Trying to fix an SMPS in a plasma TV, off topic a bit, but perhaps someone help. - diyAudio
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Old 26th August 2009, 04:28 AM   #1
orpheus is offline orpheus  United States
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Default Trying to fix an SMPS in a plasma TV, off topic a bit, but perhaps someone help.

I found a Dell W5001C 50 inch plasma out for the trash last week, and couldn't resist dragging it home to see if it worked. It doesn't, plug it in, it clicks, led's light up, but the screen doesn't even flicker. I opened it up and poked around with my multimeter, and the SMPS 5 volt supply seems to be malfunctioning. Here is what I have been able to discern.

The supply measures spot on, except for the 5 volt supply, which is 3.47 volts. The measurements are only good if I don't plug in one of the daughter boards, that only has the 5 volt supply on it.

When I measure between ground and 5 volts, I only get around 30 ohms of resistance. So far I have pulled one of the regulators, and three of the capacitors in line with the 5 volt portion of the supply, but I am still measuring a short to ground. I have checked through most of the places the 5 volt supply is connected, looking for a short between solder pads and haven't found one.

Since none of the other supplies on the board are shorted ground, I am assuming that this is my problem. Is this a good assumption? My thinking is that when I plug in the daughter board that controls the on/off function of the TV to the 5 volt supply and it doesn't get 5 volts, it shuts down the lines of the supply that power the television. If I fix the 5 volt supply, I might have fixed the problem. Unfortunately, there is no trim pot for 5 volts, only for the bigger voltages.

Should I pull everything I can in the 5 volt line and replace it? I had thought that a cap might have gone bad, especially since two of the caps looked like they had bulged a bit. I pulled those, and I still get the short to ground.

Any advice? I don't have an isolation transformer or an oscilloscope, but I do have a solid fluke multimeter. I will have to hunt and peck to fix this thing, if it is even possible.

Thank you for taking the time to read my post.

Sincerely,
Aaron.
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Old 26th August 2009, 04:41 AM   #2
col is offline col  Australia
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Be careful to discharge all High Voltage Capacitors before you start probing things, there are some lethal voltages inside SMPS. This site is very helpful:

http://www.repairfaq.org/sam/smpsfaq.htm

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Old 26th August 2009, 04:57 AM   #3
star882 is offline star882  United States
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Can you provide some pictures? What happens if you use a bench power supply to provide the 5v? The 30 ohms might be the normal load resistor. An old tape drive power supply I have (repurposed into a small bench power supply) has a 20 ohm resistor for 5v minimum load.
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Old 27th August 2009, 12:08 AM   #4
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by orpheus View Post
The supply measures spot on, except for the 5 volt supply, which is 3.47 volts. The measurements are only good if I don't plug in one of the daughter boards, that only has the 5 volt supply on it.
Not Necessarily since the daughter board using the 5V power supply make be drawing excessive current cause the voltage to drop.

Quote:
Originally Posted by orpheus View Post
When I measure between ground and 5 volts, I only get around 30 ohms of resistance. So far I have pulled one of the regulators, and three of the capacitors in line with the 5 volt portion of the supply, but I am still measuring a short to ground. I have checked through most of the places the 5 volt supply is connected, looking for a short between solder pads and haven't found one.
The 30 ohms resistance may be part of the design. Most power supply controllers need some current flow on the output for proper regulation. For instance powering up a PC power supply unconnected will typically result in clicking noises and an auto power down of the supply. Since the TV has a standby mode and the 5V is powered at all times, its a good bet that the 30 ohm resistance is built into the power supply by design.

If there is indeed a short somewhere a good place to start is to check all of the caps. The cheap Chinese caps often fail by creating a short. You can try to check them using a ohm meter to spot a short across the cap, however its possible that there is another device in parallel with the cap that is the culprit. If you have a in-circuit cap checker, you can use it to determine if you have a failed cap. If the power supply supplies enough output current you can check the temperature of each cap to see if one is abnormally warm since if its bad it will give off excessive heat.

The only other option is to desolder them and test, if you do desolder and find one of them is bad, then replace them all on the PS, since is very likely that are all part of the same batch, and sooner or later (sooner) another cap is going to fail.

One cheap option is to call Dell and ask them what part typically fails in your model, and see if they can ship you a replacement part. If the part isn't too expensive it make be a more practical solution to fix the problem. Or at least Dell might be able to pin-point the problem board for you.

