.

Calorimetry is the calibration of a power source by measuring the increase in temperature which that source produces in a given load impedance.  The 434A contains a 50 Ω load, and measures power from 10 mW to 10 W full scale from DC to 12.4 GHz.  It is quite complex mechanically, containing an oil-flow system with a pump and heat exchanger.  The design of this instrument provides much faster response than conventional calorimeters.

The 738AR is the core of the calibration system.  It provides calibration voltages from 300 μV to 300 V.  DC voltages are accurate within 0.1%.  400 Hz RMS and P-P voltages are accurate within 0.25%.  The calibrator is designed for a 3 MΩ load, and a correction table is provided for other load impedances.

The 739AR provides the high-frequency signals for calibration.  It covers 300 kHz to 10 MHz in three ranges.  The meter and output attenuator provide accurate output voltages, and are also used with the 200SR oscillator for low frequencies.