Vizio VO47LFHDTV10A - No Backlight

[dskall]

Re: Vizio VO47LFHDTV10A - No Backlight

Here is an earlier post from 2015 https://www.badcaps.net/forum/showth...t=45134&page=2 Found bad caps on inverter boards causing voltage problems

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

Check Fusible resistor R847 to se if it shows high resistance, I believe it should be < 2 Ohms, you can check it on the bottom side of the board.
Also check C610, see if the body is bulging out on its side, also check its capacitance.
BTW, you do have PS-ON when TV is turned on and also have >380VDC between the two legs of the main filter cap in the primary side?
Basically you need to have PS-ON present to turn on the PFC Voltage booster, PS-ON also turns on the 24V power supply but without boosted Voltage there will be no 24VDC.

[Derek76757]

Re: Vizio VO47LFHDTV10A - No Backlight

Thanks budm, I will check this and see what I find out. When you say to check and see if I have less than 380VDC between the two legs, do I need to put both the positive and negative leads from my multimeter on them, or do I put the negative lead on the chassis ground? Thanks

[Derek76757]

Re: Vizio VO47LFHDTV10A - No Backlight

Also, when you say the main filter cap in the primary side, is there a number for that on the board?

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

C819 large 450vdc lytics cap under the heatsink.

[dskall]

Re: Vizio VO47LFHDTV10A - No Backlight

On power boards anything on hot side the ground is referenced to neg. side of main cap lead or neg terminal of bridge rectifier

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

Hi all,

My brother's VO47LFHDTV10A tanked a couple of weeks ago and I've been troubleshooting it since.

My observations:

1. Audio, but no backlight.
2. Did the flashlight test: there is a picture, but I can't see it because there's no backlight.
3. Changed out all of the 680uF 35V caps on the master and slave inverter boards... No improvement.
4. Changed out all of the transistors and ICs on the inverter boards and the backlight flickered on for a minute or two, then disappeared and never came back.
5. With the inverters disconnected from the power supply, all the voltages to the inverters (24V) are present, but not when connected... A short somewhere?
6. Voltages going to the other boards from the power supply are all present.
7. Checked the fuses on both inverter boards... They seem ok: 0 ohms.

Seems obvious that there's a problem with the inverters, but I changed out the transistors, ICs, and caps and there was no improvement aside from that momentary flicker.

Any ideas or suggestions?

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

New observations:

1. I determined that the short circuit is on the slave inverter board, not the master.
2. Since it's not conclusive to determine that one or more of the FDD8447L MOSFETs is shorted by checking them on the board, I painstakingly removed them one by one until the short was gone.
3. Turns out three of the MOSFETs were shorted, so I replaced them.
4. Unfortunately, that didn't solve the problem: once I powered up the TV, I got a flickering picture for a moment, then it was gone and the slave had a short again... Probably the MOSFETs again... Grrr.
5. So apparently there's something else that's bad on the slave inverter, causing the MOSFETs to fail, but what?
6. I'd have to think it might be one or both of those transformers on the board... A test would be to remove both transformers, replace the failed MOSFETs, and power up the board again and see if there's another short circuit... Good thing those MOSFETs aren't terribly expensive...
7. If it is one or both of those transformers, I don't know how I could possibly find replacements...

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

But did you replace the IC at the same time? if the IC is bad then it will cause the MOSFETs to be on at the same time and cause the MOSFETs to short out.

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

But did you replace the IC at the same time? if the IC is bad then it will cause the MOSFETs to be on at the same time and cause the MOSFETs to short out.

Thanks for your reply.

I did replace the Rohm BD9887FS IC the first time around on my initial post on 1/28/18... The backlight flickered and buzzed intermittently for a few seconds and then it was gone for good.

Would/should the backlight work with just the master inverter connected? I did try that, but no backlight, either.

The voltages appears to be OK on the master: 24VDC at the caps, ~3VDC on the ON/OFF pin 12... Not sure what voltage I should see at the 2-pin connector to the CCFLs.

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

OK, I re-read your post again so all the MOSFETs and ICs for both boards have been replaced.
Can we see good clear pictures of the boards? Are you 100% sure that there is no solder bridges or bad solder works on the IC pins?

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

OK, I re-read your post again so all the MOSFETs and ICs for both boards have been replaced.
Can we see good clear pictures of the boards? Are you 100% sure that there is no solder bridges or bad solder works on the IC pins?

