Why every new horizontal output transistor is blowing in a LG CRT television?  - Page 1
Forum section: electronics / repair

author: Dhanushka
author_rank: Regular Contributor
timestamp: October 15, 2019, 07:43:48 pm
content: Dear all,I replaced leaked old fly back transformer with new one and also horizontal output transistor of my LG 21" flatron CRT TV. But it was blown when I powered up. Without H-OUT transistor, 75 W and 60 W light bulbs are parallel at B+ and ground B+ has 112.5 V and big capacitor has about 320 V. Horizontal drive transformer has not short between primary and secondary. I replaced HOT again and turn on the TV without yoke and degousing coil the TV was on with series 75W light bulb. But, When I connected only yoke and powered on, again HOUT was blown though there is the series bulb. What may be the problem?Thanks

author: wraper
author_rank: Unknown
timestamp: October 15, 2019, 07:48:09 pm
content: QuoteCapacitance measurement is of little use. Capacitance may measure fine but ESR can be 100 times higher than initial value.

author: james_s
author_rank: Super Contributor
timestamp: October 15, 2019, 09:04:18 pm
content: If a capacitor is swollen that's the obvious place to start. That's a clear indication that it is in bad shape.

author: PKTKS
author_rank: Super Contributor
timestamp: October 16, 2019, 01:41:52 pm
content: Apart from individual component health..You should check the Horizontal driver wave form input.Remove the transistor and fit a dummy load suited to haveat least the wave form... this wave is crafted to drivea very specific load so you should not have anythingfar from the manual specs. (right in the coupling transformerwhich feeds the driver)Also you must issue the proper CHASSI for any reasonableguess - there have been so many changes in that sectionacross the years which make even guessing speculation.The wave is also usually damped with some sort of FR diode.Paul

author: fzabkar
author_rank: Super Contributor
timestamp: October 16, 2019, 10:09:17 pm
content: In addition to the capacitor, another common cause of failure is dry solder joints at the horizontal drive transformer. I would reflow all the pins.One more thing is the possibility of Chinese fakes (I have been stung). Did you purchase your LOP transistor from a reputable source? Typically with fakes, the set will run for several minutes and then fail. You should run it for a short time and test the temperature of the transistor. It should not run hot.

author: Dhanushka
author_rank: Regular Contributor
timestamp: October 25, 2019, 06:48:08 am
content: This was solved after replacing exactly same transistor. Previous transistors may be fake ones.Thanks for everyone who have helped for me. Specially thanks fzabkar.

author: PKTKS
author_rank: Super Contributor
timestamp: October 25, 2019, 09:57:53 am
content: Quote from: Dhanushka on October 25, 2019, 06:48:08 amUnfortunately this problem have increased over thepast decades with prices not dropping for originalsand new brands offering clones of several wideused semi.I generally buy a more wide supply for having spareparts to change adhoc... just in case the match isreally the semi quality.Pesky problemPaul

author: james_s
author_rank: Super Contributor
timestamp: October 26, 2019, 01:01:52 am
content: Nobody is making CRTs anymore so it's likely that nobody is making the relatively specialized transistors used in horizontal scan circuits for CRTs. Certainly the output transistors are not the mass market parts they once were.