Samsung LE40m87BD ... no backlight

[Porto]

Hello,

Here I have this nice LCD television which from one day to the next gave no backlight anymore. The led in the on/off button flashes 5 times and then nothing at all. Relais clicks as usual when I push the button to power it on / off. Looked at it with a flashlight, I'm seeing the TV menu (no source connected) so the screen is there (so, T-con board OK?!) but no backlight, not even a flash. When I turn on the TV and let it stand for awhile, he switches off so I guess something is going hot and goes in shutdown. Also, if it stays a while in standby mode, it switches on, so I don't really know what is going on exactly. My priority is to get the BL back on and then to the auto shutdown/activate issue.

I have changed the four caps on the right top side of the powerboard, these are 2x 1000uF 25v and 2x 2200uF 10v ... kept the capacitance but changed the 10v ones to 25v. The originals where Samyoung brand and were not bulged or damaged in any way but changed them anyway because others had these four caps bulged when having problems with their display. The large filter cap is doing some 320v when in standby mode and 390v when powered on, so I guess that's OK and also means the PFC booster is working (I guess).

I already measured the most common voltages on the powerboard which is a power/combi board with modelnr: IP-231135A / BN44-00165B (yes, not an 'A' on the end, so it's another revision than other folks has so the schematics don't match completely).

All these voltages were taken with the TV running.

CN801, (top right, right side), 24 pins, comes from main board):

01. (pwr on/off) = 2.18v
02. (NC)
03. (stb5.2v) = 5.99v
04~06 (GND)
07~08 (12V) = 12.04v
09~12 (GND)
13~16 (5.4v) = 5.55v
17~18 (GND)
19 (13v) = 13,54v
20 (GND)
21~22 (13v) = 13,54v
23~24 (NC) = 0v

As you can see are ALL voltages accounted for, I only don't know what voltage pin 01 (power on/off) would be normally and pin 03 (standby 5.2v) is nearly 6v, a bit too high but maybe that's normal. Also, the 13v pins are also 0.5v higher.


CNI803, (top right, top row), 5 pins, comes from main board:

01. (Inverter on/off) = 5,35v
02. (DIM) = 3,45v
03. (NC) = 5.8v (??)
04. (GND)
05. (DET_5v) = 5,58v

pin 01 goes to 5,35v when the LED has flashed 5 times and an image is displayed but the BL remains off. I'm guessing this Inverter pin is the same as the BLON pin on some other Samsung TV's so the BL is getting its 'turn-on' signal from the main board.


CNI806, (top right, left side), 4 pins, going to inverter balancer board with modelnr: SSB400HA20V.

01. (VCC_12v) = 12.04v (must drop when BL goes lit... doesn't now).
02. (FB) = 0v
03. (GND)
04. (NC) = 0v
05. (LD) = 10.58v


I don't know if 'FB' has to show a voltage, what's it for anyway? And what is the 'LD' pin for? I can't seem to find the 24v for the BL, where do I have to measure for it?

The four fuses on the powerboard are OK. The inverter balancer board don't have a fuse on it. The two power mosfets next to the inverter transformer are OK.

If more (detailed) pics are needed I'll post them.

[budm]

Re: Samsung LE40m87BD ... no backlight

You do have correct signal, the FB pin is the AC feedback for regulating the backlights.
At this point the inverter is not putting out the high Voltage even though you do have all the correct DcV and the command signals.
I will have to at the inverter circuits next after dinner.
BTW, all the red round fuses on that power supply-inverter board are all OK?

[Porto]

Re: Samsung LE40m87BD ... no backlight

Hi Budm,

Yes, the fuses are OK. Measured them at their corresponding testpoints on the board (Fuse_CP) and they are measuring around 290v.

[budm]

Re: Samsung LE40m87BD ... no backlight

Hi Budm,

Yes, the fuses are OK. Measured them at their corresponding testpoints on the board (Fuse_CP) and they are measuring around 290v.

