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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.

 

testing diode

Author By : Jestine Yong

 

 

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1 Feb 2007

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