Vizio VW37 Power Supply PSON working but no 12v or 24v output

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

I have a few electrolytic capacitors that would fit, but I will only be able to fit one (I was thinking I may add up a few to get the correct values, but there is not enough space).

One is labeled 33 μF and 16v, and has a multimeter reading of 36.6 μF. The next nearest is a 22 μF and 50v which has a multimeter reading of 20.5 μF.

A picture of both caps is attached. Any opinions on the matter?

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

I have a few electrolytic capacitors that would fit, but I will only be able to fit one (I was thinking I may add up a few to get the correct values, but there is not enough space).

One is labeled 33 μF and 16v, and has a multimeter reading of 36.6 μF. The next nearest is a 22 μF and 50v which has a multimeter reading of 20.5 μF.

A picture of both caps is attached. Any opinions on the matter?

More capacitance doesn't generally do any harm.

If you use an electrolytic make sure to secure it down, for example with a weak glue. The leads won't be enough if surface mounted; if you move the TV it could fall off the board.

I would highly recommend you use a ceramic though.

Have you tried powering it up without the failed cap?

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

No, I have not tried powering it up yet. I guess that would be a good idea before I start soldering stuff onto the board. One of the symptoms was fuzziness or dimness - would this be because there is no extra filtering portion from these caps, so the voltage going into the TFT will not be very smooth?

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

No, I have not tried powering it up yet. I guess that would be a good idea before I start soldering stuff onto the board. One of the symptoms was fuzziness or dimness - would this be because there is no extra filtering portion from these caps, so the voltage going into the TFT will not be very smooth?

Essentially, yes. It may also work at colder temperatures but stop working at hotter temperatures. Or the picture might be a bit noisier, not as sharp. But there should be something there to tell you the rest is working. On one Dell LCD monitor I get a picture for 2-3 seconds which is blurry, then it shuts down with a white screen, with one of those caps missing (in a similar area.)

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Ok, I just started it up without the caps and it worked - at least for the time I left it on (about 10 seconds). It displayed the blue "stand by" screen, and had the channel number in a box in the upper corner. Nothing seemed to blurry, but I didn't really test it with much graphics - I was just really psyched it is working. I swear I am going to frame the tiny single burnt out smd capacitor as a trophy.

Anyway, now that the issue is almost completely resolved, why are you (tom66) against electrolytics for the repair and budm is recommending them? How much of a difference would it really make (especially since it will run with no caps at all)?

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Ok, I just started it up without the caps and it worked - at least for the time I left it on (about 10 seconds). It displayed the blue "stand by" screen, and had the channel number in a box in the upper corner. Nothing seemed to blurry, but I didn't really test it with much graphics - I was just really psyched it is working. I swear I am going to frame the tiny single burnt out smd capacitor as a trophy.

Anyway, now that the issue is almost completely resolved, why are you (tom66) against electrolytics for the repair and budm is recommending them? How much of a difference would it really make (especially since it will run with no caps at all)?

I'm against using electrolytics because the designers specified ceramic for a reason. Ceramic capacitors are more expensive than electrolytics. They could have used surface mount electrolytics but they didn't. Sort of like most of these boards are gold plated, because signal integrity is crucial.

On my Dell it's motion blur, so things had a 0.5 second or so trail behind them when they moved.

Oh, and it's got one remaining other capacitor, so it's running "at two-thirds capacity".

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

One of the reason they have to use the surafce mounted low profile is that there is not much height for mounting electrolytics cap, second is that you do not have to worry about the polarity of the ceramic cap, third ceramic has low ESR. You can use the scope to check the AC ripple on the cap to verify that it is filtering properly.
I use either one depend how much room I have when I design the PCB that has to fit into the chassis. T-CON board is always covered with low profile metal cover. Just a thought from designer point of view.

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

One of the reason they have to use the surafce mounted low profile is that there is not much height for mounting electrolytics cap, second is that you do not have to worry about the polarity of the ceramic cap, third ceramic has low ESR. You can use the scope to check the AC ripple on the cap to verify that it is filtering properly.
I use either one depend how much room I have when I design the PCB that has to fit into the chassis. T-CON board is always covered with low profile metal cover. Just a thought from designer point of view.

The metal cover may be important for EMI but don't worry about it for everyday use.

