Vizio DPS-167DP-1 failure

[Nr4ps]

I'm trying to troubleshoot a DPS-167DP-1 A power supply board from a Vizio M552i-B2 tv. The tv would no longer turn on, and I found the usual melting on IC901, with D950B and ZD909 shorted and R915 open. Sounds like this is a pretty common failure, so I replaced all four components expecting it to be an easy fix but it's still not working.

I checked D801 and it's fine. I get about 170V DC across the main filter capacitor C800A. I'm not getting any voltage on the 5V standby pins in connector CN101, nor any voltage on the VCC pin of IC901, which is presumably why it's not working -- the datasheet wants 11 to 25V on that pin. Can anyone tell me where that voltage is supposed to be generated? I must have another component failed in there somewhere, but I have no experience with power supplies so don't know where to look.

[nomoresonys]

Maybe check Q800.

[Nr4ps]

Maybe check Q800.

Thanks. I can't see what Q800 is under the heat sink, and I'm not sure what the correct way to test it is, but in-circuit (which I'm sure is not the correct way to test it but hoping to minimize further desoldering and resoldering) with my meter in diode test mode I get 0.63V or 1.93V across the outer legs depending on polarity. Plugging it in, the center leg is at 169V while the other two are 0V.

I'm trying to make sense of the data sheet for IC901. I assumed the VCC pin would power the chip but I think the chip is supposed to actually charge the capacitor on that pin. Attached is a "typical application" 5V supply from the datasheet. I don't know how closely Vizio matches this, but without a schematic of the Vizio board I don't have anything else to go on. Do you think Q800 might be playing the role of IC51 in the diagram? The COMP pin is evidently used to close the loop so I wonder if the IC2 photocoupler could be bad, but I see a bunch of photocouplers on the board. I don't suppose there's an easy way to test them without desoldering? Everything I mentioned in my first post was tested out of circuit, but I'm just trying to avoid pulling every single component off if I can.

[nomoresonys]

Check it in ohm mode, tv unplugged from power as all ohm checking is done, leave it in circuit, test each leg to the other two, checking for a short 0 ohms or low ohms is bad, good ones will probably be in the M and K ohm range, if you don't have an auto ranging meter you may need to adjust the ohms range to get a reading.

[lotas]

Four components were replaced, but they forgot to change the optocoupler (feedback); it, too, could have been damaged if the PWM failed.
What was the PWM (IC901) and which one did you install? Have you checked all the components in the harness of this IC901?

[Nr4ps]

Ah, thanks. Lowest reading between pins of Q800 is 9.6k.

IC901 is LD7913JGM6 and I replaced it with the same.

How to test the photocouplers?

[lotas]

What does the Q800 have to do with it? He's on a different chain!
You can't just check an optocoupler; replace it with a known good one.
Check, replace the electrolytic capacitor with 5 pin. (VCC)

[Nr4ps]

C904B is good. I believe the optocoupler in question is PC902. I don't have any LT816 on hand. Could I substitute LT814 or better to wait to get the right part? Thank you for your help.

[Nr4ps]

I decided to try the 814 with no luck. But I notice that pin 2 on IC901, the brownout pin, is connected by way of R919 (approx 130k) to VCC and R918 (approx 128k) to ground. That pin needs to be above about 1V before the chip will even turn on. And I think that's my problem. If I'm tracing that correctly, something needs to charge C904B to a couple volts before this chip will do anything. I'm trying to figure out where that's supposed to be coming from. Like I said, D902B read good out of circuit. I get 1.2 ohms across R908A, B, and C. From there, it's connected to pin 1 on T901. Is that supposed to be transforming a voltage input from the secondary side on T901 pins 5 and 7? There's no voltage across those two pins, AC or DC.

[nomoresonys]

Don't buy parts you don't need, optos are easy to test: https://www.youtube.com/watch?v=WCu3F73ief0

[Nr4ps]

Don't buy parts you don't need, optos are easy to test: https://www.youtube.com/watch?v=WCu3F73ief0

Thanks! PC902 seems to be in good working order.

Can anyone tell me what all those pins on T901 are for? Is that a multi-tap output to make a variety of voltages?

[nomoresonys]

Get the digits from it and look up the datasheet.

[lotas]

Thanks! PC902 seems to be in good working order.

Can anyone tell me what all those pins on T901 are for? Is that a multi-tap output to make a variety of voltages?

There are three windings, 1 is the power winding, 2 is where the voltage for the PWM power supply (VCC) is generated, 3 is the output for the standby mode (+5v).

[Nr4ps]

The markings on the transformer are below. I couldn't find a data sheet for the transformer, but another badcaps post suggests that the primary is 2~3, The vcc feedback is 1~4 Pins 5~7 are the secondary. Resistances measurements are no more than a few ohms across each pair, so I don't think the transformer is blown but it's not getting any AC voltage on 2 and 3.

MP-130I (01)
AV-DP11153 DELTA
DCUM 1418(06)H60

[nomoresonys]

Get the digits from it and look up the datasheet.

Nevermind, I read it wrong, thought you were still talking about PC902.

[nomoresonys]

So check all the diodes and resistor and that capacitor right by T901.

[Nr4ps]

I can't find anything wrong with any of them. I even pulled most of the diodes out of circuit to make sure it wasn't an illusion that they were working. Capacitors seem fine. Resistors seem fine. Even the tiny resistors on the back of the circuit seem roughly correct.

[nomoresonys]

Where did you get your parts, I suppose you could have received a bad IC901, good practice to test the parts that you can before installation.

[Nr4ps]

Yeah, I was thinking about that... IC901 came from eBay because it's hard to find anywhere else, and it's the one part of the four that I don't know of any way to test. Maybe if I remove it from the board again I can at least see if that somehow allows C904B to charge to a couple volts such that IC901 would start if it were present.

[Nr4ps]

C904B doesn't charge with IC901 removed either.