Am I on the right track with this repair? - Page 1
Forum section: electronics / repair

author: c.cam108
author_rank: Newbie
timestamp: September 12, 2013, 09:57:49 pm
content: A couple of weeks ago I managed to rescue a 42" Philips plasma TV from the side of the road. I took it home and it powered up and worked perfectly.However, today when I took it to my church and set it up, it wouldn't power on. When trying to turn on from standby, the relays would click on and immediately back off again repeatedly.I pulled the cover off and disconnected all other boards from the PSU board and it still wouldn't power up, clicking the relays. The status LED on the daughterboard attached to the PSU board would dim when the relays clicked.I've attached photos of the PSU board. I've annotated a few items:The red line is the isolation gap between primary and secondary.Green indicates mains input and filtering.Blue shows the relays that are switching.Yellow shows the main bridge rectifiers switched by the relays which I first suspected were shorted, but they are fine (now that I think about it, that would have blown the fuse immediately).The relays and bridge rectifiers indicated supply everything, other than the standby voltage. This is supplied through the smallest transformer, highlighted in orange. The capacitor next to this transformer is bulging a bit.My thoughts are that the cap for the standby power has failed, and when the rest of the board powers up, there isn't enough charge to maintain the standby voltage, causing it to brownout and resetting the circuit. Would this make sense?I'm not sure how I can measure if the standby rail is dipping. I've got a digital multimeter which probably wouldn't update fast enough, and I've got a scope, but it's an old analogue with no capture or storage facility. I might be able to get my hands on a analogue multimeter to try and spot the needle twitching.Thanks for the help,Colin C

author: JamesAus
author_rank: Contributor
timestamp: September 12, 2013, 10:15:23 pm
content: Some of the electrolytic caps look very sick. Replace all of them on the PSU board. Make sure you use 105 deg low esr. There's a 95% chance that will solve your problem. I wouldn't even bother looking deeper until I'd done that.

author: mariush
author_rank: Super Contributor
timestamp: September 12, 2013, 10:39:53 pm
content: Replace all small capacitors on the right side of the board, AND the capacitors on that other side of the heatsink by the transformers and the tiny group of capacitors at the center-bottom of the board.Use good quality, low esr capacitors, 105c etc as already advised.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 12, 2013, 10:40:13 pm
content: It's probably dropping out because there's no smoothing on the low voltage side. As James says, those caps at the right hand side of the board look very suspicious I count at least six that I would replace without bothering to measure them. Whist your at it you may as well replace all the other caps of similar size.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 12, 2013, 10:42:39 pm
content: Panasonic FM or FR series (low ESR) are very good for this type of application.

author: c.cam108
author_rank: Newbie
timestamp: September 12, 2013, 11:11:46 pm
content: Yeah figured the caps would all need replaced. I'm just surprised that a 6 year old TV has failed within the 2 weeks I've had it. Typical eh?I see decent Panasonics are really cheap on RS. I was worried that good caps were going to cost a ridiculous amount.

author: David_AVD
author_rank: Super Contributor
timestamp: September 12, 2013, 11:13:55 pm
content: I'd say that's the reason it was on the side of the road. The caps are borderline and happened to be on the "right side" when you first tested it.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: September 13, 2013, 12:53:31 am
content: Quote from: c.cam108 on September 12, 2013, 11:11:46 pmYou can`t complain,i think you got a bargain .

author: johansen
author_rank: Super Contributor
timestamp: September 13, 2013, 02:33:02 am
content: Quote from: c.cam108 on September 12, 2013, 09:57:49 pmlol, there's your problem right there. No, i don't mean going to church (unless its a 501C3 church, and if it is, you're name is already on the blue list lol)

author: vk6zgo
author_rank: Super Contributor
timestamp: September 13, 2013, 04:21:22 am
content: For future reference, an analog Oscilloscope can be used as a large display DC voltmeter.For instance,if you want to see if a DC voltage is "dipping",set your 'scope to "free run" (auto trigger position) at any convenient time/cm setting.Switch the 'scope to DC coupling & vertically centre the trace.With a suitable volts/cm setting for the voltage you are observing,turn on the DUT.The trace should deflect to the correct value,then dip if there is a problem.This obviously doesn't show the actual waveshape of the "dip",but neither does an analog multimeter.

author: c.cam108
author_rank: Newbie
timestamp: September 13, 2013, 08:45:05 am
content: Quote from: vk6zgo on September 13, 2013, 04:21:22 amYeah I would have to keep a close eye on it though, depending on how momentary the dip in voltage is. That's why some kind of persistent display or memory would make it easier.What's DUT?

author: David_AVD
author_rank: Super Contributor
timestamp: September 13, 2013, 08:53:07 am
content: Quote from: c.cam108 on September 13, 2013, 08:45:05 amDeviceUnderTest

author: c.cam108
author_rank: Newbie
timestamp: October 22, 2013, 03:28:24 pm
content: Just to follow up on this:I bought £20 worth of Panasonic FR caps to replace all of the output capacitors on the board (didn't bother with the smaller caps) and it now works perfectly. Not a bad price to pay for a 42" plasma.The annoying thing is it doesn't have a VGA input