Vizio VX32L HDTV10A - No backligh

[jarl]

Hello again,

I have been enjoying the TV and the monitor PlainBill and other helped me to fix some time ago... but I needed some more to do... so I have two new challenges (which, of course, I need help with)

The subject of this post is a Vizio VX32L HDTV10A LCD TV I got from Craigslist. The TV turns on but there's no backlighting. Shining a flashlight at an angle you can see the "no signal" banner.

Right after opening the chassis I connected the TV to the mains and turning it on to measure the voltages at the power supply (which, BTW, seem to be Ok), the TV actually worked for a few minutes, but after a while it started flickering every few seconds and eventually went dark again. I have been unable to reproduce this.

What I have done so far: I checked the caps at the power supply and they look fine. Voltages there look fine as well. The caps on the main board look fine too (although there are *lots* of them, several of which are not of the "blow" type), and on the inverter board there are just 6 large(r) electrolytics that look fine as well. So, what should be next in the list?

I was hoping this would be just a matter of quickly replacing a few caps and that's it (yeah...right ) It might as well be like that, but I don't like the shotgun approach at changing parts, so I was hoping the local gurus could offer me some help. Please?

Thanks!

[PlainBill]

Re: Vizio VX32L HDTV10A - No backlight

Hello again,

I have been enjoying the TV and the monitor PlainBill and other helped me to fix some time ago... but I needed some more to do... so I have two new challenges (which, of course, I need help with)

The subject of this post is a Vizio VX32L HDTV10A LCD TV I got from Craigslist. The TV turns on but there's no backlighting. Shining a flashlight at an angle you can see the "no signal" banner.

Right after opening the chassis I connected the TV to the mains and turning it on to measure the voltages at the power supply (which, BTW, seem to be Ok), the TV actually worked for a few minutes, but after a while it started flickering every few seconds and eventually went dark again. I have been unable to reproduce this.

What I have done so far: I checked the caps at the power supply and they look fine. Voltages there look fine as well. The caps on the main board look fine too (although there are *lots* of them, several of which are not of the "blow" type), and on the inverter board there are just 6 large(r) electrolytics that look fine as well. So, what should be next in the list?

I was hoping this would be just a matter of quickly replacing a few caps and that's it (yeah...right ) It might as well be like that, but I don't like the shotgun approach at changing parts, so I was hoping the local gurus could offer me some help. Please?

Thanks!

Familiar story. Odds are the problem is in the lower left corner of the last picture. To say that Darfon has a less than stellar reputation is like suggestion BP had a minor leakage problem.

You did a good job on the pictures, but the detail isn't good enough if I am going to have to give precise instructions. It looks like there might be a legend at the right end of the connector in the lower left corner. In any case, those 4 dots are test points for On/Off and brightness. If you can identify On/Off, verify it goes to the On state when the TV is turned on and stays there.

The odds are overwhelming that the problem is one of the inverter transformers. Check the continuity between the pins on the transformers. The secondary should have a resistance in the range of 1K, the primaries would be under 10 ohms. Compare the resistances of the secondaries of all transformers. They should be the same to within 5%.

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Hello PlainBill,

Glad to have you here

I measured the transformers, but the primaries seem to be in parallel or something like that. The resistance for all of them register as being 0.4~0.6 ohms, but that's just too close to the lower accuracy limit on my digital multimeter (i.e. I found my testpoints are just crap).
All the secondaries are close to 1.5 Kohm (do I need to measure this with the lamps disconnected, btw?)

I'm adding the picture of the connector's surrounding area... I'm not sure which of those pins you are referring to (and there's one marked PWM... my personal nightmare)

[retiredcaps]

Re: Vizio VX32L HDTV10A - No backligh

I measured the transformers, but the primaries seem to be in parallel or something like that. The resistance for all of them register as being 0.4~0.6 ohms, but that's just too close to the lower accuracy limit on my digital multimeter (i.e. I found my testpoints are just crap).
All the secondaries are close to 1.5 Kohm (do I need to measure this with the lamps disconnected, btw?)

I like this video on how to measure transformers "Testing for Faulty Inverter Transformers Tutorial"

http://www.youtube.com/watch?v=eNuGWBPRGKA

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Uhm... that answers my questions. However, I would rather not disconnect the transformers for testing purposes... at least not yet. The pins on theses ones are more crowded than the ones on the video. Is there any test that can be done (easily) to check for the presence of the high voltage output? (one that can be survived, I mean )

Going back to PlainBill suggestions, I'm guessing the pin labeled BLON is the backlight-on signal. If that's the case, it shows 2.64 volts when the TV is on. The power supply doesn't say how much should it be, though. What should I be measuring there?

On a side note, while looking for information about the inverter board (Darfon V070-W01) I found a message sent by someone looking for a replacement IC for this board. The ICs (there are 8 of them) are Niko-sem P2804ND5G (dual FET). Any suggestion on how to test these? Some insight on why there are just 8 of these, but 14 transformers?

