The Model 3905C Mainframe is used in conjunction with the Krohn-Hite plug-in modules, that are operational only when inserted in the five module Model 3905C Mainframe. The Mainframe includes a Model 39A-05 microprocessor plug-in module. The modules are easily accessible from the rear of the Mainframe. The Mainframe provides local/IEEE-488 programming, indicators for the input and output gain, cutoff frequency setting, channel selection and overload detection. Non-volatile, battery-backed, CMOS memory permits storing and recalling of 85 selectable groups. Storing and recalling group settings is accomplished with only one command. Self-testing of the digital circuitry occurs upon power-up. This Operating and Maintenance Manual is for the Model 3950A and its Model 39A-05 microprocessor module only. Separate manuals are provided for each type of module.