TEST EQUIPMENT AND FAULT DIAGNOSIS
Several of the previous chapters in this book contain servicing and setting-up hints relating to the equipment dealt with there. It is the purpose of this chapter to concentrate on servicing and diagnostic matters entirely, since many readers are likely to be concerned with maintenance and repair to a greater or lesser degree.
SAFETY
For all equipment which operates from domestic mains supplies the first concern in servicing should be safety, particularly from electric shock hazard. While a risk of shock from internally generated volt- age is present all the time the covers are removed, the most dangerous shock hazard is that of completing a circuit from mains live to earth. Since TV aerial systems can be expected to be earthed, and many other surfaces and equipment which surround the engineer may be earthed, it is essential that the main supply to the equipment under service is isolated. A double-wound and fully insulated isolating transformer is required, and to provide a suitably ‘stiff’ (low imped- ance) supply a 500 W type is recommended. Suitable fuses are a 5 A HRC type in the primary circuit, and a 2.5 A anti-surge type in the secondary circuit.
For field engineers the shock hazard on site is greater than in the workshop. Possible solutions are a large rubber mat on which to work, or the use of a portable isolating transformer: provided service is confined to reasonably modern TV sets a 250 W type suffices for diagnosis work. Test equipment for TV field work should not be earthed; it should be tested regularly for insulation integrity.
A second aspect of safety is the need for consumer equipment to maintain BS EN60065 and BEAB safety standards, as it did when it left the factory. These standards are mainly concerned with user safety, and are implemented by the OEM (Original Equipment Manufacturer); the requirement of the service engineer is that he does nothing to compromise the equipment safety. In practical terms, shock, fire and implosion risks are avoided by using OEM-sourced and approved components wherever a safety component (clearly marked by the symbol _! or shading on the circuit diagram and/or parts list) is replaced, and by mounting it in the same way as the original. Particular care is necessary in the event of repairs to – or replacement of – back covers, cabinets, picture-tubes and user controls.