Model bwd 521/521R is a true double beam, precision oscilloscope possessing by virtue of its range of plug-in Amplifiers and Time Bases a wide measuring range of both voltage and time. 1 .1 The instrument is 100% Silicon Solid State design employing no valves or neons other than the double gun CRT and panel indicators. It incorporates three identical plug-in compartments for the series '5' interchangeable plug-ins providing facilities for single trace, dual trace, with similar or dissimilar amplifiers , delayed sweep, identical X-Y operation or raster type displays. As no partition exists between the plug-in compartments special twin or triple units can be supplied for specific applications. 1 .2 The main frame consists ot the 5" double gun CRT, all low voltage power supplies, EHT supplies, two <.30MHz vertical amplifiers,< 1.5MHz horizontal amplifier, calibrator and plug-in cage, etc . All DC supplies are electronically regulated to accommodate input voltage and lead variations. 1 .3 An additional feature of this model is in the complete DC isolation of all circuits permitting the input COMMON terminal to be connected to DC voltage rails up to ± 400V from ground, making isolated measurements as simple as clipping a meter across the circuit. 1 .4 Where signals require measuring between two points, both of which are varying with respect to a reference or ground rail, Differential Amplifiers Models 5A or 5C can be employed in which either input can be separately AC or DC coupled to further increase their measuring flexibility. 1 .5 All inputs are protected against overload. However, care must always be exercised to not exceed the specified voltage limits of each input and, in particular, do not attempt to connect the COMMON of each plug-in to different voltages or damage may result to the power supplies or printed circuit ground line tracks. 1 .6 It has been designed for reliable long-term use and has been subjected to environmental tests. Each instrument is heat soaked and vibrated as part of its alignment procedure. 1 .7 For maximum reliability it is advisable to replace the power supply protection fuses every 2000 hours of operation to guard against thermal stress failure. Additionally, if the instrument is to be left non-operating for long periods and is stored in a dusty atmosphere, it is wise to drop a plastic protection cover over it to minimise dust ingress into switch wafers, etc . A storage cover and a carrying case are available from B.W.D. Electronics piy. Ltd., together with a full range of accessories (see catalogue).