First steps on Fault Finding

Hi Neil, Fault finding is not very easy. Especially if you don’t have schematics of the device. There are a few hints that might help. First use your eyes and nose and look for discolored components, components that look different and components that smell different. For instance electrolytic capacitors that are expanded. Measure the supply voltages first with a voltmeter and also with an oscilloscope. Measure the voltage across fuses to look for open fuses. That is easier then taking the fuses out to measure them. For many components the supply-pins and supply-voltage can be found in the datasheet. If you find a missing supply-voltage you know why the device is not working. Next thing to do is find out why this supply-voltage is not there. For processor controlled devices measure activity on all clock, data and address-lines. If a device connected to the data or address bus is broken, often one or more data and address-lines have a much lower logic level. Next thing to do is find out which component is shorting the bus. Components that often fail are electrolytic capacitors. For older equipment it does no harm to replace them all. Next are high power components like audio output amplifiers and power-supplies. Heavy components like transformers and components like connectors and potmeters might have broken connections on the circuit board. The Leappad that when you switch on the screen flashes and then the unit goes off might suffer from high ripple on the supply of the processor caused by an electrolytic capacitor that has no capacity anymore. If you measure the supply-voltages with an oscilloscope you can see if this is the case or not. Another reason for the device to switch off is a protection circuit sensing a too high current. The tablet that will not charge can have different causes. Most probably with a new one is a faulty connector. Care must be taken with charge-circuits for lithium batteries. Lithium batteries can start a fire that is difficult to extinguish if not properly charged. Never change the charging and protection circuits for a lithium battery. Very often the charging and protection circuits are build inside the enclosure of the battery. Follow the wires from the charging connector to the battery with a voltmeter. Sometimes you will measure a voltage at one end of a wire and zero at the other end.

Hi Neil, Fault finding is not very easy. Especially if you don’t have schematics of the device. There are a few hints that might help. First use your eyes and nose and look for discolored components, components that look different and components that smell different. For instance electrolytic capacitors that are expanded. Measure the supply voltages first with a voltmeter and also with an oscilloscope. Measure the voltage across fuses to look for open fuses. That is easier then taking the fuses out to measure them. For many components the supply-pins and supply-voltage can be found in the datasheet. If you find a missing supply-voltage you know why the device is not working. Next thing to do is find out why this supply-voltage is not there. For processor controlled devices measure activity on all clock, data and address-lines. If a device connected to the data or address bus is broken, often one or more data and address-lines have a much lower logic level. Next thing to do is find out which component is shorting the bus. Components that often fail are electrolytic capacitors. For older equipment it does no harm to replace them all. Next are high power components like audio output amplifiers and power-supplies. Heavy components like transformers and components like connectors and potmeters might have broken connections on the circuit board. The Leappad that when you switch on the screen flashes and then the unit goes off might suffer from high ripple on the supply of the processor caused by an electrolytic capacitor that has no capacity anymore. If you measure the supply-voltages with an oscilloscope you can see if this is the case or not. Another reason for the device to switch off is a protection circuit sensing a too high current. The tablet that will not charge can have different causes. Most probably with a new one is a faulty connector. Care must be taken with charge-circuits for lithium batteries. Lithium batteries can start a fire that is difficult to extinguish if not properly charged. Never change the charging and protection circuits for a lithium battery. Very often the charging and protection circuits are build inside the enclosure of the battery. Follow the wires from the charging connector to the battery with a voltmeter. Sometimes you will measure a voltage at one end of a wire and zero at the other end.