It is designed to be adjustable and serviceable in the field, thanks to its simple circuit design, component markings and the enclosed circuit diagram indicating test points and voltages. Furthermore, the meter can be used as a voltmeter with the internally provided test probe. A number of electronic components are enclosed for easy servicing. The machine can be powered from either an external supply (ATN-4 Order # 70 14 378 000) or internal batteries (12 D cells). Specifications THD at 400 H