Is that the reading with black probe of the meter connected to the negative leg of the main filter cap which is the GND ref for the DC primary circuit?
If you are getting only 290VDC then then the PFC Voltage booster is not working, please recheck the DCV right between the two legs of the main filter cap, it should be close to 400VDC.
May be typo, 390V is the correct number, right?

http://www.shopjimmy.com/samsung-bn4...pply-unit.htm#
Pictures provided by shopjimmy.

[budm]

Re: Samsung LE40m87BD ... no backlight

Can you read the P/N of one of the inverter circuit 8-PIN DIP IC UI802S? FAN7382? This IC drives the Gate drive transformer TI802S which drive the two Power MOSFETS MI801s, MI802S which then drive the Inverter transformer.
So far the A version SCH seems to be pretty close as fas as the circuit topology and the parts designators in the inverter section.

[Porto]

Re: Samsung LE40m87BD ... no backlight

I rechecked, the voltage on the large filter cap is 390v when TV is powered on.
The voltage on the two red fuses is also 390v.... I measured it wrong the first time, I used the housing as ground... OK for the secundary side but not the primary.

That UI802S IC is a FAN7382VH36.

Only pins 1 and 8 have 12v, the other pins range from 0 to 0.15v.

[budm]

Re: Samsung LE40m87BD ... no backlight

OK, so the driver IC does have VCC to run but not putting anything out to drive that Gate drive transformer.
So if we use SCH of rev A, you should check the DCV of IC UI807 pin 16 for 5VDC, and >2.5VDC on the pin 9 ENable pin.
I am hoping that the SCH will be close enough to use.

[chrismurphy]

Re: Samsung LE40m87BD ... no backlight

It think you are going about this the wrong way
If you can see the menu using a torch ,then all is probably ok with the power supply
Measure the back-light voltage , if OK i suspect that you have failed leds

[vinceroger69]

Re: Samsung LE40m87BD ... no backlight

It think you are going about this the wrong way
If you can see the menu using a torch ,then all is probably ok with the power supply
Measure the back-light voltage , if OK i suspect that you have failed leds

Is this a led set i thought it was a ccfl type as has the inverter transformes etc.

[tw2005]

Re: Samsung LE40m87BD ... no backlight

Looks CCFL, it's 2007 era.

[Porto]

Re: Samsung LE40m87BD ... no backlight

OK, so the driver IC does have VCC to run but not putting anything out to drive that Gate drive transformer.
So if we use SCH of rev A, you should check the DCV of IC UI807 pin 16 for 5VDC, and >2.5VDC on the pin 9 ENable pin.
I am hoping that the SCH will be close enough to use.

Those pins are outputting 4.9 volts, so that IC is OK.

[Porto]

Re: Samsung LE40m87BD ... no backlight

It think you are going about this the wrong way
If you can see the menu using a torch ,then all is probably ok with the power supply
Measure the back-light voltage , if OK i suspect that you have failed leds

This could be failed CCFL's but it's very suddenly they broke, it's possible but I still hope it's something more repairable.

Which is the BL voltage for this screen?
I'm looking for a 24v but I only found a 12v going from the powerboard to the inverter balancer board. And I'm not comfortable in poking around those two high voltage CCFL cables that could kill my DMM.

Is there a way to externally connect the CCFL's to a powersource only to check if they light up?
Could there be a problem with the inverter balancer board itself?

[Nick's Tvs]

Re: Samsung LE40m87BD ... no backlight

Usually if the CCFL's are out, you can just go to the menu go down to brightness and turn that baklight all the way down. This can be tricky if you have to hold a flashlight to the screen but not impossible. Once the backlight/brightness is turned all the way down. Turn the tv off and then back on again.
But honestly I doubt your CCFL's are the issue here unless the tv was violently banged around and caused one to break.

[Porto]

Re: Samsung LE40m87BD ... no backlight

Usually if the CCFL's are out, you can just go to the menu go down to brightness and turn that baklight all the way down. This can be tricky if you have to hold a flashlight to the screen but not impossible. Once the backlight/brightness is turned all the way down. Turn the tv off and then back on again.
But honestly I doubt your CCFL's are the issue here unless the tv was violently banged around and caused one to break.

