Vizio VT420M No backlight but seems to turn on

[Nick's Tvs]

I have a Vizio VT420M that seems to turn on but I can't see picture.
I suspect Power supply part number: PWTV9QH1GAC9

The logo at the very bottom goes from dim orange to bright orange when I press the power button, and the backlight never comes on. The flash light test yields no picture.

Using a DMM I got perfect voltages for all pins going to the main. So far it sounds like a bad main, but I wanted to test the PS before ruling it out. Checked all caps, all fuses, and diodes. After my check I found something a little odd on 2 diodes sitting next to eachother.

D9404 was showing voltages passing both ways at 0.210V
D9403 had 0.276V passing the way its supposed to and .590v passing when it should show OL
I measured them out of circuit, and both measured .301v one way, and OL the other way. So the diodes are good. I check the board again, and it looks like the measurement if coming from something else on the board

I buy a PS from Payless components, they have been good to me in the past, and before installing it, measure the diodes on it due to curiosity. I got the exact same result as on my board, I plug the thing in anyways, and I get the same symptoms.
Is this just the way this PS is designed, and my issue is indeed coming from the main board, or is this a common failure on these Power supplies and I just got a bad power board sent to me?

Thanks to everyone that can help me figure out if my main is bad or if my power supply is bad

[budm]

Re: Vizio VT420M No backlight but seems to turn on

Does the connector on the power supply board that goes to the Main board have pin lables on them to tell you which pins are for?
At least it has standby power otherwise you will not see LEDs for the logo to come on.
You need to find the PS-ON pin to see if the main board is sending the ON command signal to turn on the rest of the power supply or not.
Good clear pictures of the whole back side of the TV and the boards?
See basic LCD TV troubleshooting guide below.

"Using a DMM I got perfect voltages for all pins going to the main." What does that means? All the command signals are all OK?
Do the command signals toggle HI and LO when power switch is toggle on and off?

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

budm
Thanks for the reply.

The connector going from the PS to the main is indeed labeled with all voltages and all voltages appeared properly. Pson showed 3.3 volts when tv was on.
How do I know if the main board is sending the turn on command to the power board?
As for pictures I don't have those right now, but I can do that soon.


"Do the command signals toggle HI and LO when power switch is toggle on and off?"
Well I am not sure what that means. The PSON pin stays at a constant 3.3 when the tv is on and doesn't fluctuate if that's what you mean.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

"Pson showed 3.3 volts when tv was on. How do I know if the main board is sending the turn on command to the power board? " PS-ON is the command signal from main board to turn on the main power supply circuits.
PS-ON is normally LOW, when TV is OFF, it goes high and stays HIGH when power switch is activated, this signal is generated by the main board.
This PS-ON is used to turn on the PFC Voltage booster if this power supply has one, and the rest of the power supply.
the main board also sends out the BL-ON (Backlights ON) signal and also the Dimming command signal.
I cannot tell much until I see the pictures.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Attached below are the pictures

[budm]

Re: Vizio VT420M No backlight but seems to turn on

Is this the exact match of your power supply?
http://www.shopjimmy.com/vizio-pwtv9...pply-unit.htm#
The SJ pictures have wires in the way so I cannot read the label of each pin.
Please provide good clear pictures so I can each pin name.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Here are additional pictures

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

The second set of pictures are of the connector going to the main, and of the diodes that are bad. They are the 2 that are screwed into the silver heat sink. I put 2 little blue pen marks above them.
And yes the SJ power board you just linked previously is the same board that I have
http://www.shopjimmy.com/vizio-pwtv9...pply-unit.htm#

[budm]

Re: Vizio VT420M No backlight but seems to turn on

Can you please give clarification to the two bad rectifiers? Are they shorted out?
They are Common Cathode dual rectifier that have two outer pins (Anodes) tied together to form 1 Diode (parallel Diodes).

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Ok so this is how they are set up.
The middle pin is pushed forward and the outside pins are sitting back straight down from the rectifier. The middle pin that is pushed forward is the Cathode, and the outer pins are both anode. -->!-!<-- is as close as I can get with a computer keyboard to mimic the symbol on the circuit board. And actually if you look closely on the picture of the rectifiers, you can actually see the symbol.
On the both rectifiers, I believe both of the anode pins are shorted together.
Now what I am getting that is strange is that when I put my black lead on cathode and red on anode, I am getting 0.210V on the DMM and when I switch my leads to black lead on anode and red lead on cathode, I am still getting the same exact reading of 0.210V when normally I think I should be getting OL. This is for the D9404 rectifier.
Now on the D9403 rectifier, I am getting 0.276V when I have black on cathode and red on anode, and I am getting .590v with red on cathode and black on anode. I believe I should be getting an OL on one of those no??

