In monitor power supply where pulse width modulation (pwm) ic UC3842 are used. Pin 3 of this power ic is called I-sense which means current sense and this pin is connected to source pin of POWER FET. The purpose of this current sense circuit is for overcurrent protection. This circuit usually samples the voltage drop across a resistor in series with the switching fet transistor. If the current rises abnormally due to some short circuit in the secondary side such as a shorted secondary diode or horizontal output transistor (HOT), the voltage will exceed a reference level and shutdown the pulse generator which is the PWM ic. This circuit provides protection to components in the event a problem occurs. In some cases where overload happen-the power section will blow and most of the time the components changed are fuse, power fet, pwm ic, zener diode and current sense resistor.
Do you know that a small change in this current sense resistor value have a great impact on the secondary output? A little change in value say from 0.22 ohm to 0.44 ohm can caused your ouput voltage to drop into half! I have seen a lot of repairer can’t detect this type of problem (low output voltage) due to that they used analog or digital meter that don’t have the low ohms range. You can’t differentiate if the low ohms resistor has its value changed by using normal meters. I personally use the Dick Smith ESR Meter which have the capabilities to check resistor value down to 0.01 ohm! If you don’t have this special meter you may just direct replace the current sense resistor to see if the generated secondary output will back to normal voltage.
Another tip is that if the current sense resistor in the monitor B+ circuit have changed value it will caused the output in the boost circuit to become low and you will have a small width display and of course the high voltage will become low as well. Instead of 24Kv it becomes 15-18Kv.
Jestine Yong is a electronic repairer and a writer. For more information on electronic repair please visit his website at http://www.noahtec.com/electronic-repair-articles.htm