Notes on Safety Contents: o Chapter 1) Introduction: o Chapter 2) General Safety Guidelines: ----------------------------------------------------------------------------- Chapter 1) Introduction: Excerpts on SAFETY from Samuel M. Goldwasser's, 'Notes on TV repair and diagnosis' Author: Samuel M. Goldwasser Corrections/suggestions: sam@stdavids.picker.com Copyright (c) 1994, 1995, 1996 All Rights Reserved Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. ----------------------------------------------------------------------------- Chapter 2) General Safety Guidelines: General Safety Guidelines when working on line powered equipment including: o TVs o Monitors o Microwave Ovens These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage - there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe! 1. Don't work alone - in the event of an emergency another person's presence may be essential. 2. Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system. 3. Wear rubber bottom shoes or sneakers. 4. Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts. (From the Editor: There are people who have lost fingers when their wedding bands/rings shorted a 5VDC but high-current buss. Do not assume that just because it's low voltage, it poses no risks!) 5. Set up your work area away from possible grounds that you may accidentally contact. 6. Know your equipment: o TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. o Microwave ovens use the chassis as ground return for the high voltage. o In addition, do not assume that the chassis is a suitable ground for your test equipment! 7. If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks - use plastic or wood. 8. If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 100-500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K-100K ohm resistor). Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter. In a TV or monitor, if you are removing the high voltage connection to the CRT (to replace the flyback transformer for example) first discharge the CRT contact (under the insulating cup at the end of the fat red wire). Use a 1M-10M ohm 1W or greater wattage resistor on the end of an insulating stick or the probe of a high voltage meter. Discharge to the metal frame which is connected to the outside of the CRT. 9. For TVs and monitors in particular, there is the additional danger of CRT implosion - take care not to bang the CRT envelope with your tools. An implosion will scatter shards of glass at high velocity in every direction. There are several tons of force attempting to crush the typical CRT. Always wear eye protection 10. Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations. 11. If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand. 12. Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. 13. Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac(tm) is not an isolation transformer! The use of GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. A circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. 14. Don't attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity. 15. Finally, never assume anything without checking it out for yourself! Don't take shortcuts! -----------------------------------------------------------------------------