Vizio M261VP Powers On Has Sound But No Video

[jimg1948]

I have a Vizio M261VP 26" LED/LCD HDTV that will turn on/off and has sound but no video. The LED backlight lights bright and then dims to about half the brightness after 5-10 seconds. There is no video and I can not see the menu when selected.

The TV is powered from a 19Vdc AC/DC Adapter that checks ok.

There are basically only 2 boards in this tv. See attached pictures. The main board part number is 3626-0082-0150(3E)). It contains all power supply circuitry (including for the LED Backlight), audio circuits, video circuits, and all external input ports. The second board is the "t-con" display board. It looks like Vizio uses an LG display assembly part number 6870S-1103A. The "t-con" display board seems to be and integral part of the LG display assembly. There is also a small wireless daughter board for wireless connection to a network.

I checked the 3 obvious fuses, and I have checked the voltages at various points on the main board/display board and they appear to be normal. I looked for swollen caps and any other damage, but everything looks ok.

There is a 10 pin ribbon cable on the upper right of the main board that connects to the upper right of the LCD. I think this is the LED backlight cable. Voltages in the upper right area are around 108Vdc on the Power MOFET and Schottky Diode. Should I assume the backlight is ok since it turns on with the power? Is the dimming normal after 10 seconds?

From the symptoms is there any clue where the problem may be i.e. main board or t-con display board? Is there any test that can isolate it to which board has the problem and where on the board to look?

Any help on where to go from here would be appreciated.

[Agent24]

Re: Vizio M261VP Powers On Has Sound But No Video

The TV is powered from a 19Vdc AC/DC Adapter that checks ok.

How did you verify it? If you simply powered it on and checked output with a voltmeter that may not be enough. You should really load it to a normal working load and check voltage then, also ripple if you can.

Failing that, swap it for another one that is known good.

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

Pretty sure the power adapter is ok because it powers up the tv and I measure the 19v with the TV on. This problem is somewhere in the video processing. Either on the main board or the display board. I checked the ac ripple as you suggested and none was detected.

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

Thought I would post update on diagnosing no video on M261VP. Since audio and other functionality of the main board seem to be working ok I decided to focus my attention on the T-con board. The T-con board is getting the 12Vdc source voltage ok from the main board. The board has additional power supply components to get the proper voltages to drive the LCD display. 2 chips appear to be doing most of the power conversion the first is a TPS62110 17-V, 1.5-A, SYNCHRONOUS STEP-DOWN CONVERTER and the second is a Maxim 17113E (US1) Low-Cost, Multiple-Output Power Supply for LCD TVs. I checked the voltages on the TPS62110 (US2) and found that the output is zero. I was not sure why so I decided to do a diode check on the various diodes in the circuit. One Schottky Diode marked B340A (D4) connected to the input of the maxim chip measured zero ohms in both directions indicating a short. I guess this is a typical failure mode for Schottky diodes. Not sure whether the shorted diode is causing the 0 volts output on the TPS62110. I will replace the diode and see if the voltage returns to the correct levels. I will provide updates once I have replaced the diode.

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

I thought I would post a progress update on diagnosing no video problem in case anyone has the same problem in the future. I removed what I thought was a shorted Schottky diode from the video display board power supply. To my surprise The short was still in the circuit. the diode checked ok. So after more diagnoses I determined bypass capacitor C71 on the display board (t-con) was causing the problem. Removed the capacitor from the circuit and the short disappeared. Reassembled the set without the bypass capacitor and I now get video. The video however is flickering and producing some black horizontal lines. The missing capacitor may be causing to much ripple in the power supply or there may be another problem yet to be discovered.

[Agent24]

Re: Vizio M261VP Powers On Has Sound But No Video

It sounds like you're getting along pretty well with it! What type of capacitor is C71, ceramic?

[budm]

Re: Vizio M261VP Powers On Has Sound But No Video

Is the cap in parallel with the diode for snubber function? it sounds like one of those MLCC which will fail in shorted mode.

