Vizio VT470M, no display, components seem OK

[Damfino]

A couple months ago my VT470M decided to just stop working. I replaced it with a new TV then began looking into repairing it to mount it in another room. The symptoms are that the screen itself never comes on, but the Vizio light at the bottom does change brightness normally when you turn it on and off. I don't see any dim pixels at all.

I took it apart and based on my initial research, it sounded like an inverter board problem, so I bought a new one from ShopJimmy. "CMO 27-D029966 Backlight Inverter" When taking the TV apart before I ordered the inverter, I had taken out the old one to examine it, put it back in, and the TV began working again. It kept working as I replaced the back cover of the TV... until I put the last screw in! It was then back to its initial problem. When I installed the new inverter, it made no difference at all.

I've checked the +5, +12, and +24v leads on the power supply and they all seem fine. The caps all appear to be undamaged. I'm OK with buying a new power supply if I had any confidence that it would fix the problem, but with good voltages, I am hesitant to throw more money at it.

Any ideas what else could be causing this problem?

Thanks!

[Fwrothy]

Re: Vizio VT470M, no display, components seem OK

The first step: https://www.badcaps.net/forum/showthread.php?t=39740

[Damfino]

Re: Vizio VT470M, no display, components seem OK

The first step: https://www.badcaps.net/forum/showthread.php?t=39740

There's not much to see, that's why I didn't bother originally. Anyway, here's the whole TV, the power supply, and the inverter that was replaced.

[budm]

Re: Vizio VT470M, no display, components seem OK

"There's not much to see" may not much to you but for us we have no idea which version of the boards are used in your TV, we do not have the TV in front of us.
When when the screen goes black, is the 24V still preset at the inverter fuses?
How about the BL-ON and Dim control signals at the inveter board.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

Just for clarification, the screen is always black. I haven't been able to get any picture since it failed except for a brief time after removing and re-adding the original inverter (not the new replacement.)

I only see two fuses on the board, one large one (F2, 12a/125v) by the cable (as shown in the photo), and a small one (1v, 63v) about 2/3rd of the way to the right on the bottom. Both read 24.3v on both sides.

On the inverter board, where the cable comes in:
The 5 VBL lines all read 24.3v.
Status and E_PWM are 0v. (Understand as there's no wire coming to them!)
I_PWM is 3.88v.
BLON is 0.1v.

Down at the CM11 connect, DM_OUT, SEN_CB, and DM_IN are all 0v.

I checked the other end of the cable from the power supply and got the same numbers.

I tried the original inverter and have the same numbers.

The BLON reading seems a bit low - I checked it while turning the TV on and it just goes from 0v (off) to a steady 0.1v (on). A little research seems to indicate that it should be switching between 0 and 5v when on.

It looks like probably a power supply issue?

[budm]

Re: Vizio VT470M, no display, components seem OK

"BLON is 0.1v." is bad, >2.5V = ON, <1V = OFF , it is generated by the main board, so that is where the problem is.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

"BLON is 0.1v." is bad, >2.5V = ON, <1V = OFF , it is generated by the main board, so that is where the problem is.

By main board, you mean the main power supply?

[budm]

Re: Vizio VT470M, no display, components seem OK

By main board, you mean the main power supply?

No, main board is the board where the tuner and the input jacks are.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

Yuck. Not only is it the most expensive part, it's apparently pretty hard to find. Is this probably likely caused by just a bad cap, or something more serious? I'm all for trying to repair the board before just replacing it.

[budm]

Re: Vizio VT470M, no display, components seem OK

You can try forcing on the the BL-ON pin by using 500~1K resistor between BL-ON pin of the inverter board and the 5VSYBY, but you MUST cut or disconnect the wire first because we do not want to back feed the forced on signal back to the main board, then see if the TV will have pictures and all other functions are still OK or not.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

Did you mean 5v STBY? I can't find any clues as to what SYBY could mean.

Just for checking, I cut the BL_ON wire going to the inverter, soldered on a 1k resistor, and connected it to an old 5V cell phone plug. In this configuration, the backlight comes on fine but the TV is completely non-functional - the normal blue screen doesn't appear and none of the buttons (other than power) do anything. That certainly lends further credence to the main board being the problem...... maybe I need to start testing caps, or just hunt around more to see if I can find a replacement.

[budm]

Re: Vizio VT470M, no display, components seem OK

"Did you mean 5v STBY?" Yes, sorry, typo.
So the bottom line is you do have bad main board.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

Yup. OK, thanks for your help.

[Damfino]

Re: Vizio VT470M, no display, components seem OK

To wrap this up, I finally found a main board (thanks to eBay alerts), and the TV is back working now. Thanks again!