Sanyo DP42849 - Backlight Not Turning On

[wcsd106]

Hi again,

Regarding: Sanyo DP42849 LCD TV

After fighting with that DPS-260LP Power supply I had started a thread regarding for a couple of days, I opted to order a replacement power supply board as I was not making any headway on getting even the 5v standby voltage.

To make a long story short, I ordered a supposedly good power supply board, and after installing it the TV turned on and functioned correctly with one exception: The backlight would not turn on. (Killing the room lights and shining a flashlight at the lcd screen allowed me to verify that the movie was indeed being displayed on the lcd)

I returned the board and ordered a replacement from a different vendor. It arrived today and after installing, I still have no backlight.

I have checked my voltages at the 4 pin connector on going from the main board to the power supply (see pics attached)

STB 5v: 5v
BL_ON: 2.46v
BL_DIM: 3.3v


I'm going to tear it down to where I can inspect the bulbs. I read in another thread here that some tvs have ballast capacitors near the bulbs.

Any other things I can look for?

[CapLeaker]

Re: Sanyo DP42849 - Backlight Not Turning On

I am not sure which version you have but for this TV are few backlight inverter boards, including a Master/Slave inverters. Maybe worth checking the caps and fuses on there. Your main board is sending BLon, so make sure to also check the power going to the inverter.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

Thanks, Capleaker.

Best I can tell though, the lamps are driven straight from the main board. I've disassembled the lcd panel to check for bad ballast caps. I have attached pictures of the panel to this post.

I don't see any obvious signs, but since my BK meter doesn't read capacitance below about 50nf, I don't have a good way to test them either. There are some darker areas near the ends of the ccfl tubes, but should I really expect that each bulb has gone out?

As far as testing inverter output, should I be testing the primary and secondary sides of the transformer right before the connectors that go to the backlights? If so, what voltages should I be looking for there?

I don't discount that it could be a problem with the inverter section of the board, but thought it was unlikely that two boards could have the same problem.

[budm]

Re: Sanyo DP42849 - Backlight Not Turning On

You look for resistance leakage on those ballast caps, the cap should have infinite resistance.
Bad one will usually has brown tint to it.
http://s807.photobucket.com/user/bud...?sort=3&page=1
Do not try to measure the High Voltage on the output side of the Inverter transformer unless you want to blow up your meter, the startup Voltage is about 2000V.
Look and see if the all the lamps light up with the same intensity, see if the end of the lamp has blackened.
http://s807.photobucket.com/user/bud...?sort=3&page=1

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

Thanks, bud. Testing now.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

All ballast caps are reading infinite when tested for resistance leak. At this point, would I be good to reassemble the LCD panel? If it is indeed the bulbs, it would probably make this TV not economical to repair anyway.

[budm]

Re: Sanyo DP42849 - Backlight Not Turning On

you can leave the LCD glass panel out, no T-CON borrd connection needed so you can see how all the lamps light up, may be you can see which one does not light up properly.
The lamps are also socketed so the lamps may have bad connection to the sockets.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

These lamps actually appear to be soldered onto the pc board. Best I can tell, they're not socketed.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

So at this point, I should probably reassemble but leave the lcd glass panel out so that I can get a good look at the bulbs while turning the tv on?

[CapLeaker]

Re: Sanyo DP42849 - Backlight Not Turning On

yes

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

OK. Getting ready to do just that. Will report back with my findings.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

Update:

I assembled the panel and installed only the main board & power supply / inverter board. I left the TCON board disconnected and left the lcd panel and glass setting aside on the bench.


Approximately 5 seconds after hitting the power button on the TV, all of the CCFL tubes turn on and immediately turn back off. They all light, but they only stay on for a fraction of a second. It's more of a single flash. Then they go back out and do not come back on.

I'll be doing some searching, but would appreciate any advice on where to go from here.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

Found this thread: https://www.badcaps.net/forum/showth...hlight=dp42849

Looks like the same problem I'm having. Unfortunately, the OP quit before he got it resolved.

To save some time, I have tested: R322 (The Fusible resistor) with a reading of 0.33 Ohms.

