Should I throw away CRT TV's PCB which has damaged soldering point? - Page 1
Forum section: electronics / beginners

author: bshi02
author_rank: Regular Contributor
timestamp: May 07, 2024, 02:35:37 am
content: By refering to below tutorial,I measured voltage of power regulator ic(STR-S6707) with switching on, which is a crucial component of CRT TV's PCB in HOT area.https://www.electronicsrepairmadeasy.com/2010/11/solving-power-supply-problems-smps.html?lr=1715042500018I successfully measured voltage pin#1(collector 370-400DCV)#9(vccin 6.2-6.6DCV) of STR-S6707 with analogue meter.But I noticed that when I measured voltage between the biggest capacitor(main filter capacitor),It was 300-320DCV,But when I measured voltage of pin#1 of STR-S6707 It was 370-400DCV.In order to raise the accuracy of voltage measurement, So I tried to measure it with hioki CARD HiTESTER 3240 which my father ,who has already passed away, bought me more than 30 years ago.To my surprise,As soon as I placed hioki CARD HiTESTER 3240's black and red probe to negative pin of the biggest capacitor and pin#1 of STR-S6707,I saw big spark from soldering point of STR-S6707(pin#1)This big spark cause to peel off some of upper soldering pad(pin#1 of STR-S6707) like below screenshot.And I doubt that this big spark would spoiled component(FB802,FB804) which is very close to pin#1 of STR-S6707.I think that If I connect this PCB of CRT TV to cathode ray tube and switch on, There would be a possibility to malfunction or even catch a fire as high voltage(370-400DCV) pass through this damaged soldering pad(pin#1 of STR-S6707).Should I throw away CRT TV's PCB which has damaged soldering pad of power regulator ic(pin#1 of STR-S6707)?Or Although some of soldering pad(pin#1 of STR-S6707) have been damaged ,where high voltage(370-400DCV) pass through, Is it safe to mount STR-S6707 power regulator by soldering and continue to test it?

author: CaptDon
author_rank: Super Contributor
timestamp: May 07, 2024, 01:00:32 pm
content: Did you cause that destruction by placing probes of a grounded tester onto the HOT side of a line operated power supply? I am guessing the destruction goes beyond what is visible now. Do you have a schematic of the supply or just fumbling around in the dark? If there was stored energy after the regulator and you shorted the input side of the regulator then energy may have traveled through the regulator backwards and probably ruined it along with other possible destruction. Flash+Bang = it's toast.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 07, 2024, 01:10:52 pm
content: The Hioki card tester is a battery operated handheld DMM.https://www.ebay.co.uk/itm/355049598788It does not have current ranges but maybe it was set to resistance or diode mode when the OP tried to measure voltage across the capacitor after the mains rectifier. If so, I suspect the DMM is also now kaput.

author: CaptDon
author_rank: Super Contributor
timestamp: May 07, 2024, 01:16:11 pm
content: Since battery operated tester that would eliminate the ground-short possibility. Sure looks like current conducted through the probe tip however as the 'divit' from the tip seems to be visible. I agree, testing device may also be ruined now.

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2024, 01:23:48 pm
content: All of the threads that you have started on the forum have been related to repairing this TV. From reading through them, I have the feeling that, although you are trying to learn and do things properly, you are out of your depth. I am worried about your safety.Regarding the PCB, as CaptDon says, the STR-S6707 PSU IC is probably blown and maybe other parts too, but I think you expected this. To be able to replace it in the PCB, you need to determine whether you can safely removeallof the evaporated metal from the large spark that has now been deposited on the surface. After ensuring that ALL capacitors are safely discharged, you need to use cotton buds and Isoropyl Alcohol (IPA) to scrub the surface of the PCB to try to remove it. If you can restore it to the point of shiny green solder resist then it is probably safe to re-use. If there is any material burnt into the surface, then not.Please be more careful.

author: bshi02
author_rank: Regular Contributor
timestamp: May 07, 2024, 02:24:31 pm
content: Thank you very much for all of reply!In fact,I wrote a thread to reddit about this 29 inch CRT TV which contains a very primitive rough schematics of STR-S6707 and some of components which connected to all pins of STR-S6707https://www.reddit.com/r/ElectronicsRepair/comments/1cb7m6y/comment/l1fonex/https://i.ibb.co/RpXqKjB/schematics.jpgFrankly speaking,Before voltage measurement with switching on TV, I always put on leather glove and plastic glove in layers.I think If I measure this voltage without put on any glove at all,I would be severely electrocuted..https://www.dropbox.com/scl/fi/oeympdwmrw946bk386a2q/IMG_20240425_113830.jpg?rlkey=lpjret0yqdtmos4p9s6ax4w9x&e=1&st=ko5ecy5h&dl=0I examined probe of Hioki card tester and I found that red probe tip is somewhat broken and its surface is blacken,But TO my surprise,It still can be able to quite correctly measure 1.5V battery voltage and carbon resistor's resistance.And It seems that fuse on the PCB of CRT TV not yet to be blowed.I think that When I measure DC voltage with switching on TV, I placed black probe of Hioki card tester to negative pin of main filter capacitor in the HOT area and red probe to soldering point of STR-S6707(#1 pin) in the HOT area .I already have clean off all of debris from soldering pad(#1 pin of STR-S6707) by rubbing cotton bud which was soaked 99% Isoropyl Alcohol.Although some of upper soldering pad have been peel off,resistance between soldering pad(#1 pin of STR-S6707) and adjacent components(FB802 and FB804) is less than 0.2 ohm

author: bshi02
author_rank: Regular Contributor
timestamp: May 07, 2024, 09:48:13 pm
content: Quote from: Gyro on May 07, 2024, 01:23:48 pmThank you very much for your reply!By the way,I finished to scrub most of deposit from damaged soldering pad(#1 pin of STR-S6707) and It looks like below screenshot.It seems that some soldering pad(#1 pin of STR-S6707)have been peeled off and It revealed looking burnt plastic PCB surface.But Aside from that,I think there is no more damage on this soldering pad(#1 pin of STR-S6707).Then, Is it probably NOT safe to re-use?