TV repair, special mosfets? - Page 1
Forum section: electronics / repair

author: willie_phistterbut
author_rank: Newbie
timestamp: May 17, 2017, 03:52:22 pm
content: A friend of mine has recently had the a$$ blown out of his tv. On the secondary side three caps had exploded and I'm guessing went short. I don't have an ESR meter or an LCR to have measured them with, but the magic smoke escaped for sure. I got those three caps replaced, rated 35v, 470uF and a couple 680uF. Replaced them all with 35v rated 680's, which according to one of Dave's repair vids, should be fine to go higher capacitance. Got it back together and it still didn't work. Pulled apart one last time and measured some 252 packaged p-channel mosfets labeled "apm4050b". They looked okay but the board underneath in that area was stained brown and on the underside are two heat sinks for them. Did some in-circuit measurements of them and got continuity between the big drain tab and everything else. They must be shot right? Well the only datasheet I can find on these things lists negative voltage specs. How would I source new parts for these? I've checked around digikey and the search doesn't list negative voltages. Just trying to save my buddy some cash shelling out for a new tv and learn something about repair at the same time. Might even get a new soldering iron out of the deal XDHere is a link to the only datasheet I could find for that part number.http://www.datasheetbay.com/pdf/1107191/APM4050BPU.htmlThanks for taking the time to read this wall of text folks, cheers!

author: keymaster
author_rank: Regular Contributor
timestamp: May 17, 2017, 09:22:00 pm
content: you see negative voltage specs because the mosfet is p-channel (all the p-channel mosfets have negative voltage specs). Any p-channel mosfet with close to that spec will do .Be careful ,your mosfet is a logic level mosfet. So the replacement must be too.Also check all the diodes , most of the time they die with the mosfets too.edit:FDD4685 seems to be a close match.(http://eu.mouser.com/ProductDetail/Fairchild-Semiconductor/FDD4685/?qs=sGAEpiMZZMshyDBzk1%2fWi1qxDdW%2fXBi2K9EVN7eRvUc%3d)

author: Armadillo
author_rank: Super Contributor
timestamp: May 18, 2017, 03:49:19 am
content: Quote from: keymaster on May 17, 2017, 09:22:00 pmExcept that the Rds is at least 16 times higher but will still work [if that is the smoking gun].I would actually suggest salvage pmos from junk board for trial first because I don't endorse trial and error repair fixes anyway. Don't need to go all out to look for few cents component for it now.

author: willie_phistterbut
author_rank: Newbie
timestamp: May 18, 2017, 06:16:34 am
content: Thank you for the responses. Yes, I'd rather know more about repairing TV's before actually doing it, but what's the fun in that?Before I waste my friends money, I'll check my voltages for sure. Since drain is common I should have a lower voltage on the gate than the source right? Also, how can you tell it's logic level?

author: Armadillo
author_rank: Super Contributor
timestamp: May 18, 2017, 06:40:52 am
content: I am sorry but I think you should first discover why 3 capacitors in the secondary would explode before anything else otherwise you would be searching and buying in vain.And if 3 capacitors really exploded, it wouldn't be just simple voltage measurement.But I think there is distinction between wanting to repair versus having fun, I am more incline to think the latter.So have fun discovering

author: willie_phistterbut
author_rank: Newbie
timestamp: May 18, 2017, 07:06:43 am
content: A bit of both really, you're right. Saying they exploded was a bit of an over statement I guess. They had either just bulged at the top, or had semi-punctured the vent. Other than that and the stained, over heated looking board, I didn't see any thing too obvious.

author: keymaster
author_rank: Regular Contributor
timestamp: May 18, 2017, 11:53:46 am
content: Quote from: Armadillo on May 18, 2017, 03:49:19 amfdd4685 datasheet (http://www.mouser.com/ds/2/149/FDD4685-1007311.pdf)Rds(on) = 27mOhm Vgs=-10vRds(on) = 35mOhm Vgs=-4.5vAPM4050BPU datasheet(http://www.1mos.com/upload/file/SINOPOWER/APM4050BPU.pdf)Rds(on) = 33mOhm Vgs=-10vRds(on) = 59mOhm Vgs=-4.5vThe fdd4685 has smoller Rds(on) than the APM4050BPU. not 16 times higher.Am i missing something Armadillo?Quote from: willie_phistterbut on May 18, 2017, 06:16:34 amIt will be mentioned in the datasheet , at the beginning . But some times they don't. Then the only clue is when at the first page of the datasheet , you will see 2 values for Rds(on) . One at 10v and another for 4,5v (or 5v). Then the mosfet is a logic level .

author: Armadillo
author_rank: Super Contributor
timestamp: May 18, 2017, 02:21:59 pm
content: mw or mohm,http://www.datasheetbay.com/pdf/1107191/APM4050BPU.htmldatasheet provided by Willie. Confused by the differing datasheet.

author: keymaster
author_rank: Regular Contributor
timestamp: May 18, 2017, 08:51:58 pm
content: its the same datasheet .But datasheetbay.com shows to you an html version . the conversion program (pdf->html) couldn't understand the greek leterand put in place the letter 'w' .

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2017, 10:30:56 pm
content: Check whatever drives the mosfet too, since the mosfet shorted, the driver is probably bad, unless you got lucky and there's a gate resistor that went open.

author: Shane711
author_rank: Newbie
timestamp: September 01, 2017, 10:12:24 pm
content: So is the FDD4685 compatible or not? I have the same problem. I have 2 blown mosfets APM4050B & APM4030B. I also had 3 blown caps but replacing them did not fix the TV. I am putting this TV back together until I get parts.

author: james_s
author_rank: Super Contributor
timestamp: December 11, 2017, 10:27:44 pm
content: That is a typical failure mode of many modern electrolytic capacitors, I've replaced hundreds of bulging/leaking capacitors where nothing else was wrong with the circuit. The discolored PCB is not necessarily an indication of a fault either, lots of components run hot enough in normal operation to discolor the PCB over time.