Well, I don't know exactly what happened to this screen in the meantime.
The screen was first brought to a repairshop (some guy in garage) but they said the 'board was damaged' and a new one costing about $250 so that was a clear no-go for such an old screen; they wouldn't say which board was broken, just to make things difficult for hobbyists like me (and they losing money over it ofcourse).

I said the screen suddenly turned off but the owner (my uncle) said the BL did work till the last moment and after he got it back from the repairshop and gave the TV to me, I never saw the BL active. I don't know if that repairshop guy is to be trusted, fact is the powerboard was missing when I opened the housing and I had to get it back from that guy, had a heavy argument with him in the process because he first said he threw it away but later, when he suspected I was not going to leave without the powerboard, he was agitated to some degree and he suddenly had a 'similar' board lying around. Yeah right, it seems it was the ORIGINAL board for this TV he had lying around... very doubtfull guy who now has a customer less. He thought the TV was taken directly to the trash so he 'conveniently' removed the powerboard for his collection because he was sure it was not broken, but he 'forgot' to tell my uncle about it.. that's no practice for a professional, right?

But back on-topic, I don't know if that repairguy did more to this screen than to look for faulty caps. Maybe he switched one of the other working boards also for a faulty one, or damaged the CCFL's and 'forgot' to tell it, we can't prove anything and so there's no way to tell.

I have a CCFL tester but I only can test it directly, not with this balancer board in between.

[budm]

Re: Samsung LE40m87BD ... no backlight

Those pins are outputting 4.9 volts, so that IC is OK.

The EN pin is the receiving pin to turn on UI807 PWM. So you do have EN signal for sure right?

[Porto]

Re: Samsung LE40m87BD ... no backlight

The EN pin is the receiving pin to turn on UI807 PWM. So you do have EN signal for sure right?

Yes, I have a 5v output on the EN pin 9.

It also could be the inverter transformer TI801S, but I don't know exactly how to measure a transformer without a ringtester.

[budm]

Re: Samsung LE40m87BD ... no backlight

Even one or more of the CCFL are broken, the circuit will not know that so the Firing Voltage will still be applied to ALL the lamps which then you should see the backlights flash on for seconds then the protection circuity will kick in to shutdown the high Voltage.
You can easily test if the high Voltage is coming out from the connector CNI802 or 801 by using the old lamp as a probe.
https://www.badcaps.net/forum/showth...x+dx-40l261a12

[budm]

Re: Samsung LE40m87BD ... no backlight

Yes, I have a 5v output on the EN pin 9.

It also could be the inverter transformer TI801S, but I don't know exactly how to measure a transformer without a ringtester.

UI807 outputs at pin 2 and pin 3 drive the input pin of UI802S pin 2 and pin 3.

'That UI802S IC is a FAN7382VH36.

Only pins 1 and 8 have 12v, the other pins range from 0 to 0.15v.'

UI807: 'Those pins are outputting 4.9 volts, so that IC is OK.'
So something does not make sense to me if the output pins of UI807 has 4.9V, then the pins on UI802S cannot have only 0.15V unless both inout pin of UI802S are shorted out.

[Porto]

Re: Samsung LE40m87BD ... no backlight

Even one or more of the CCFL are broken, the circuit will not know that so the Firing Voltage will still be applied to ALL the lamps which then you should see the backlights flash on for seconds then the protection circuity will kick in to shutdown the high Voltage.
You can easily test if the high Voltage is coming out from the connector CNI802 or 801 by using the old lamp as a probe.
https://www.badcaps.net/forum/showth...x+dx-40l261a12

I have a few working CCFL's lying around which came out of a another screen. I tried one on one of the CCFL outputs but the lamp rapidly burned out; it did light up for a second though so I guess there is a high voltage present but I don't know if it's still outputting enough to lit the original lamps. Also, those onboard connections have both pins connected to eachother while my testlamp has a pink and a black wire... I guess that could be the cause of burning out the lamp. I only saw they were tied together when I flipped the board over.

[budm]

Re: Samsung LE40m87BD ... no backlight

What we need to know is if the Inverter circuit is completely dead or not, if it is active then we need to find out if it goes into protection shutdown or what by looking at the signals for the Inverter circuit to operate.