[Dgtech]

Re: Vizio VX32L HDTV10A - No backligh

Datasheet - one N-channel and one P-channel in the same package.

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Thank you Dgtech... unfortunately I'm not familiar with this kind of circuit, so I'm not sure what the signal should be on each pin. I used my (crappy) oscilloscope to test the signals and they look the same on all the 8 ICs, though...

When I put the oscilloscope on pin #1 of the transformers, though, I found something interesting (see images). The first image shows the signal on each of the 14 #1 pins... and one of these looks *very* different from the others (they are ordered by rows, so the first two images correspond to the two transformers for lamp #1, the next two to lamp #2 and so on). The 2nd image shows the signal for #3, #4 and #5...

So, I think I know where the problem is located... But I don't know yet what the problem might be

Edit: I just realized that the amplitude of the signal on the 1st transformer of the 2nd lamp (T3) is larger than any other... some crosstalk between T3 & T4, maybe?

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Elaborating on the previous images:
Image 1: pin #1 for the two transformers on each of the three first lamps
Image 2: what I think are the two pins for the primary of the transformers one to four

I guess I'll have to desolder T4 after all ?

What exactly is it that the board detects that makes it shut down the lamps?

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Hello PlainBill,

Glad to have you here

I measured the transformers, but the primaries seem to be in parallel or something like that. The resistance for all of them register as being 0.4~0.6 ohms, but that's just too close to the lower accuracy limit on my digital multimeter (i.e. I found my testpoints are just crap).
All the secondaries are close to 1.5 Kohm (do I need to measure this with the lamps disconnected, btw?)

I'm adding the picture of the connector's surrounding area... I'm not sure which of those pins you are referring to (and there's one marked PWM... my personal nightmare)

I hadn't looked at the inverter carefully. All the primaries are in parallel, don't worry about them.

One likely cause is shorted turns in a secondary. It's not necessary to disconnect the lamps - they won't conduct at the voltage of your DMM.

I don't think I was emphatic enough. Measure all the secondaries, write down the numbers. Compare them. What is the difference between the lowest resistance and the next lowest? The first time I tried this I found the readings were all about the same - until I wrote them down. Then I noticed they ranged from 934 to 984. However, when I ignored the 934, they ranged from 955 to 984. Using a ring tester I found that the 934 was shorted.

Now, that's the easy fix. If you look at the components around the lower end of each primary, these measure the current through each CCFL. One of those might be the cause, too. If you don't notice any anomalies with the transformer secondaries, what is the part number of the 20 pin IC?

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Hello PlainBill,

Sorry I didn't post the numbers (ordered by resistence):
T# KOhm
10 1.465
4 1.471
6 1.472
9 1.473
5 1.476
7 1.478
12 1.478
8 1.483
13 1.484
1 1.493
2 1.494
11 1.494
14 1.494
3 1.509

I think the difference is around 3% or so...

The 20 pin IC is an O2Micro OZ964SN... looks like an interesting device

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Hello PlainBill,

Sorry I didn't post the numbers (ordered by resistence):
T# KOhm
10 1.465
4 1.471
6 1.472
9 1.473
5 1.476
7 1.478
12 1.478
8 1.483
13 1.484
1 1.493
2 1.494
11 1.494
14 1.494
3 1.509

I think the difference is around 3% or so...

The 20 pin IC is an O2Micro OZ964SN... looks like an interesting device

The only one with an extraordinary difference is transformer 3. And I'd expect a shorted secondary to result in lower resistance, not higher.

In the meantime, I forgot the primary rule - check the fuse!! The fact that it worked once does indicate the fuse is PROBABLY good.

I'll be looking at the OZ964 datasheet later. Perhaps there is a test we (that means you) can perform that will indicate the nature of the problem.

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Thanks again Plainbill
Yep... the fuse is ok Allright... it *was* Ok... I blew it when measuring something on the PS. I just put another one

Do the oscilloscope traces I posted yesterday hint at anything? It seems evident (I know... nothing's really evident when talking about these things...) the problem is related with the #4 transformer, but the resistence seems to be Ok. So... ? Maybe the circuit that senses the condition of the lamp might be bad? I dunno

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Thanks again Plainbill
Yep... the fuse is ok Allright... it *was* Ok... I blew it when measuring something on the PS. I just put another one

Do the oscilloscope traces I posted yesterday hint at anything? It seems evident (I know... nothing's really evident when talking about these things...) the problem is related with the #4 transformer, but the resistence seems to be Ok. So... ? Maybe the circuit that senses the condition of the lamp might be bad? I dunno

I'm sorry!!!! I didn't even notice those waveforms. That certainly illustrates the problem.

I see several different approaches. If this is like other Darfon inverters, the transformers are surface mounted. My preferred approach would be to remove T4 and test it with a ring tester. Removal is easy if you use Chip-Quik.

