Bad LCD TV power supply board - Page 1
Forum section: electronics / projects

author: tayexdrums
author_rank: Contributor
timestamp: December 08, 2011, 08:09:47 am
content: My Insignia 32" tv went out about a month ago. I can hear audio but no video. From research, I have concluded that it is the power supply board. The power supply board also has a burn mark near a resistor, but I have tested the resistor and it is good. I can't figure out what is wrong with it. There are 3 large transformers and 1 smaller transformer on the board. Power transistors are mounted to heat sinks and the solder joints on the leads have become brown and crusty from getting hot. I think it a transformer but I am unsure how to test them. I've tried hooking the primary side up to a function generator and the secondary side up to the scope. I'll gradually increase the frequency until I see a waveform appear on the scope. I'm not an expert (yet) but what exactly shouldn't the waveform resemble the input waveform. I used a sine wave.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 08:46:56 am
content: Does the power supply board have a written output voltages at the connector? Do you have a service manual? Measure the voltage outputs that way you will know surely

author: tayexdrums
author_rank: Contributor
timestamp: December 08, 2011, 08:54:06 am
content: Yes, there are labeled output voltages and I have measured them. Some of them do not output what is written. That is one of the main reasons I think that the power supply board is the suspect. I don't have a service manual...the tv is about 5 years old

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 08:56:30 am
content: Service manual with test points would be verry helpful. I guess you will need to do some reverse engineeringTesting the transformer without knowing its specs is not possible (atleast not easy) since you dont know its internal construction. However they fail verry rarely

author: tayexdrums
author_rank: Contributor
timestamp: December 08, 2011, 09:10:35 am
content: It's funny you should mention reverse engineering because that's what Ii just started doing. I doubt it will help.lolI have my meter set to check continuity and I'm probing the windings of the transformer to see if there are any shorts between the primary and secondary.

author: Psi
author_rank: Super Contributor
timestamp: December 08, 2011, 09:16:08 am
content: Have you had a good look at all the capacitors?

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 09:29:35 am
content: Quote from: tayexdrums on December 08, 2011, 09:10:35 amThat wont be a good method too because some windings can be sharedQuote from: Psi on December 08, 2011, 09:16:08 amThat is a good advice too, especially if you noticed alot of heat damage on the PCB. Definately check them first then look at faulty voltages and trace them backwards

author: tayexdrums
author_rank: Contributor
timestamp: December 08, 2011, 09:32:11 am
content: Yes, I have physically inspected them and they all appear to be fine. I don't have an ESR meter to test them though=[

author: ElektroQuark
author_rank: Unknown
timestamp: December 08, 2011, 09:46:05 am
content: Are you sure there is no video?Check the screen with a flashlight, if there is an image then is the inverter board is bad. Sometimes the video is there but the CCFLs do not light up.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 08, 2011, 10:00:19 am
content: If it's obviously been running hot, the first thing I'd do is replace all the electrolytic caps on the PSU board. Make sure you use 105 or 125 degree ones.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 10:11:45 am
content: Quote from: tayexdrums on December 08, 2011, 09:32:11 amUnfortunatley looking at the capacitors you cant know are they good or bad

author: saturation
author_rank: Super Contributor
timestamp: December 08, 2011, 12:11:24 pm
content: If you've identified what line supply has failed, you can use a lab bench supply to provide the missing output. Are you sure there isn't a stage supplied by the PSU that trigger the failure? If so, the internal protection of a lab supply should protect it against shorts or equivalent, it which case there is more to do that just repair the PSU.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 02:02:38 pm
content: Quote from: saturation on December 08, 2011, 12:11:24 pmThat could be a good idea but he should be careful on current setting.

author: saturation
author_rank: Super Contributor
timestamp: December 08, 2011, 02:16:04 pm
content: Yes, but with a fully variable constant current output the user can slowly ramp up the current or voltage output until the device works. If a user has a proper lab PSU: at least short circuit protection, overload/overheat protection, constant current, constant voltage capability, one needn't worry. It also should be cleaner, lowest noise and strictest regulation, than any PSU installed in products so the output noise or regulation should not be a factor in complicating the diagnosis.Quote from: hacklordsniper on December 08, 2011, 02:02:38 pm

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 02:49:27 pm
content: I agree, the only problem if there is problem in the mainboard and he uses too large current on the lab PSU

author: Zad
author_rank: Super Contributor
timestamp: December 10, 2011, 09:26:23 pm
content: The LCD backlight inverter has failed. Talk to your local independent TV repairer and ask them how much a replacement is likely to be. This is the most common fault in LCD TVs. Chances are that it is cheaper to replace the inverter module than for you to source the individual components.