Among the domestic electronic equipment, power supply PWM control circuit is the most commonly used model is the integrated circuit UC3842 (or KA3842). That is because often encountered, there are also some of it was, following a brief UC3842 good or bad way to judge:
the complete replacement of the damaged peripheral components, do not install the first switch transistor (MOSFET), power-measuring 7 feet UC3842 voltage, if voltage fluctuations between 10-17V, the rest of the foot were also fluctuations of voltage, then the circuit has been start-up, UC3842 normal; if the 7 pin voltage is low, and the remaining pins or no voltage fluctuations, the UC3842 is damage.
In the UC3842 7.5 feet between the left and right external +17 V DC voltage, measuring 8 feet if the +5 V voltage, 1,2,4,6 feet have different voltages, the UC3842 normal, current is small, its easy to damage . It is the most common cause of damage to the power switch transistor (MOSFET) circuit, the extremely high voltage from G added to the 6 foot walls to burn. And some models eliminates the need for the protection of the G grounded diode, the power switch transistor (MOSFET) is damaged, UC3842 and G very external current limiting resistor must be bad. At this point a direct replacement.
Note that the power switch transistor source (S pole) usually take a small resistance, the resistance of high-power sense resistor as the overcurrent protection. This resistance is generally between 0.2-0.6, there will be greater than this value can not afford to load with the phenomenon (that is, sub-low voltage).
As the UC3842 (KA3842) operating voltage and power output are related to UC3843 (KA3843) falls far short of 3842 series and 3843 series off voltage in the starting voltage and also there are large differences between the former start-up voltage of 16V, turn off voltage 10V; the latter starting voltage of 8.5V, off voltage 7.6V. The two series of IC can not be directly substituted. If substitution is necessary to use the latter when the former should be to transform the circuit. Therefore, this point in the maintenance work must pay attention to.