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

I think it is a ceramic. It is an 0805 size SMD but is unmarked.

This capacitor according to the MAX17113 datasheet is to minimize ripple. It looks like a value from 10-22uf would work ok. The higher the capacitance the lower the ripple. ESR also contributes to the ripple and that has to be less than 43 milliohm. It is connected to the output side of a 10uH coil and ground. There are a couple of other caps downstream to ground so I did not think it would hurt to run the tv without the shorted cap until I could buy one to replace. I will have to measure the ripple to see if that is causing my flicker problem.

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

I checked the voltages on the display board. The following is what I found.

Vcc = 2.75V no ripple (this is where bad cap was removed)
Vgh = 25.7V
Vgl = -7.5V
DPV = 2.5V
POL = 1.4V
SOE = 0.2V
VcomL = 4.0-5.5V
VcomR = 4.0-5.5V
VcomRFB = 4.0-5.5V
VcomLFB = 4.0-5.5V

Vcc seems a little low. I expected around 3.3V. It is still within the operating range of the chips that use the voltage.

The 1.5V swing on the Vcom voltages was not expected. From my understanding of Vcom, it is used to prevent memory or image retention problems on the LCD panel and give the LCD longer life. This contributes to image retention, commonly known as a sticking image. Flicker can result if the Vcom voltage is not centered exactly around the video signal. I read that they use Digitally Controlled Pots to adjust the Vcom voltages on sections of the LCD to be exactly half of the video swing voltage to eliminate flicker. The video swing voltage is typically 0-10V so a 5V Vcom is about right.

Three chips are used to generate the Vcom voltages a BUF08832 Programmable Gamma-Voltage Generator and High Slew Rate VCOM with Integrated Two-Bank Memory, M24C08 is an 8-Kbit I2C-compatible EEPROM and a MAX9651 High-Current VCOM Drive Op Amp.

Can anyone tell me if a 1.5V Vcom voltage swing is typical or should it be a more stable voltage?

[tom66]

Re: Vizio M261VP Powers On Has Sound But No Video

Definitely ought to be more stable - can you locate AVDD to see if it is measuring around 2 x VCOM?

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

I find a VDD =15.2V and a HVDD = 7.5V. Is HVDD the same as AVDD?

Where would I find AVDD?

[tom66]

Re: Vizio M261VP Powers On Has Sound But No Video

VDD is AVDD on this, that is fine. I am guessing HVDD means 1/2 VDD.
VCOM should be close to HVDD, from what I've read, not certain though. Still, poor VCOM would usually only cause a distorted/flickering image...

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

Thanks

I found that the Vs (Pin 19) connection to the BUF08832 chip also exhibited unstable voltage (around 11.0-15.2V). I was able to trace that back to the MAX17113 power supply chip on the positive-charge pump regulator circuit (pin 2 DRVP). This pin is connected to a capacitor and dual diode. One side of the dual diode produces the AVDD 15.2V voltage and the other side the VGH 25.2V voltage. Both voltages are intermittently unstable. I found if I lay the TV flat on its face the voltages stabilize to 15.2V AVDD and 25.2V VGH. When I lift the TV on edge I see the bottom 25% of the screen become unstable. Twisting the screen a little I can get horizontal black lines to appear. Twisting some more and the bottom 1/4 of the screen disappears. Lay the TV back down and the voltages stabilize and the screen appears normal.

Does this sound like I have a bad ribbon cable connection to the LCD Panel? Would a bad ribbon cable connection cause the AVDD and VGH voltages to become unstable? If it is the ribbon cable connection I assume the TV is toast. Could it be an insecure solder on the display board somewhere?

[jimg1948]

Re: Vizio M261VP Powers On Has Sound But No Video

Success of sorts

I noticed if I pressed on the display board the voltages would stabilize and the abnormalities in the video would disappear. I made a clamp and attached it under an LCD mount screw to hold the board tight. TV has run for a couple of hours without any problems. Can not guarantee how long it is going to last but at least it is ok for now.