When I get home, I'll take some close ups of each board.

I'm pretty sure that there are no solder bridges on the IC, but I'll double check.

I removed all of the MOSFETs from the slave board, trying to determine which ones shorted out... They were Q106 and Q108. I'll need to get some more before I can do any further testing.

I suppose the BD9887FS IC that I replaced could be bad, but I'm not sure how I can determine than other than replacing it again.

There's a thin ribbon cable that plugs into the inverter board... A control cable? Could that possibly be causing the IC to not work properly?

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

Here are the photos of the slave and master inverter boards.

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

The flat cable that link between the Master and Slave board is for syncing the Mater and Salve inverters because they are driving the lamps in PUSH-PULL setup.
The 4 MOSFETs for driving the primary side of the Inveter transformer are set up a full bridge drive and only two of them should be turned on at the time otherwise you will have MOSFET blown up if more than two are turned on at the same time. I always replace all 4 at the same time.

https://cdn.badcaps-static.com/pdfs/...5628d3ac52.pdf

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

The flat cable that link between the Master and Slave board is for syncing the Mater and Salve inverters because they are driving the lamps in PUSH-PULL setup.The 4 MOSFETs for driving the primary side of the Inveter transformer are set up a full bridge drive and only two of them should be turned on at the time otherwise you will have MOSFET blown up if more than two are turned on at the same time. I always replace all 4 at the same time.

So if the master and slave don't sync, that would cause the slave to short out?

Is there a way to determine if the master is functioning correctly and not causing the slave to fail?

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

So if the master and slave don't sync, that would cause the slave to short out?

Is there a way to determine if the master is functioning correctly and not causing the slave to fail?

No, the MOSFETs are driven by the IC not directly by the Master, if they are not synced then the lamps will not light up properly, if the Slave board is bad then the lamps will on be on half the length of the tubes, the dead half will be the one with the bad Salve inverter.

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

No, the MOSFETs are driven by the IC not directly by the Master, if they are not synced then the lamps will not light up properly, if the Slave board is bad then the lamps will on be on half the length of the tubes, the dead half will be the one with the bad Salve inverter.

Both times after I changed out the failed MOSFETs on the Slave, the backlight flickered on for a moment then died... Is that a true indication the the IC is not functioning properly, causing the MOSFETs to fail or possibly some other issue with the various tiny surface mount diodes and transistors sprinkled throughout the board?

Not sure how much it would help, but it would be nice if there was a schematic for this freaking thing.

I guess I'll have to replace that IC with another one just to see if that makes a difference, though I'd hate to damage another couple of MOSFETs to find out.

[budm]

Re: Vizio VO47LFHDTV10A - No Backlight

So you have 8 MOSFETs per inverter board.
http://www.shopjimmy.com/lg-philips-...repair-kit.htm
http://www.shopjimmy.com/lg-6632l-03...verter-kit.htm
4 MOSFETS per one transformer, so you have some shorted MOSFETs for each transformer on the SLAVE inverter board?

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

So you have 8 MOSFETs per inverter board.
http://www.shopjimmy.com/lg-philips-...repair-kit.htm
http://www.shopjimmy.com/lg-6632l-03...verter-kit.htm

Yup, I ordered that repair kit a couple of days ago.

I had purchased a similar kit earlier from eBay and replaced all those MOSFETS and ICs on both boards previously... And replaced all the 680uF caps before that.

4 MOSFETS per one transformer, so you have some shorted MOSFETs for each transformer on the SLAVE inverter board?

The two MOSFETs that were shorted (Drain to Source) were Q106 and Q108, located on the left side of the Slave board (see the pics I posted yesterday).

I checked the other six MOSFETs that I removed and they're not shorted.

[bmwracer]

Re: Vizio VO47LFHDTV10A - No Backlight

Some encouraging progress:

1. On the Slave inverter, I replaced the IC, reinstalled the good MOSFETs, and replaced the two MOSFETs that shorted out.
2. Now half of the backlight works, the side that's driven by the Master inverter.
3. One thing though: one of the transformers on the Master inverter is buzzing... A loose winding, maybe?
4. The backlight side driven by the Slave is still dead, but at least nothing on the Slave inverter has shorted out... So far. *fingers crossed*
5. Is it possible that one or more of the "good" MOSFETs that I reinstalled on the Slave inverter isn't really good, but not shorted, either?