Intermittent Diode Open Circuit
Today afternoon a customer sent
me a china made monitor for me to repair and the complaint is no power. First I discharge the big filter
capacitor with a 2.2kohm 10 watt resistor before using the dick smith flyback tester to measure the primary
winding of the power transformer. This is my usual technique to troubleshoot any power supply problem. The meter
shown more than 5 bars and this prove that the primary winding is ok, power fet not shorted and of course all
the secondary power diode in good condition. For your information if any of the secondary diode short, it can
pull down the meter's reading to 1 bar or totally goes off. Flyback, yoke coil, b+ fet and horizontal output
transistor tested to be ok. I power on the monitor to measure the voltage .
I found that one of the secondary
diode have low voltage and sometimes no voltage at all but the other line voltage all seems to be normal. When I
took out the diode and check the reading was ok so I suspect it could be something along the line that have
problems but no components found to be defective along the line. I took out the diode and recheck again and this
time it have no reading at all even forward bias! This diode is an ultra fast recovery diode and the part number
is HER302. Replaced a new diode solved the no power symptom. Conclusion-our normal multimeter is not enough
power to check this type of intermittent problem, fortunately diode seldom breakdown like that and it is either
short circuit or open circuit but not intermittent.
That's why when you go to search
engine and key in the word 'diode checker', it came out some list or company that is specially selling only
diode checker or tester that cost thousands of dollars. I believe this expensive diode checker can detect the
intermittent diode problem.
Author By : Jestine Yong