I picked up this LCD tv at a yard sale for $10. There was a sticker on it that said "Needs Work", and because I've been wanting a new project, I picked it up. I asked the guy selling it what was wrong with it, and all he said was that it needed a new power board. (I didn't think to ask him what kind of testing he'd done to get to that) I had just fixed up a Gateway monitor that needed new caps on its power supply, so I figured I'd either be able to swap out caps and it'd be a quick fix, or worst case scenario, I'd have a good educational experience. I should point out at the beginning that I know just enough to get myself into trouble, so if I'm testing something the wrong way, or if I'm making an assumption I shouldn't, please holler at me. Also, I'll put up some pictures in a second post.
What happens when you give it power is, you hear a relay click, and a red light comes on by the power button. If the set was working, it would turn green, and the tv would come on.
After taking it apart, and doing what initial testing I know to do, here's what I've found:
1. All the caps on the power supply board *look* good. Nothing is bulging or leaking at all. That being said, 2 of the three brands of caps on the board, from what I can tell at least, are known to fail without any signs. The caps are Teapo, CapXon, and Jamicon.
2. There is a mini power board, with a relay and some other components. I'm getting 120VAC coming in to this board, and when the relay clicks on, i'm getting 120VAC out, going to the main Power Board. There is also a line going out, outputting 5.4VDC and 2.2VDC (I'm assuming this is the standby power) Can I assume this board is working correctly?
3. After looking over the board, I realized that a mystery component was actually a fuse. I didn't recognize it at first, because I had never seen a fuse like this before. It's a reddish/brown cylinder, labeled as 6.3A 250V. Anyway, I tested the fuse for continuity, and got nothing. So this gave me a starting point for trying stuff out.
I ordered replacement fuses (I got 10, since I had a good idea what would happen) and put one in this morning. Sure enough, I give it power, the relay clicks on, and I smell a faint burning smell as the new fuse blew.
4. While waiting on the new fuses to come in, I was just messing around trying to test what I could. At some point, for some reason, I decide to feel the heatsinks to see if anything is getting hot. (I realized after that there's no reason for the heatsinks to get hot if the board's fuse was blowed, but I did it anyway).
There are two heatsinks running down both sides of the board, connected to two lines of diodes and transistors. I touched the back of a finger to the bigger one, and it felt like a sharp bit of metal poked my finger. I looked at it, and there was nothing there. Touched it again (carefully, again with the back side of a finger, and with my free hand behind me), and there was a vibrating feeling, followed by the sharp pain again. Nothing super painful, just like touching a finger to the point of a nail. I unplugged the tv, gingerly touched it again, and there was nothing at all.
To try and figure out where the shock was coming from, I powered the set back up, and used my multimeter to see if I could find out how much voltage the heatsink was getting. With my black probe on the chassis ground screw, I measured for AC, while running the red probe down the length of the heatsink. For most of the heatsink, I read about 1V, but next to a transister labeled Q7, I'm getting a reading of 120VAC. ...I'm assuming this is bad
Just to reiterate, this is with the power plugged in, the set turned on (120V into the main power supply), but with the power supply's fuse blown.
5. I have taken a look at the other boards, and everything there looks ok, at least to my eyes. I did notice that most of the other boards have their own fuses, with lower amperages than the power supply fuse. Can I assume that this means anything bad on these boards that could blow the PSU fuse would have blown the local fuse first? In other words, I should still focus on the PSU?
What would be my next step? I know something is blowing the fuse on the power supply, and I know that there's 120V going somewhere it probably shouldn't. How do I track down the cause (preferably without electrocuting myself...) This is already the most in depth I've ever gotten with any project, but I really love learning this stuff.
What happens when you give it power is, you hear a relay click, and a red light comes on by the power button. If the set was working, it would turn green, and the tv would come on.
After taking it apart, and doing what initial testing I know to do, here's what I've found:
1. All the caps on the power supply board *look* good. Nothing is bulging or leaking at all. That being said, 2 of the three brands of caps on the board, from what I can tell at least, are known to fail without any signs. The caps are Teapo, CapXon, and Jamicon.
2. There is a mini power board, with a relay and some other components. I'm getting 120VAC coming in to this board, and when the relay clicks on, i'm getting 120VAC out, going to the main Power Board. There is also a line going out, outputting 5.4VDC and 2.2VDC (I'm assuming this is the standby power) Can I assume this board is working correctly?
3. After looking over the board, I realized that a mystery component was actually a fuse. I didn't recognize it at first, because I had never seen a fuse like this before. It's a reddish/brown cylinder, labeled as 6.3A 250V. Anyway, I tested the fuse for continuity, and got nothing. So this gave me a starting point for trying stuff out.
I ordered replacement fuses (I got 10, since I had a good idea what would happen) and put one in this morning. Sure enough, I give it power, the relay clicks on, and I smell a faint burning smell as the new fuse blew.
4. While waiting on the new fuses to come in, I was just messing around trying to test what I could. At some point, for some reason, I decide to feel the heatsinks to see if anything is getting hot. (I realized after that there's no reason for the heatsinks to get hot if the board's fuse was blowed, but I did it anyway).
There are two heatsinks running down both sides of the board, connected to two lines of diodes and transistors. I touched the back of a finger to the bigger one, and it felt like a sharp bit of metal poked my finger. I looked at it, and there was nothing there. Touched it again (carefully, again with the back side of a finger, and with my free hand behind me), and there was a vibrating feeling, followed by the sharp pain again. Nothing super painful, just like touching a finger to the point of a nail. I unplugged the tv, gingerly touched it again, and there was nothing at all.
To try and figure out where the shock was coming from, I powered the set back up, and used my multimeter to see if I could find out how much voltage the heatsink was getting. With my black probe on the chassis ground screw, I measured for AC, while running the red probe down the length of the heatsink. For most of the heatsink, I read about 1V, but next to a transister labeled Q7, I'm getting a reading of 120VAC. ...I'm assuming this is bad
Just to reiterate, this is with the power plugged in, the set turned on (120V into the main power supply), but with the power supply's fuse blown.
5. I have taken a look at the other boards, and everything there looks ok, at least to my eyes. I did notice that most of the other boards have their own fuses, with lower amperages than the power supply fuse. Can I assume that this means anything bad on these boards that could blow the PSU fuse would have blown the local fuse first? In other words, I should still focus on the PSU?
What would be my next step? I know something is blowing the fuse on the power supply, and I know that there's 120V going somewhere it probably shouldn't. How do I track down the cause (preferably without electrocuting myself...) This is already the most in depth I've ever gotten with any project, but I really love learning this stuff.
Comment