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

There are a total of four opto-couplers (one is further up on the board), and they all face the same way (which I guess means the high voltage side can only receive signals but not send them, correct?). I have attached a hand drawn schematic that I am still in the process of completing.

[tom66]

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

There are a total of four opto-couplers (one is further up on the board), and they all face the same way (which I guess means the high voltage side can only receive signals but not send them, correct?). I have attached a hand drawn schematic that I am still in the process of completing.

Yep, that seems right.

If the 2N7002 has a gate-source short it could be the problem. Check that.

Q302 makes little/no sense in its configuration with a ground on both collector and emitter. Also, there needs to be a pull-up for the base.

[Tetranitrate]

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

99.7k ohms between gate and source on 2n7002.

I do not see a ground on the collector side of the Q302, do you mean the arrow I drew going up? That is a connection that goes to some components further away from this section of the board.

[tom66]

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

99.7k ohms between gate and source on 2n7002.

I do not see a ground on the collector side of the Q302, do you mean the arrow I drew going up? That is a connection that goes to some components further away from this section of the board.

I meant the emitter. I don't understand why there would be 5x330 ohm in parallel (=66 ohms) and then an NPN which may or may not short them out. Doesn't make much sense to me, but I didn't design this circuit.

So the optocoupler is IC303. Can you try bridging the two pins on the primary, and seeing if the PSU turns on?

[Tetranitrate]

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

IC303 is one of the opto-couplers, and the only one that I have gotten up to so far in my drawing of the schematic.

Is bridging the two contacts on the primary side of the opto-coupler necessary, since I know that I can turn on the power supply with a pair of AA's?

Also, the Q302 is a PNP transistor not an NPN.

I turned the power board on (while disconnected from everything), and took some voltage measurements on both the secondary and primary side of IC303. The voltage on the secondary side is 1.2v and the voltage on the primary is .112 volts.

When the AA batteries are disconnected (PSON low), the voltage on the secondary side is .202v and the voltage on the primary side is 15.92 volts.

[tom66]

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

Okay, skip that opto check for now. It seems to be working. The big test will be with the low 3.184V PSON from the main board. What voltage does that induce on the primary of the optocoupler?

[Tetranitrate]

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

The thing is when the PSON line is disconnected between the main board and the power board (but the rest of the lines are connected, e.g. the main board is still receiving the 5v from the power board) the main board will correctly output 3.3 volts. It is only when the PSON line is connected between the two that the voltage drops. The AA batteries are also not a perfect 3.3v but actually 3.2 volts.

This makes me think that the PSON line voltage being slightly low is a result of the whole system force-shutting itself down to prevent damage.

Budm suggested before that there may be a short on the inverter board and it automatically senses the excess current draw and shuts itself down. If I knew where the safety portions of the circuit were (main board, power board, or inverter board?), I might be able to do some testing on those portions to see if it reveals anything.

[Tetranitrate]

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

I know I have said this about half a dozen times before in this thread, but I feel like I am closing in on the final problem.

When the J7 line is disconnected (shown in the attached block diagram), the power supply will work correctly using the power button. There must be some problem (most likely short) on the far side of that connection that is telling the power supply to shut down.

The J7 line disappears into the next layer in the TV, so I will need to pull off some of the large metal support structure to see what it goes to.

[tom66]

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

There will be a current sense resistor on the primary, connected from one end of the transformer to the ground of the cap. Same for PFC and standby. For PFC it will be connected between mosfet source and ground.

[Tetranitrate]

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

I just took the TV apart even more to find the board that J7 was connected to. It is shown in the two attached pictures.

In the first picture the J7 line is shown connected to the main board. To take the second photo I pulled out a few dozen more screws and cut my hands up on the metal framework, but was able to expose the display board underneath.

Since the power supply would work normally with the J7 line disconnected, that means that if there was a problem it would be on the far side of the J7 line (on the display board) - is this correct? I was a little disappointed with how small and insignificant everything on the display board seems. I was hoping to see some burn marks or bulging caps, and not just a few chips and really thin traces. Am I looking in the right place? Would a short on this board really draw enough current to cause the power supply to do a safety shut down?

[Tetranitrate]

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

Upon further inspection of the display board, I saw a chip that looks like it has some damage on the bottom left side (two attached photos).

It is an Atmel 2-wire serial EEPROM (attached datasheet).

Does anyone know how I can test this chip?

[Tetranitrate]

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

I just tested the inverter board and the backlight (with the janky setup shown in the attached photo) and they both work. The power supply will switch on correctly using the power button as long as the J7 line is not attached.

So I now feel 99.9% sure that the issue definitively resides on the display board. There is a short or something else that is drawing too much current and causing the power supply to shut down. I was able to find a datasheet (attached) for the display board. It shows that the Vcc is 12v (and not the 5v I was initially thinking), which makes the idea of a short pulling too much current seem more plausible.

I feel like I am on the homestretch of getting to the root cause.

I have gone over all the diodes I can see on the display board with my multimeter, and they all seemed to work. The fuse at the 12v input is working. There is no visible indication of anything that may be at fault, except for the small amount of exposed metal on the Atmel chip (shown in a previously attached picture).

-What is the best way to find out where a faulty short may be on the board?

-If the issue was the Atmel EEPROM chip, would I even be able to replace that? Has anyone ever heard of a company selling already programmed chips like that for doing refurbish work, or would I have to buy a whole new board?

It would really suck to have done 100+ hours of troubleshooting only to find that, even though I tracked down the issue on a component level, I would still have to do a board replacement.

[retiredcaps]

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

Upon further inspection of the display board, I saw a chip that looks like it has some damage on the bottom left side (two attached photos).

If you are referring to the grayish part, I believe that is either a felt marker or spray paint from the factory to denote that the part is an eeprom where the user settings or code is stored. It is normal and I can't see any physical defects from the photos.

[tom66]

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

Looks like you've found the short.

This is a T-con board.

It has a lot of ceramic capacitors on it; and as I've discovered on a 17" LCD, these can fail shorted.

Several tests to perform:

green/blue: Diode test this device. It *may* be an open circuit; in its current position it looks like a TVS. (ESD/EOS protection.)

yellow: test the fuse (I don't expect this is blown)

red/black: test these caps (polarity unimportant) should read open circuit on ohms (don't worry about capacitance)

This board is available from ShopJimmy for $50, should the EEPROM be at fault: http://www.shopjimmy.com/auo-55-37t0...-con-board.htm It's also available from the UK for £24.99 ($40), I'm willing to ship that to you if you want.

I note that the picture looks different, but it might be a newer revision. Oh, and the ShopJimmy pic clearly shows a marked EEPROM so I doubt that's a fault if that board is tested/working.

EDIT: Found an exact substitute from China: http://www.aliexpress.com/fm-store/9...70XW02-V5.html. $53.42 including shipping (EMS.)

[Tetranitrate]

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

If you are referring to the grayish part, I believe that is either a felt marker or spray paint from the factory to denote that the part is an eeprom

Oh, and the ShopJimmy pic clearly shows a marked EEPROM so I doubt that's a fault if that board is tested/working.

That is great news, because I figured if it was a burnt out EEPROM it would be impossible to get a new one with the same code on it (unless the code is just customized system settings input by the user). Also the silver felt marker thing makes sense for a few reasons. The first is that the chip is a plastic case and would not have burnt out to reveal a shiny metal interior (I should have realized this sooner ). The other is that the there is a bit of the silk screened code visible under the paint. The last is I did a continuity test and nothing was internally shorted.

Test results:

Green/Blue: With the multimeter anode probe on green there is .115v drop with the anode on blue there is a .5v drop.

Yellow: The fuse is good.

Black/red: The capacitors have 293 ohms resistance across them. You are saying that it should be open? Does that mean that one (or multiple) of these little smd caps has internally failed short? What would be the best way of verifying this?

This board is available from ShopJimmy for $50

I was able to find it stocked a bunch of places, the cheapest being ebay for about $40 + 5 shipping. I am glad I now have the option of just buying a board and being done with the whole thing (I am lucky that the cheapest of board failed; the inverter board, power board, or main board failing would have set me back much more); however, I would still really like to solve this thing at the component level.

I have spent so much time troubleshooting that it would feel like giving up if I did not see it through to the bitter end (e.g. identifying the exact component/point of failure).

[retiredcaps]

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

I have spent so much time troubleshooting that it would feel like giving up if I did not see it through to the bitter end (e.g. identifying the exact component/point of failure).

I agree and since you are doing an EE, I think this real world lab test is helpful for your studies.

Having said that, momaka is written several excellent responses in regards to tcon board testing. This is a sample.

https://www.badcaps.net/forum/showpo...5&postcount=18

PS. I have a similar problem with a short on a main board causing the SMPS to shutdown and after many months of putting it away and then inspired by momaka's shorted capacitor thread I *think* I may have a shorted capacitor (it measures 2 ohms), but waiting for supplies before I can verify.

[tom66]

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

Okay, check all the brown caps. All of them. And the diodes. 245 ohms is fine; it's around 20mA, so probably just the substrate of one of the ICs. If one of the brown caps is failed on the output to a boost converter this won't show with a normal test across the 5V and ground because the diode won't be biased but it will show when 5V is applied.

These boards draw around 1.5-2A at 5V to drive the LCD panel and failures are fairly common.

I didn't say this, but when I was locating a short on a board I eventually gave up and applied 3.3V straight to it through a supply limited to 500mA. (One of my projects... not a TV board.) The component that was failed got warm enough to find but didn't melt because it was current limited. You could try with a series power resistor and a power supply. 5V with a 10 ohm 1W resistor in series (100mA) is enough for a component to get warm if it is shorted, but not enough to melt anything.

[Tetranitrate]

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

Okay, check all the brown caps.

All the caps look brown, although some look more beige than others. Is there a slight color difference I should be looking for, or were you just saying to test all the caps on the board?

Is there any relation between the caps size and its likelihood of failing? Should I work my way from larger to smaller / smaller to larger, or does it not matter?


I had previously went and checked every diode I could see and they all seemed to be within working range.

These boards draw around 1.5-2A at 5V to drive the LCD panel and failures are fairly common.

This board actually has a Vcc of 12v (don't know if that makes a difference, but I just thought I would mention it).

[retiredcaps]

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

All the caps look brown, although some look more beige than others. Is there a slight color difference I should be looking for, or were you just saying to test all the caps on the board?

I personally would test them all. It should not take more than 15 minutes.

On my board with the shorted SMD ceramic cap, it measures 1.2 ohms (I just re-measured it). On my Fluke 12, which can measure up to 10,000uF, it shows as 0L. A good cap on the same board gives a valid reading.

Of course, none of these types of caps have any markings so when I go to replace that shorted cap, I'm going to take replace it size for size off a junk board.

[Tetranitrate]

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

So I found two capacitors relatively close to the Vin that both have a resistance of .5 ohms. The first photo shows their position relative to where the previously shown portion of this board, and the second photo shows them a bit more clearly. The bottom part of the caps is connected to the ground plane.

If these are the failed components (or some of them at least), how will I ever be able to know what their intended values were so that I can order replacements?