First steps on Fault Finding

Hi Neil,

Most of the time, with hardware that you have not designed yourself, some of it is just familiarity with some common issues as you've no doubt found; e.g. if something is powering on but immediately switching off, that could be as simple as a battery which is not charging, to in theory as complex as a fault detected by the microcontroller software which is then promptly powering off the device. If it is a kids toy then yes, loose connection would be a good thing to check for (although regarding the leappad, leapfrog stuff is very well made - I have a couple of nephews).

You've probably got a lot of experience in detecting symptoms and then troubleshooting them already. The only thing I've learnt is that if you spot a symptom, even a small one that is a departure from normal behavior, then to mentally note it (i.e. add it to the clues) and not to ignore it. And to sleep on it :-)

With general things not charging/powering up, the power connectors can sometimes work loose and need re-soldering.

Many devices are so integrated that it is hard to diagnose the fault. With simpler devices if you can spot generally available ICs with markings (not always possible), then the datasheet helps, because then you know what the expected behavior of the parts is to an extent. That helps loads with kids toys repair - especially RC cars (nephews again : ).

Hi Neil,

The flashing display could be due to a low battery or poor battery connection.

Charging issue could be due to dirty connector or possibly the charging voltage is a bit low. I have one device that is supposed to charge from a USB port, but it won't start charging unless the voltage is 5.6V. There are some tips online how to clean USB connectors.

Doug

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.

Thanks for all the tips i'll try these over the next few days while i wait for my Arduino Kit to arrive

Most of the new Gear come out to the market are SMD parts inside, unless you have all the good SMD tools, otherwise just for half to one year, you will find it not worth to repair any cheap equipment compare the damage to your eyesight.

> ...a Leappad that when you switch on the screen flashes and then the unit goes off.

>  So i'm thinking Fuse? Loose screen connection (see this on a DS)...

Taking a guess that the Leappad in question is one of the newer ones with rechargeable batteries, the first step to take is to check the charger/power adapter - if it comes with one. Only equipment you need for this is a multimeter; if the charger/power adapter is found to be good (working voltages are usually always listed on a label on that unit), then the next step will involved working on the Leappad, again not knowing exactly what model you refer to makes it difficult to suggest more definitive troubleshooting info, but with luck these devices will have a cover accessing the battery pack, if not then it is time to open the device and carefully check the insides minding any connection wires or cables between components, battery pack should be easily identified - try finding any part number imprinted on it, could be just simple AA sized rechargeable batteries which would be not much of a hassle to replace, recharge and try powering up the unit. It is rare to find fused power connections on devices like the Leappad, if any fuses are to be found they would probably be inside the charger/adapter.

> Also to a cheap Ebay tablet that will not charge - Thinking Charge Port damaged?

Same scenario as above - start with the charger/power adapter. If that's good next thing is to review the state of the rechargeable battery pack, replace with spare if possible and try the device.

-- RP