Another cheap option is to blow down all of the circuit boards using an air compressor. There could be a small piece of loose solder or other small metal foreign object, that is creating a short somewhere.

Best of Luck.
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Old 28th August 2009, 06:18 PM   #5
orpheus is offline orpheus  United States
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Quote:
Originally Posted by col View Post
Be careful to discharge all High Voltage Capacitors before you start probing things, there are some lethal voltages inside SMPS. This site is very helpful:

http://www.repairfaq.org/sam/smpsfaq.htm

col.
I appreciate your warnings, I know I am taking a chance working on this thing. The caps discharge very quickly, I measured the voltage while I unplugged it to see, and in less than a minute there is no voltage. So far I have managed to get shocked by the 22 volts that runs through the heatsinks on the first stage of the supply. Thank you for the link, that is a great resource.

Quote:
Originally Posted by star882 View Post
Can you provide some pictures? What happens if you use a bench power supply to provide the 5v? The 30 ohms might be the normal load resistor. An old tape drive power supply I have (repurposed into a small bench power supply) has a 20 ohm resistor for 5v minimum load.
I'll put up some pics if it is raining tomorrow and I can't go to the beach . There are a lot of voltages on this supply, and a lot of connections that carry the 5 volt line. I don't think a bench supply would be feasible because of the number of connections I would have to disconnect and reconnect, plus I don't have one, but I do have batteries and adaptors. Thank you for letting me know about the 20ohm to ground on your old tape drive supply, that makes me think that it is designed that way. Unfortunately the manufacturer put white rubber(?) caulk on some of the parts in the 5 volt supply, so I can't easily pull the resistor or read the color bands.

Quote:
Originally Posted by TechGuy View Post
Not Necessarily since the daughter board using the 5V power supply make be drawing excessive current cause the voltage to drop.



The 30 ohms resistance may be part of the design. Most power supply controllers need some current flow on the output for proper regulation. For instance powering up a PC power supply unconnected will typically result in clicking noises and an auto power down of the supply. Since the TV has a standby mode and the 5V is powered at all times, its a good bet that the 30 ohm resistance is built into the power supply by design.

If there is indeed a short somewhere a good place to start is to check all of the caps. The cheap Chinese caps often fail by creating a short. You can try to check them using a ohm meter to spot a short across the cap, however its possible that there is another device in parallel with the cap that is the culprit. If you have a in-circuit cap checker, you can use it to determine if you have a failed cap. If the power supply supplies enough output current you can check the temperature of each cap to see if one is abnormally warm since if its bad it will give off excessive heat.

The only other option is to desolder them and test, if you do desolder and find one of them is bad, then replace them all on the PS, since is very likely that are all part of the same batch, and sooner or later (sooner) another cap is going to fail.

One cheap option is to call Dell and ask them what part typically fails in your model, and see if they can ship you a replacement part. If the part isn't too expensive it make be a more practical solution to fix the problem. Or at least Dell might be able to pin-point the problem board for you.

Another cheap option is to blow down all of the circuit boards using an air compressor. There could be a small piece of loose solder or other small metal foreign object, that is creating a short somewhere.

Best of Luck.

Thank you very much for your suggestions. I should have been more specific in what happens with the supply, I was trying to be brief, but wasn't specific enough. The 5 volt supply always measures 3.47 volts, whether it is plugged into the daughter board (not the only daughter board, but seemingly the only one that makes a difference). Some of the other voltages on the board go to almost zero when the daughter board (perhaps the controller? It is below the AC input and near inputs/outputs/and switches). The +-190, and 198 turn off when I plug in that particular board.


So, it seems from the response that the 30ohm resistance to ground isn't the problem as I thought it might be. Once eliminated, and after reading the repairfaq and responses, I am going to replace the caps, at least the ones in the 5 volt line at first. Two of them had slightly bulging tops, which I know isn't a good sign from blowing up a few capacitors myself in the past. I don't know if my Fluke 115 will do in line capacitance measurements accurately or not. I looked in the manual, but it wasn't clear. I'll google to see if I can find an answer to that. I have pulled the two bulging caps, when I start working on it again, I'll measure them and see if they are working. I also pulled a regulator thinking it might change the resistance to ground (and since the 30ohms to ground began there). It didn't change anything, so I'll have to put a new regulator, and new caps in before I can test the board again.

