if a circuit board has a defective component but the physical condition is good no burns or carbon on it than how to find the defective electronic component..secondly is there any way to check capacitors without extracting it from PCB????

 

One is to reverse engineer the circuit in the absence of a schematic, in order to trouble shoot with various measuring equipment, take voltage/waveform readings etc, capacitor can be checked depending on the associated circuitry, most time removal is the only way.
Max.

 

if a circuit board has a defective component but the physical condition is good no burns or carbon on it than how to find the defective electronic component..secondly is there any way to check capacitors without extracting it from PCB????

Most ESR meters check capacitors in situ, most digital ones read capacitance too - but useless for anything under a few uF.

Shorted caps are easy enough - but leaky ones can cause all sorts of trouble. best to lift one end so other components can't taint the reading.

Electrolytics often show visible signs of distress that helps a fair bit.

 

Yes......but it takes training and a developed skill. All one needs is a small ac voltage and a scope set on X-Y mode. And some skill or practice. Any component can be tested in circuit.

Hardly anyone trouble shoots and repairs anything anymore. It's because of the time required. And the cost of replacement board.

If one can cut the time required to find the bad component......then one might make money. But you have to determine quickly.

 

Testing capacitors in-circuit can vary from quite easy to virtually impossible. Where capacitors are used across power supplies, such as local "bulk" capacitors on a computer motherboard or decoupling capacitors on any analog or digital board, there are likely to be many of them in parallel spread out over the board. The power and ground may be distributed on inner layer copper planes which makes the DC resistance and AC reactance of the distribution very low. This can make any attempt to locate a single bad capacitor by trying to measure any property in-circuit nearly impossible.

Aluminum electrolytic capacitors are the only type that frequently fail due to "wear out" which is essentially loss of the solvent in the electrolyte. Plastic film capacitors rarely fail unless subjected to severe stress (I've seen some split open as a result of lightning strike). Ceramic capacitors can fail occasionally. I've seen a leaky (i.e. conducting a small DC current) monolithic ceramic capacitor, which is extremely rare. Surface mount ceramic capacitors are vulnerable to cracking from physical stress due to thermal effects or bending of the circuit board or having be hand soldered with an iron. Wet slug tantalum caps fail due to dry-out like aluminum caps and occasionally due to growth of metallic dendrites ("whiskers") but they are quite rare these days in consumer and industrial products (expensive!). Solid tantalum caps can fail due to voltage or current overstress which may result from a failure in something else or because they weren't the appropriate type for the application.