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Introduction This service manual is written primarily for the service engineer who is performing a standard recalibration or who is undertaking repairs when the instrument has developed a fault. The system overview will help the engineer to understand the inner workings of the 400 Digital storage Oscilloscope (DSO). The fault finding procedure given in this manual takes a systematic approach. Starting with the symptoms, the engineer is led to the area or areas at fault by a series of questions in the form of several flowcharts. These require no knowledge of the system, although a basic knowledge of electronics is essential. Throughout this manual a reasonable level of understanding is assumed. A list of abbreviations and mnemonics used is shown in section 9. Many of the complex operational features of the 400 DSO are performed at chip level. The system includes one large semi-custom gate array which generates the ten phase system and related timings and handles the acquisition of data. On the analogue side there are three semi-custom analogue I.C.'s, two of which are channel preamplifiers providing virtually all the input signal processing, the third is the trigger signal processing circuit. When fault finding, they can be treated as 'Black Boxes' so alleviating some of the more difficult servicing tasks. Should a problem arise whilst servicing the instrument expert help and advice is available from Gould (see section 10 for details).