I also tried connecting the BL_ON to the 5VSB at CN6 through a 1k resistor. Still only get a flash of the backlight.

[CapLeaker]

Re: Sanyo DP42849 - Backlight Not Turning On

look at the caps and transformers on the inverter board. Compare one transformer to the other.

[wcsd106]

Re: Sanyo DP42849 - Backlight Not Turning On

I got an email response from the place I bought the board from. Their tech was nice enough to send a few emails back and forth with me. He stated that the board was tested in a working TV prior to being sold. He offered to exchange the board for another that he has, so I don't think he's really out to rip me off.

He did mention, however, that the Sanyo TVs were known for having problems with the CCFL bulbs themselves. According to him, a bad bulb can cause the single flash problem I'm experiencing.

If it's the bulbs, this TV is not worth putting any more time or money into. The bulbs are indeed soldered to their respective boards and would be interesting to try and replace if I could even find the replacement bulbs.

That being said, I would appreciate any advice as to how to proceed... Should I accept his offer to exchange the board, or assume that since two supposedly good boards have failed to light the ccfl, that there is a problem with the bulbs themselves?

Would there be any way to simulate the bulbs for a short period of time using a large resistor network in order to test the output of the inverter?

[johnboy1313]

Re: Sanyo DP42849 - Backlight Not Turning On

I once repaired a DP42848 with a bad bulb. The bulb would light, but had a spot that was glowing red at the end. It was difficult to notice the failure due to how quickly the lights went out. I was able to get the lights to stay lit for a little longer by unplugging a four wire connection marked K8L on the main board. Perhaps you have the same connection. Otherwise, use a camera set to burst mode, or one that will let you take several pictures in a short amount of time. Try to photograph the lit bulbs and then examine the picture closely for any odd looking spots in one or more of the bulbs.

[CapLeaker]

Re: Sanyo DP42849 - Backlight Not Turning On

I'd go for the board... but first make sure that the solder connection is good on the ccfl... maybe resolder those first anyway.

[coreyand]

Re: Sanyo DP42849 - Backlight Not Turning On

I have the same model tv with the same issue only I dont think I have any problems with my power supply/inverter board. When I shine a flashlight on the screen I get nothing but when I jump a wire from the 5vstb to the BL_On the backlight on the screen comes on for about a second or 2 then goes back off.

All voltages from the power supply look good and in their respective ranges. I suspect something is wrong with the main board. Doesnt the 3v for the BL_On come from the main board? Also when I jump that wire from the standby to the bl_on there is nothing on the screen and it is completely blank, even when pushing the menu button or any other button prior to putting the jumper in place. I would hate for the mainboard to be out because even as old as this tv is the mainboard is about $150.

The person I am fixing this tv for I dont think wants to spend, or has that kind of money for a repair of replacing the mainboard. I suspect a failed IC on the mainboard somewhere but I have not had a chance to trace it back from the BL_on pin just yet.

I thought I would get any ideas or suggestions from you all before proceeding further with the symptoms this tv has.

Thanks, you guys have all saved me in a bind in the past and its been about a year since I have done any tv repairs as I got out of the scene for a minute but I am back in it now and I love this forum and the support you all give. I contribute when I can also.

[CapLeaker]

Re: Sanyo DP42849 - Backlight Not Turning On

The BLon and PSon does come from the main board. So if you have either both, or one of them not going, that's not good. You also have a second problem with the back light turning off, that would have to be investigated.

I suppose you could check the service manual on the main board and trace around. Most of the time it turns out to be the main IC, in which you are gonna be SOL unless you got skills and proper soldering equipment.

Try this: disconnect everything from the PSU but the backlight. Take two 1k resistors and jump STBY to PSon and andotherone from STBY to BLon. When you plug the TV in, the backlight should be on and all the voltage rails on the PSU. If the backlight doesn't stay lit, there is a inverter problem too.

[coreyand]

Re: Sanyo DP42849 - Backlight Not Turning On

Is there anything I can use in place of the 1k resistor? I have none on hand at the moment. I dont think, I might need to do some digging. Isnt the color pattern on that brown, black, red, yellow?