I'll try the compressed air and contacting Dell as well.

When I measure the pulled caps, am I looking for shorts and seeing if they have their specified capacitance? Anything else I should look for?

Thanks again to all of you for sharing your expertise. I appreciate your help.

Best,
Aaron.
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Old 29th August 2009, 12:07 AM   #6
col is offline col  Australia
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I fixed a broken Nakamichi BX-100 tape drive that I pulled off the scrap heap via this forum.

Nakamichi BX-100 repair

One of the members (Stevecole) actually spotted what was wrong with it via one of the photos that I put up. You might want to take some large detailed photos and load them into one of the free photo sites and then link to it.

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Old 29th August 2009, 01:17 AM   #7
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by orpheus View Post
Iof them had slightly bulging tops, which I know isn't a good sign from blowing up a few capacitors myself in the past.
Replacing the bulging caps is a no brainer. It would be the very first thing I would do. If there are other caps with the same brand on that board, even if they aren't bulging, just replace them all. Otherwise you'll end up playing Wack-a-Mole but with caps!

Assuming you get the PS running again, after replacing the caps, If the PS has a output spec in the silkscreen ie (+5V @ 700 ma), check to make sure that the load isn't near the top or over. If it is at or over the max output current than you probably have more caps to replace. As Electlytic caps age, the dry out, which causes a rise in ESR and current leakage, which cause higher current demand. Cheap aging caps start becoming less like capacitors and more like resistors.

I do have one question, after you fix it, are you going to keep it or sell it?

Best of luck to you!
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Old 31st August 2009, 07:18 PM   #8
orpheus is offline orpheus  United States
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Quote:
Originally Posted by TechGuy View Post
Replacing the bulging caps is a no brainer. It would be the very first thing I would do. If there are other caps with the same brand on that board, even if they aren't bulging, just replace them all. Otherwise you'll end up playing Wack-a-Mole but with caps!

Assuming you get the PS running again, after replacing the caps, If the PS has a output spec in the silkscreen ie (+5V @ 700 ma), check to make sure that the load isn't near the top or over. If it is at or over the max output current than you probably have more caps to replace. As Electlytic caps age, the dry out, which causes a rise in ESR and current leakage, which cause higher current demand. Cheap aging caps start becoming less like capacitors and more like resistors.

I do have one question, after you fix it, are you going to keep it or sell it?

Best of luck to you!

I haven't put up pictures before, so I need to sort that out.

Thanks again for taking the time to respond. I am going to order some parts from digi-key soon (hopefully today), caps and the one regulator I pulled when I thought the 30ohm connection to ground was the problem. I want to get the supply working properly before I desolder and replace all of the caps, so that I can see if the screen is burnt in anywhere, and if the TV is even resurrectable before I get super crazy with preventative maintenance.

To measure current, I just connect my Fluke 115's leads to the correct terminals, set it to current measurement, and then put the leads on two +5volt places, right?

I don't have any intention of selling the TV if I can get it working. Right now I have a 27 inch CRT circa 2000 (I spend my entertainment money on my stereo and recordings). My hope is to bring it back to life and use it. If it isn't possible to bring it back from the dead, I might sell some of the parts on ebay to recoup the cab fare bringing it home and the parts that I purchased, and to keep things out of the landfill.

Thanks again for your time and help.

Aaron.
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Old 3rd September 2009, 08:33 AM   #9
h_a is offline h_a  Europe
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Quote:
Originally Posted by orpheus View Post

To measure current, I just connect my Fluke 115's leads to the correct terminals, set it to current measurement, and then put the leads on two +5volt places, right?
Hi Aaron, I'm hesitating a bit to recommend you to continue your work on this TV if you are not sure about these basic things.

To measure current, your multimeter has to be in series with the circuit. That usually means you can only measure between modules that are connected by wires (like power supply->wire->Multimeter->wire->Module) as you need to break them up. On pcbs you would need to cut traces and repair them after the measurement, so not really an option.

Have fun and take care! Hannes
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Old 3rd September 2009, 09:35 AM   #10
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You should certainly replace ALL bulging capacitors, and (as already suggested) replace any similar make ones, or as a minimum check them - to do this you need an ESR meter, measuring capacitance or using a multimeter is useless.

If the 5V rail is low, it's most likely that the capacitors on that rail are high ESR.

High ESR electrolytics is by FAR the most common problem in modern electronics.
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