I don't have experience with these flat panels (have worked on regular tube sets for about 5 years), so I'm hoping for a little help. This set powers on, and you briefly see the philips logo on screen but the image is shaky, then after maybe one second, this display drops out and all that's left is a dim blue screen. Haven't seen anything on the web concerning problems with this set. Would this be a back light inverter problem? Without a schematic, I'm sorta of lost on this. If it may be the backlight, how do you go about troubleshooting before replacing parts? Thanks for any help group.
If the mainboard attempts to 'boot' (logo displays) but then it drops back to a blue screen, I would feel inclined to be looking around the power supply first. Just about all LCD TVs suffer from problems with bad electrolytics on the power supply, and also bad joints on the dreadful lead-free solder that's used now to build them. First, carefully examine all of the electrolytics, but particularly the secondary side ones, and especially any near or under heatsinks, for signs of their tops bulging. If none immediately leap out at you as being faulty in this way, then run over them all with an ESR meter. After that, examine the solder side of the board very carefully under a strong light and with a powerful magnifying glass. Be advised though that bad joints made with lead-free solder can be very difficult to spot ...
Because unless you remove better than 95% of the lead free solder the resulting alloy is too far from the eutectic and vastly increases the risk of a bad joint!
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Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
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Absolutely correct, quite aside from the fact that it is illegal for a professional repairer in the EU to repair a post-June 2006 piece of equipment that's built using lead-free, with anything other than lead-free. As a professional repairer myself, I observe this, much as I hate doing so and, without looking into the status and location of a poster too closely, I would not recommend that he or she does any different.
Also, have you ever tried doing a successful blanket resolder on a complex board that is covered in surface mount as well as conventional components? On an average LCD power supply, there is probably better than 1000 joints ...
Thanks to all for the advice. Sorry for the late reply (been working night shift). I have looked over the power supply (checked some of the components) since the set does startup, but immediately drops the display like when it is demanding power. I haven't ESR'd any caps yet, but don't see any visibly distressed either. I disconnected two ribbon cables from the power board to check a few voltages. I have 5.2 volts where it says it should be, but am missing the 12 volts on the two seperate connectors. I was trying to trace the PCB backwards from the connector pins to locate the source, but it was getting late. I checked several surface mount resistors and transitiors on the bottom of the board, but haven't located a fault yet. I will continue with looking over the power supply, and update this post if I find the problem. Thanks again group.
Thanks to all for the advice. Sorry for the late reply (been working night shift). I have looked over the power supply (checked some of the components) since the set does startup, but immediately drops the display like when it is demanding power. I haven't ESR'd any caps yet, but don't see any visibly distressed either. I disconnected two ribbon cables from the power board to check a few voltages. I have 5.2 volts where it says it should be, but am missing the 12 volts on the two seperate connectors. I was trying to trace the PCB backwards from the connector pins to locate the source, but it was getting late. I checked several surface mount resistors and transitiors on the bottom of the board, but haven't located a fault yet. I will continue with looking over the power supply, and update this post if I find the problem. Thanks again group.
Most LCD power supplies are not just one supply, but actually three. One is the standby supply and this typically produces either 5v, 3v3, or both. I would guess that the 5v2 that you are seeing is your standby supply. The other two supplies are the PFC supply, and the main supply. The PFC supply is a 'front end' one responsible for producing the raw supply to the main section with a power-line draw that doesn't upset the electricity supply company. The main supply usually produces +12v,
24v and +34v for the tuner (though not always that one). Sometimes, there will also be additional +5v and +3v3 rails. All of these rails are switched on by the system control micro on the main board.
But here's where it then gets tricky ...
Most LCD power supplies are covered in protection circuits, and if a 'problem' is detected on any second level rail - that's over-current or over-voltage - the whole TV is immediately reset by momentarily interupting the standby supply, which removes the supply to the system control micro, which then removes the "on" signal to the PSU ...
All of which makes it difficult to troubleshoot, unless you have spare boards to try, and detailed schematics. One thing that is quite common is a problem with the backlight inverter, which causes it to draw too much current, although as you say that you can see a logo come up before a blue screen, I'm a little loath to go down that route. Usually, if you disconnect the plug going to the backlight inverter board, and the fault that's causing the power supply to shut back down is on there, then the TV will come on and stay on, and you will hear normal sound, and if you shine a strong light at the screen at an angle, you may also be able to see a picture.
Beyond these basic checks, there is likely little more that you will be able to do. Oddly enough, I was discussing this just last Friday with a colleague who has a shop specialising in LCD and plas repairs, and I asked him how he goes on with problems of sets that won't come out of standby where the cause is on the mainboard itself, which is also common, and may well be the case on your TV. He told me that he keeps all of the boards from insurance write-offs with broken screens and cabinets, and then repairs, where possible, by using them to board-swap. Beyond that, he said, it's pretty much scrap 'em.
I was hoping not to see that last sentence, but I do thankyou for the info. Perhaps I should clarify.....I don't loose standby power supply since the screen is still lightly illuminated after the logo dissapears, and the green power light remins lit. The menu display comes up when commanded, but intermittently gets really shaky, sometimes goes to just black and white vetical lines, sometimes comes back to normal, etc. When I power up the TV, I always get an onscreen message saying "locked" after the Philips display goes away, so I'm not sure if all this is related to one problem. Iv'e cleared the V- chip ratings in the user menu (when the display hangs in there), but no change. I managed to change the video source to TV (have a VCR connected), and try to put on channel 3, but I get a message saying channel unavailable. I pulled the power supply board and checked all caps for ESR. I did find one faulty (102+ohms) and replaced. No change. I also resoldered several joints that just looked really dull and thin on solder. I would really like to figure out what the "locked" problem is. Philips was not any help, and web searches turned up nothing except using a four digit code to unlock, but referring to a menu item that this TV doesn't have to use this function. Other than accessing a service menu that I don't know how to do, is there an electronic means of "resetting" the TV? Could all this be a software problem? Thankyou again for advice.
I was hoping not to see that last sentence, but I do thankyou for the info. Perhaps I should clarify.....I don't loose standby power supply since the screen is still lightly illuminated after the logo dissapears, and the green power light remins lit. The menu display comes up when commanded, but intermittently gets really shaky, sometimes goes to just black and white vetical lines, sometimes comes back to normal, etc. When I power up the TV, I always get an onscreen message saying "locked" after the Philips display goes away, so I'm not sure if all this is related to one problem. Iv'e cleared the V- chip ratings in the user menu (when the display hangs in there), but no change. I managed to change the video source to TV (have a VCR connected), and try to put on channel 3, but I get a message saying channel unavailable. I pulled the power supply board and checked all caps for ESR. I did find one faulty (102+ohms) and replaced. No change. I also resoldered several joints that just looked really dull and thin on solder. I would really like to figure out what the "locked" problem is. Philips was not any help, and web searches turned up nothing except using a four digit code to unlock, but referring to a menu item that this TV doesn't have to use this function. Other than accessing a service menu that I don't know how to do, is there an electronic means of "resetting" the TV? Could all this be a software problem? Thankyou again for advice.
To be honest, it could be almost anything. Understood now that your power supply is not actually going off. Have you checked the actual voltages on the second level supplies, and run a 'scope over them, just to make sure that none are 'hashy' ? Somehow, your fault description still seems to have something of a power supply 'feel' about it. I'll see if any of my buddies in the trade have had anything similar with the "locked" message on any Philips LCDs
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