Help with LCD TV Power Supply - Page 1
Forum section: electronics / projects

author: skytoastar
author_rank: Contributor
timestamp: October 17, 2012, 11:07:30 pm
content: I'm trying to repair an Insignia LCD TV Power Supply problem. Let me describe the symptoms first. When in standby mode, nothing happens (which is normal). When powered on, the TV works normally for about 35 seconds and then it turns off with the sound giving a pop. It stays off for about 3 seconds (but the blue "power on" LED will stay lit) and then it turns back on again with a screen that says "Please Wait." After about 5 seconds sound and video are restored. After another 35 seconds of normal TV function this repeats. (The timing was pretty much the same after 15 minutes of operation, also). There are two boards connected to the power supply board. The first is the main board and the second is the board controlling the LCD backlight. This problem happens even with the backlight board disconnected. It appeared there was a capacitor on the main board with white crusty material near the top edges of the cap but with no bulging. I replaced that cap but it had no effect. I disconnected everything I could from the mainboard to eliminate other possible problem parts. Only the three composite inputs, the speakers, power, and the control buttons were attached to the main board and it still had this power cycle problem.With everything attached, here are the voltages I read from the power supply output:Names: 24V 12V 5V ADJ BKOn Voltages: 23.3, 12.3, 5.1, 3.3, 5Off Voltage: 24.5, 11.8, 5.1, 0, 0On voltage is during normal operation, off voltage is the voltage read during the 3 second blackout period.When I have the power supply board disconnected from everything, I don't know how to turn it on. The main board's STB line goes to the power boards PSON line. As far as electrolytics go on the power supply board, there is one big 100uF 450V cap on the "hot" side, along with a few small caps, and on the "cold" side there are a bunch of 470uF and 220uF caps (25-35V). They all visually check out as being okay. The 100uF cap has about 365V across it during operation and is glued to the board. The cold side caps would be more difficult to replace because there is an L-shaped heat sink covering them. The heat sink is for two FCH20A10 and one MBR2045CT power rectifiers. It wouldn't be impossible though. Many of the cold side caps are also in parallel, making an ESR check more difficult.Any ideas for things to check or do? Is this a problem someone has come across before? Any help is appreciated.

author: skytoastar
author_rank: Contributor
timestamp: October 25, 2012, 04:31:26 am
content: I just got some replacement boards in the mail and it turns out it was the main board and not the power board that was bad. I had enough caps so I decided to replace every electrolytic cap on the main board. No help. I checked the temperatures on the heatsinked chip on the board and they weren't crazy, like 38 C or so after running a while, and about the same as my good replacement board. I'm stumped. If anyone has any brilliant idea I'm open to hear it but I think it may get junked.

author: amyk
author_rank: Super Contributor
timestamp: October 25, 2012, 06:00:34 am
content: Possibly a bad voltage regulator to the CPU that's temperature-sensitive.Post high-res pictures if you want more info.

author: skytoastar
author_rank: Contributor
timestamp: October 25, 2012, 03:34:32 pm
content: My board that doesn't work. You'll notice the caps are different than the next one since I switched them out. (PA250875.jpg)The board that does work. (PA250877.jpg)The main power connection is on the lower left. On the bottom of the board, from left to right, I'll describe the white connection terminals. First is the speaker output, then power for the built-in dvd player, usb plug "for service only", one I'm not sure of (it wasn't connected), and finally the volume/channel/input/menu/power buttons on the side of the tv. I've also attached the first image to this post.

author: skytoastar
author_rank: Contributor
timestamp: October 26, 2012, 04:03:34 am
content: A quick update. I checked the voltages on all the big voltage regulators that I could find on the main side of the board. The voltages are the same on both boards and don't change during the 3 second off period, that I can tell. One did by 0.02 V. There are some very small three terminal components that will change much more on two of the three pins. For example, for pins A,B,C I might see before/after voltages of 0,5,0/0,0,0.67 or 0,0,0.67/0,0,11.8. There are a bunch of those on the back side of the board I can't access when powered on. Kind of a side note. One thing I noticed when probing around was that when I probed in a certain place with my multimeter, marked in the picture below, I would get distortion in the sound and the off period could happen sooner. The sound would kind of be permanently distorted until the reset occurred, when it would go back to normal. One time I did it the left speaker became louder than the right. Another time I got it to stick on a solid tone until reset. The marked device has 8 pins and is marked "560" (ohms I assume?). If I check the top four pins against ground, I see 0.8V DC and no distortion in the sound. If I probe the bottom (circled in red), I also see 0.8V but get distortion. I can probe with my red probe and this happens whether ground is referenced with black or floating. I thought this was interesting.

author: amyk
author_rank: Super Contributor
timestamp: October 26, 2012, 06:12:58 am
content: Those look like a differential pair going from the main CPU (under the heatsink) to what looks like a RAM chip(?). 0.8V suggests that sort of signaling. The part numbers are too small to be readable. Probing with a multimeter probably introduced noise into the signal, which can cause the symptoms you mentioned. I doubt it has anything to do with your original problem, however.There is what looks like a serial EEPROM diagonally at the upper left of the CPU. If it is one, you could try reading out the contents on both boards and comparing, then trying the good code on the bad board. (What is the part# of the CPU? It might help finding more information about it.

author: skytoastar
author_rank: Contributor
timestamp: October 26, 2012, 02:47:32 pm
content: I guess the reason I even brought up the signalling corruption was to point out that it caused the reset to happen sooner than the typical 35 seconds, but not change it permanently. I found this out on the good board and just about passed out when it reset in the same way, worried I had ruined it as well. That doesn't seem to be the case, though, and I think it was just the one time.That diagonal chip is a Macronix MX25L3205D 32M-Bit Serial Flash. I'm really not enamored with the idea of reading those chips, however. I don't have a ton of tools available to do that. The best I can think of is that I have an MSP430 launchpad that could provide a SPI interface to read from/write to it but I frankly am not in the mood to do that coding and interfacing. What if we assume they are the same?And I don't really want to pull that heatsink off, either, although I'm more tempted to each day. The two metal pins holding the heatsink to the board are held to the back of the board by a glob of solder on each one. I'm just afraid I wouldn't be able to solder them back while applying the right amount of pressure.I know it's not helpful to not be willing to try things when doing diagnosis but I'm hoping there might be somewhere easier to start. Maybe?

author: skytoastar
author_rank: Contributor
timestamp: October 26, 2012, 10:44:45 pm
content: I pulled the heatsink off. It's an MStar MSD229EL-LF.

author: Psi
author_rank: Super Contributor
timestamp: October 26, 2012, 11:24:06 pm
content: The last LCD main board i repaired turned out to be a 3pin TO-92 style device which was on the reset line of the main IC.It would delay pulling reset High (run state) until the power rail had stabilized. You could see its output (which was supposed to be digital) drifting all over the place.But from what you say i think your issue is with a regulator on the main board. It's output is probably fluctuating and causing the audio to clip and IC's to reset.I would watch all the power supply outputs with the scope and see if you can identify one which is behaving strange when or before the problem occures.