There are also descriptions of how to improvise a ring tester using an oscilloscope and a square wave generator - or even the scope's calibration signal. Since this works best with a reference, remove T4 and T5, then compare them. Or simply swap them.

Heck, if you have a square wave generator, try applying a square wave to the secondary of each transformer (with the CCFL disconnected). Observe the waveform on the scope and you will quickly establish if the problem is with T4, or an external component.

If (when) you establish the transformer is the problem, Google the part number. The prices on these transformers vary over a wide range from different sources. When I needed one I discovered what 'LCD Outlet Parts' was selling for $27, 'Electronica' was selling for $12.

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Interesting...

Can you elaborate on "try applying a square wave to the secondary of each transformer (with the CCFL disconnected). Observe the waveform on the scope and you will quickly establish if the problem is with T4". What should I look for?

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Interesting...

Can you elaborate on "try applying a square wave to the secondary of each transformer (with the CCFL disconnected). Observe the waveform on the scope and you will quickly establish if the problem is with T4". What should I look for?

OK, you asked for it. The following drawing gives a rough idea of what to look for. If you feed a square wave into the windings of a transformer the transition will result in a series of oscillations. The number of oscillations is an indication of the integrity of the transformer. By comparing a known good transformer (T5) with a suspect one (T4) you can determine if the problem is the transformer or the load. On a good transformer I would expect 5 or more rings. On a transformer with shorted turns I would expect 1 or 2.

Negative comments about my artistic abilities will be ignored. As will the critic.

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

So, we can add "line-drawing artist" to your signature?
Life is catching up with me (or vice-versa) so the TV will probably have to wait until the weekend. I'll keep you posted

THX!

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Hello again

I finally got to test your suggestion, and I'm afraid I managed to misunderstand the instructions

If I understand correctly, the idea is to connect the output of a signal generator (are the amplitude/frequency critical in any manner?) to the two terminals on the secondary of the transformer (there are just two, right?) AND the oscilloscope probe and probe ground to the same two terminals?

Assuming the problem is with T4, I tried to locate a replacement, but I can't seem to find the right parts.The only marks on the transformers read "6502G 74AA07 (GP) 04". Who makes these? More importantly: where can I get a replacement part?

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Hello again

I finally got to test your suggestion, and I'm afraid I managed to misunderstand the instructions

If I understand correctly, the idea is to connect the output of a signal generator (are the amplitude/frequency critical in any manner?) to the two terminals on the secondary of the transformer (there are just two, right?) AND the oscilloscope probe and probe ground to the same two terminals?

Assuming the problem is with T4, I tried to locate a replacement, but I can't seem to find the right parts.The only marks on the transformers read "6502G 74AA07 (GP) 04". Who makes these? More importantly: where can I get a replacement part?

First of all, my instructions may have been incomplete. Toasty was kind enough to provide this link.

Second, you appear to have a V0708.501 inverter. There are a number of them for sale on eBay. What I would suggest is Googleing various parts of the numbers from the transformer. Some of the numbers are date codes. Many of the transformers for Darfon inverters are available at fairly reasonable prices. Or buy a good inverter, salvage the good transformers from your inverter and sell them on eBay for $15 each.

PlainBill

[jarl]

Re: Vizio VX32L HDTV10A - No backligh

Uhm... I think the test has been a failure on my part
I'm using a 2.2uF, 250V capacitor (if I'm reading the markings correctly). My oscilloscope has a built-in function generator, and I have set it for a square wave 18V peak to peak, at different frequencies from 100Hz to 10KHz. However, when I connect the leads to what I think are the two terminals of the secondary nothing happens... the trace on the oscilloscope stays the same (the amplitude may get a bit smaller, but just barely). And this is testing what I think is a good transformer (T1)

I may end up buying the board as you recommended... But it would be nice to know what on earth am I doing wrong

[PlainBill]

Re: Vizio VX32L HDTV10A - No backligh

Uhm... I think the test has been a failure on my part
I'm using a 2.2uF, 250V capacitor (if I'm reading the markings correctly). My oscilloscope has a built-in function generator, and I have set it for a square wave 18V peak to peak, at different frequencies from 100Hz to 10KHz. However, when I connect the leads to what I think are the two terminals of the secondary nothing happens... the trace on the oscilloscope stays the same (the amplitude may get a bit smaller, but just barely). And this is testing what I think is a good transformer (T1)

I may end up buying the board as you recommended... But it would be nice to know what on earth am I doing wrong

Would you believe - I don't know for sure. I seem to remember someone (Toasty?) suggesting using a low voltage signal, but I've always relied on my Sencore ring tester. I keep trying to find the time / energy to try the oscilloscope experiment on a transformer that has one good and one bad winding. So far other projects are more interesting. If you are unable to locate a transformer, it would appear your only choice is to buy a complete inverter.

PlainBill