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

I usually leave the cover off, the IC's on the board can get very hot, some of them do not even have venting holes, sicne the board is mounted on the top center of teh chassis, all the heat will rise up and flow through the T-CON board to the top vent of the TV. And yes the compliance testing was part of the FCC EMI radiating test. Just had a couple of our products tested for EMI for Class A and B compliance per FCC, expensive testing!

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

And one last thing when work on PCB with a lot of MLCC on the board, DO not flex the board too much or too much pressure on the MLCC.
Learn more here:

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Oh, another tips when try to find out which cap is bad when there are a bunch of them in parallel, I use microscope and look for cracks on the body of the MLCC, when you remove them, you can easily crack the body also and it will be shorted out, so do not reuse the caps you had removed.

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

On some Samsung T-cons I see a heat spreader included between the main processor and the shield. So in some cases the shield might act as a heatsink. Being at the top of the screen isn't ideal. I don't see why it couldn't be at the bottom. It would stay cooler.

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Yes, some of them has big heatsink and it does use the cover to transfer the heat, for I.E.
SONY TV:
http://s807.photobucket.com/albums/y...%20KDL-40XBR4/

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

I soldered in a small electrolytic capacitor (picture) after testing to make sure it would fit; my soldering job wasn't pretty, but it worked (or so I thought).

I watched a short music video on it full screen and figured, since the TCON board was able to handle it, I had fixed the problem. Today I reassembled the case and everything, wiped down the screen, and put it in my room. I then hooked up my old SEGA and attempted to play some Sonic 2, but the screen started flickering. It would go black for a couple of seconds, and then the image would reappear, then it would go black again. It did this for a couple of minutes before going black completely.

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Also, is there any reason why I can't use a much bigger capacitor, tape it down elsewhere, and just run a pair of wires to the traces?

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

That area of the board is the switching power supply (the IC, inductor, diode next to the caps) section, make sure to use low ESR type capacitors, also connect the DC Voltmeter to check the DC voltage on the cap to see if it comes and goes when the pcitures black out.

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Also, is there any reason why I can't use a much bigger capacitor, tape it down elsewhere, and just run a pair of wires to the traces?

Inductance.

Adding series inductance to the circuit may introduce more instability. You will form a resonant LC-tank circuit if you are not careful. The R will damp some energy leading to extra heat dissipated in the cap but most importantly the control loop may be unstable.

This is why I recommended ceramics. Low inductance and ultra low ESR.

You don't need -more- capacitance. You need less ESR. The goal of the capacitor is to kind of act as a sink for all that high frequency noise from the switching action of the MOSFET, and to provide energy to the load during the off-time. It also forms part of the transient absorbing circuit, providing energy during sudden load changes. A capacitor helps do all of these things but in all cases the ESR is effectively in series with the load or FET leading to higher ripple, poorer transient response and lower output voltages.

The ESR of the cap is probably several ohms, so it's effectively useless.

While the TV is working make a note of the output voltage. Then find the largest capacitor in 1206 you can find with the nearest voltage. E.g. if you measure -10.5V, choose a 16V 22uF ceramic in the 1206 package.

[budm]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

By the way, I will not use any long wires for capacitor connection since you want it t o have as low ESR as possible, at high switching frequency the wire is seen as inductor.

[Tetranitrate]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

Is there a way to identify low ESR capacitors by their package type (ESR can only be measured with a special meter right?). If they can be identified by their package types, does it look like there are any of that type in the attached photo (like the aluminum cased smd ones)? All the orange ceramics I have on hand are in the nF range.

Then find the largest capacitor in 1206 you can find with the nearest voltage.

I know I should probably just break down at this point and buy a smd ceramic with known and correct characteristics, but it is just so appealing to think about fixing this problem with some component I heat gunned off a random circuit board years ago with the hope of one day having it find a new home.

Anyone know what type of frequencies that these capacitors are dealing with?

[tom66]

Re: Vizio VW37 Power Supply PSON working but no 12v or 24v output

I know I should probably just break down at this point and buy a smd ceramic with known and correct characteristics, but it is just so appealing to think about fixing this problem with some component I heat gunned off a random circuit board years ago with the hope of one day having it find a new home.

Anyone know what type of frequencies that these capacitors are dealing with?

Roughly 70kHz - 3 MHz, probably around 500kHz.

You could scanvenge one off an old Pentium 4 (or later) computer motherboard; near the 12V DC VRM input (4 pin P4 connector) there are sometimes a couple of 1206-size ceramics.

Or a graphics card, I have an old X1900 which has bad BGAs and plenty of ceramics near the 12V input.