[budm]

Re: Vizio VT420M No backlight but seems to turn on

There are bleeder resistors in the circuits.

So are you getting 12VDC and 24VDC at the connector CN9102?
If you look at the bottom side of the board, you will see lots of bleeder resistors.
If the Diode is shorted, then it should show <1 Ohms when test them in Ohm mode.
The 12Vdc and the 24Vdc, and the PFC Voltage booster are turned on by the PS-ON signal from the main board.

Post 1 "Using a DMM I got perfect voltages for all pins going to the main." Then that cannot be true if those two diodes are shorted.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

Please give me the DCV readings of each pin of CN9102 with the name of each pin and the Voltage reading when TV is OFF and when TV is ON.
For example:
O/F TV OFF = xxx V, TV ON = xxx V
PDM TV OFF = xXX V, TV ON = xxx V
..........
..........

I need to see the readings to verify the function of the main board.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

I will get the exact readings for you in a second
but I do believe I am getting the 0 volts across the board when the tv is off except for the 5 volt line, and then when turned on I do believe I get 24v for the 24v line, 3.3 for the ps on, 11.94 for the 12 and still 5 volt for the 5VDC.

But I am going to double check all the above information one more time to make sure, but I do believe everything is accurate.
Now keep in mind, as stated in the original post, I took the resistors out of circuit, and they both tested .301v with black on cathode and red on anode, and OL the other way with black on anode and red on cathode. The solder joints on the back side of the board them selves still showed similar numbers of 0.210V going both ways and what ever number I posted earlier for the other, and this is with the rectifiers taken out. This ties in with the original question, the diodes themselves are not bad, however is the board set up that way and the numbers I am getting are correct, or is this indeed a problem, and the board should not be acting like that?

[budm]

Re: Vizio VT420M No backlight but seems to turn on

You are seeing the bleeder resistors, that is why it is showing conduction in both directions.

"I took the resistors out of circuit" So you removed all 9 SMD bleeder resistors? and the caps are discharged and the main board ( the load) is not connected?
What do you get in Ohm mode?

[budm]

Re: Vizio VT420M No backlight but seems to turn on

BTW, what do you get at the O/F (BL-ON) pin and PWM pin?

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Ok, so I made a big mistake, I have been working on 2 different Vizio tvs with the exact same symptoms, but one is a 32 inch. I may have crossed over some of my data from that 32 inch onto the 42 inch we are currently discussing.
All the info I did write on the previous post is correct, except for the voltages that I took of the connector.

So I am getting with tv off:
5v from 5vdc
0v from PS_ON
0v from 24vdc
0v from 12vdc
70 Millivolts from PDIM
0 from ON/off

I am getting with tv turned on:
5.17v from 5vdc
5.05v from PS_ON
0V from 24 vdc
1.1v from 12vdc
and I wrote it down somewhere at the shop but I am getting I think 3.3 from PDIM or on/off not sure. I wrote it down when at my work station, but i can't find the paper and I don't have the tv in front of me right now.

But regardless of those last 2 voltages, I think I am fairly confident that we have a bad power supply now and that indeed the spare one that was sent to me was also bad.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Also I made a mistake earlier in my post I stated I took out the resistors from the board, but I actually meant to say I took out the diode / rectifier. Not sure why I said resistors. That is my bad.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

So you do not have 12V or 24V after all, but I thought you do have all the Voltages as posted in post 1.
OK, the ps-on is present but no 12V or 24V which both them comes from the same circuit and same power transformer and this circuit requires boosted Voltage from the PFC Voltage booster to run, same for the inverter circuit.
So you need to find out if you do have >360VDC between the two legs of main filter caps or not.
So you get the new power supply board for $29 or try to fix it.

[Nick's Tvs]

Re: Vizio VT420M No backlight but seems to turn on

Yeah its so cheap, at this point I think I am going to get a new one from SJ
I just wanted to make sure that it was indeed the problem, and the the main board was not causing the problem. I just made a couple mistakes by working on several vizio tvs at the same time and confusing symptoms.
budm, thank you so much for helping me out through everything. This is my first time getting a diagnostic from someone on the website, and I have learned a lot.

1: Post pictures of connectors, boards up close, and overview.
2: more accurate description of exact voltages coming out of what pin when both on and off
3: review posts prior to posting so you don't say you took out resistors when in fact you took out diodes. haha.

Once again thanks for your patience and working through with me.

[budm]

Re: Vizio VT420M No backlight but seems to turn on

OK, just make sure to do the follow-up on the result. Good luck with the new board, I was surprised it is that cheap.