Tuesday, February 28, 2006

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.

Monday, February 27, 2006

Repaired Sony TV KV-1485GE

Last week my brother in law sent me a Sony TV for repair, he's in the cctv industry. Though I rarely repair TV, the experienced that I gained from repairing monitor can be apply to repair TV. The complaint was no power, upon checking found quite a number of defective components in the power circuit. The fuse was blown into pieces, protection resistor 3.3 ohm became open circuit, resin type ceramic capacitor 222-2kv split into two, 47uf 25v electrolytic capacitor esr has increased and finally the power ic strs6307 also need to be replace. Using my flyback tester measuring the primary winding of the power transformer found to have more than 5 bars which mean the winding is good and there is no secondary diode shorted at the secondary side of the power supply. Optoisolator checked to be ok together with other components. Replacing the defective parts above solve the no power symptom. From the outlook of the components that got burnt, it seems that it had been hit by a high surge.
When I call him saying that his TV already repaired, he added that he got another 29" goldstar TV for me to repair. The complaint is picture comes out only after couple of minutes. I suspect it could be the tube or the heater circuit. I will let you know how is the outcome when the TV is repaired. Well it seems that I'm now into TV repairing!

Sunday, February 26, 2006

It's been a long time

Sorry for not posting for such a long time. If you read my previous post you will noticed that my kids is schooling now and they need more attention compare when they were young. Most of my time have been taken up by sending my kids to school and wife to work. Then have to pick them up again after school and work. At night send my son for Mandarin course. At office, there's ton of repair work need to be done. Although this routine job made me real tired,i have the joy of seeing them happy and drawn closer to them. I knew that i can delicate this routine job to other such as hiring a school bus and get another car for my wife but somehow i just like doing it. After adjusting the schedule, now only i have the time to continue to do my work at night which is writing electronic articles and updating my electronic repair blog. Thanks to those who wrote and concerned about me and my family.
For the past few days ,i'm actually updating my website www.electronic-repair-guide.com by importing the electronic repair articles from noahtec.com. I will do it regularly and when finish i will be posting new electronic repair articles to the website. Feel free to log on to electronic-repair-guide.com from time to time to check for any update on electronic repairs. God bless!
Check out site archives for more, or search in the Net: