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

Are there any buck converters on the board that might get the 5V standby?

U6 is a buck converter, right? U5 is the same chip - AP1513.

I have attached an image of the back of the main board. There are no chips on the back of the main board, just a few SMD inductors and caps.

Should I work backwards from the PSON pin on J1? There are a pair of BJT NPN transistors (MMBT3904L) directly connected to J1. Would my best best of finding the problem be to follow the traces leading into the PSON pin?

[tom66]

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

Yes that's a buck converter. Is U5 working or not?

Re- tracing PSON - I would try that. I wish I could help more...

[Tetranitrate]

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

The first thing I hit that J1 is connected to is R2, whose value matches the expected value from the service manual (560 ohms). There are then 2 MMBT3904L general purpose transistors. What would be the best way to check if they are functioning properly?

[Tetranitrate]

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

I traced PSON from the the "start" at the jumper pin location to the the "end" at the main chip on the board (MediaTek MT5371). I have attached a hand drawn schematic. If you cant read something on it let me know.

The PSON is connected from the jumper pin to a pretty basic NPN Darlington pair, and from there is connected to MT5371.

All the resistors are within range of their expected values.

What readings should I start looking for?

The service manual has a few pages on the MT5371 which I am currently going through. I will report if I find anything indicating where the base of Q26 is connected, or what value it should be at.

[Tetranitrate]

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

I just took voltage readings at the various nodes. An updated circuit schematic is provided.

I am hoping that these voltage values will instantly set off some alarms for someone reading this. I will have to dig out my old Electronics 2 notebook and will attempt to analyze the currents in the meantime.

The 3.305v at the MT5371 makes me feel confident that I have isolated the problem to this very small portion of the circuit.

So, ideas anyone?

[tom66]

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

The circuit doesn't make much sense; Q26 pulls the base of Q1 low when Q26 is switched on, but what about when it is off? The base has no current to ever turn it on.

Is PSON naturally high when disconnected from the main board? Otherwise, where does the 3.3V come from?

[PlainBill]

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

The circuit doesn't make much sense; Q26 pulls the base of Q1 low when Q26 is switched on, but what about when it is off? The base has no current to ever turn it on.

Is PSON naturally high when disconnected from the main board? Otherwise, where does the 3.3V come from?

Good points. Time for more troubleshooting. Adding a pull up resistor may be the solution.

PlainBill

[tom66]

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

The only thing I can think of is *maybe* it's a prebiased transistor with an internal resistor. If so, I can't see any way that circuit would have ever worked.

Maybe the 3.3V comes from the PSU, and this part is faulty?

Try tracing the PSON on the power supply - look especially for pull-up resistors.

[Tetranitrate]

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

I have updated the photos and schematics to include (what I think are) Vcc's now. I have also attached a simplified schematic (just showing the relevant voltage nodes) with voltage measurements for when the system is turned "off".

There are two through hole traces that I am pretty sure must be the Vcc's for the transistors. It looks like tom66 is right that when Q26 receives the 3.305v from the MT5371 it pulls the base of Q1 low. Pulling the Q1 base low pulls up the voltage at Collector 1 from .014v to 3.184v. Is this correct?

I have been having a real hard time finding out what the Vcc's are connected to though. I have swept a whole bunch of the board using the continuity measurement on my multimeter, but have not found any connections. I have been thinking of making a test probe device out of a metal wire brush soldered that I could use to check for continuity on large portions of the board, and then from there further localize it. Thoughts?

[tom66]

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

No; there must be a resistor there, not just a trace to Vcc, otherwise the transistor will short the rail when it turns on (and it will melt.)

[Tetranitrate]

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

Is it possible that the pull up resistor is located at a different spot on the board? I have no idea what the traces I labeled as VCC are connected to. Could there be the pull up resistor at the exit hole and then the connection to VCC?

[tom66]

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

Is it possible that the pull up resistor is located at a different spot on the board? I have no idea what the traces I labeled as VCC are connected to. Could there be the pull up resistor at the exit hole and then the connection to VCC?

Yes, but it would be rather strange. Is there any continuity between that node and the 3.3V or 5V rails?

[Tetranitrate]

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

Actually that may be the reason the exit hole is so hard to find. If my input to that mystery hole were connected directly to a voltage supply, I figure it would be relatively easy to find since it would probably be coming off a chip / covering a large area / connected to multiple things; however, if the pullup resistor were at the exit to the mystery hole, there would only be a really small area (one side of an SMD resistor) that would register as showing continuity.

How does this logic/reasoning sound? Is there anything I should try before extensively attempting to find continuity to the two mystery holes? Which mystery connection should I be more concerned with? I am thinking the one that is connected to Q1, since it is the one closer to the incorrect low voltage. Any general areas of the board I should look to try and find the exit to this hole?

[tom66]

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

I'm beginning to think you've got a prebiased transistor, but there could be some resistors under the board, or elsewhere on the board. It's possible the signal is used elsewhere i.e. to turn on/off the buck converters, so placement of the resistors elsewhere could be a layout decision.

One way to check if this circuit is working is to remove or lift one leg of the resistor from the MLTxxxx to the base, and inject a signal into it via a resistor of similar value. It should be possible to get it to vary from 0-3.3V.

[Tetranitrate]

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

That is pretty delicate surface mount work. It is doable, but I would rather try and not do something like that for diagnostics unless it is a last resort, since there would be a high chance of me messing something up.

We know that when the base of Q1 is pulled low, the collector voltage goes high. Base 1 goes from .689v to .013v when the TV is turned on, and this makes Collector 1 go from .014v to 3.184v. The base of Q1 is connected to ground through R4 - a 9.89k ohm resistor. What if I were to add another resistor of similar or lower value in parallel with R4 to make the base of Q1 go even lower. If I were to add another 9.89k ohm resistor in parallel with R4, the voltage should theoretically drop by a half - making the base voltage of Q1 be around .006v, then if the voltage at Collector 1 rises at all, we will know that the problem resided with Q26(or its associated resistors).

If the voltage at Collector 1 does not rise, even with its base value being halved, then we will know that the issue has to do with whatever VCC is supplying Q1 not being high enough. The only problem I see with this test is putting twice as much current through the collector of Q26, in order to half the voltage at Base1.

The only other thing I can think to do would be to somehow find out whatever is connected to the mystery hole that is providing voltage to Q1, and seeing if I can diagnose anything from that end. If I were to do that would there be a method to finding out where it is connected, or just do random continuity tests on everything on the board?

All the help everyone has provided me has been hugely appreciated.

[tom66]

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

Okay, hold off removing the resistor.

I don't think 0.014 vs. 0.006 is going to matter to the transistor. And 3.184 is enough to saturate the transistor, provided the base resistor is not too high.

Try tracing that hole - it is going to involve random probing unfortunately, unless you have a schematic.

There is also the possibility that some component on the power supply is dragging down this voltage, although it would be a rather strange failure.

If you get tired trying to fix that circuit, you could always wire up a pair of logic inverters on a little piece of stripboard to replace them. Make sure the inverters support a ~1.5V DC input.

[tom66]

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

Okay, I just remembered a 42" LG plasma I had, with a somewhat similar problem. It came in with bad caps. When plugged in, the relay would click, it would start outputting the right voltages, the screen would flicker, but then it would shut down. And it would repeat this cycle every few seconds.

I replaced the caps... no luck...

Even replaced the main board and AV board... no luck...

It turned out to be the AC detect! D'oh, I should have checked that... but it was a little signal coming FROM the power supply to the main board, which was going low when it started up, which was causing the main board to shut the PSU down.

It looks like there are another few transistors on the other side of the connector; maybe there's an AC detect signal here?

The purpose of AC detect is to shut down the speakers and main processor before power fails, to prevent any glitches on power failure (like strange patterns on video, weird sounds.) Older tube TVs used to flash when powered off, due to the supply voltages dropping out.

[Tetranitrate]

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

If there were an AC detect would it have to be a connection between the power supply and the main board?

I have attached a list of all the connections between the mainboard and the power supply, and it does not seem like there is an ac detect signal. There are only three connections besides the power rails and the ground pins:

POWRSW_ON/OFF
PWM_DIM
BL ON/OFF

The POWRSW_ON/OFF is the one that I am having a problem with. I assume PWM_DIM is a brightness control for the inverter board, and BL ON/OFF is for turning the back light on or off.

[tom66]

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

Hmm, okay discount that theory then - clearly there's no AC detect. Except, are the A and B wires connected to anything power wise?

I suggest looking into wiring up a circuit using a CD4069 to replace the transistors.

[Tetranitrate]

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

The "A wire" is connected to pin 12, which corresponds to PWM_DIM, and the "B wire" is connected to pin 13 which corresponds to BL_ON/OFF. I am not sure if that is what you were asking.

I have a handful of things I need to test later when I get the chance, and will post results, but in the meantime should anything be added to this list or should these tests be done in a particular order (whatever is most likely to be helpful first?)

• Trace out the mystery hole connected to the collector of Q1. This is going to be rather tedious. I have already done a quick swipe of various connections on the board looking for continuity and have not found any. I will need to come up with a system and device to test large areas of the board at once. Hopefully one of those large areas will give me a continuity beep, and then I will be able to further "zone in" on the continuity portion from there.
• Disconnect the pson connection from the power board to the main board, but leave the rest of the connections untouched. While turned on, measure the voltage at the PSON line on the mainboard to see if it goes to 3.3v - if this were the case, it means that there is voltage leaking on the power supply board.
• Take some more measurements while the PSON rail is artificially (from a pair of AA batteries) being raised to 3.3v. I have done this in the past, and saw that it correctly turned on the power supply lines (12v and 24v); however that was all that I tested when I artificially drove them high. I should see if I can get the TV to full working condition, or if not at least what I can get working. Will the back-light turn on, and if it does not, what is the voltage at the BL ON/OFF pin. Will the sound work, etc. Is there anything else I should check for while artificially activating the 12 and 24 volt lines?
• Build a test inverter pair to see if Q1 and Q26 are functioning properly. Is this still worth doing? If .014 vs .006 wouldn't make a difference to the base of Q1, then can't it be assumed that the collector of Q1 is correctly going as high as possible? If it is going as high as possible and the max voltage possible that is being output at the collector is 3.184, then wouldn't the issue reside somewhere after the transistors - e.g. whatever is supplying the VCC through the mystery hole is not high enough, or the voltage is leaking out through the power board somewhere.

Questions, comments, suggestions?