UN55D6900 BN44-00428B partial led driver circuit failure

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Update:
- The first replacement power supply I received was all rusted & corroded. All I would get was a relay click on, then immediately off. No LED lights.
- The company sent a replacement board and the replacement was also defective. It would click on, make a high pitched whine, the LEDs would come on momentarily, then relay click off.
- Finally, the company sent a 3rd board and 3rd time's a charm, as they say! It works perfectly.

Took a month to get a working board, but...TV fixed. Thanks again for all the help.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Was this supposed to say Q9253 and source resistor would then be R9265?

Yes, you are correct, it should be Q9253. I think R_J had a typo, but I understood what he was looking for.

First of all there is no way you can have 130 to 219V across R9265 unless it is open.

You're right, this was a mistake when I wrote my reply (sorry, I was running on 20 minutes sleep from the night before ). My statement should have read:

"...When I get the higher (130V-219V) voltage across R3+ to L3-, the voltage across R9265 is the 2.5V-2.6V reading."

And perhaps another explanation is that IC9151 from either being defective or not receiving the proper feedback is boosting the voltage to an uncontrolled elevated level.

I did some visual inspection and found that one of the surface mount caps and one surface mount resistor had shifted when I moved the IC between circuits. I resoldered those to make sure there was a good solder joint and was able to get 159V driving those LEDs on the circuit we've been working on. But it is not stable. When I power off the TV and power it on again, sometimes the voltage on that circuit across R3+ to L3- appears to be fluctuating between -2.5V and >100V. The other circuits are still ok. And if I plug in the main board and power on the TV, I only get 80V across each LED strip pair (eg R3+ to L3-, etc) (half the expected voltage drop), and the LEDs are obviously very dim.

And finally I would say yes to getting a new Power Supply...You said you weren't very experienced so kudos to you for even accomplishing the task with the etch / components still in tact.

I ordered the replacement power supply. Should arrive Saturday. I'll give a status update once I've received it. And thanks for the kudos! I am pretty happy with what I learned and my soldering accomplishments here, even if I didn't end up fixing the board. It's a big step forward for me skill-wise. Thanks again for your help!

[RDC55]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

I am not quite sure how to comment on those voltage reading but this is my best guess, maybe R_J has a theory also.

Is the voltage across Q9223 source resistor high like before (2.27v)

Was this supposed to say Q9253 and source resistor would then be R9265?

Interestingly, sometimes when I read it it is fluctuating between 2.5v-2.6v. Other times it is 0V. I do notice the LEDs flickering when this happens. More below...

And as before this meant R9265 source resistor, but see more below says 100's of volts or zero volts across R9265.

the voltage across R9265 is 0v. When I get the higher voltage across R3+ to L3-, the voltage across R9265 is the 130V-219V reading.

Ok, I checked across R3+ to L3- and it sometimes it reads -2.5v, other times it reads 130v and then steadily rises to 219V. I notice the LEDs flickering, and the timing of the flickering seems to correspond to the change between the two voltage profiles I'm seeing. When I get the -2.5v reading across R3+ to L3-, the voltage across R9265 is 0v. When I get the higher voltage across R3+ to L3-, the voltage across R9265 is the 130V-219V reading.

First of all there is no way you can have 130 to 219V across R9265 unless it is open. At the lower number of 130V you are still talking thousand of watts so the resistor would have to be open, or the etch connectivity is not right.

As for 219V directly across the LED's again this is really unsustainable as it would mean over 4V of Vf across each LED. I don't know the exact numbers of what is sustainable across an LED, but I would guess that would overheat and open them fairly quickly.

One explanation for 219V across the LED's is that the LED's are defective, and when driven by full current one or more is going high resistance and the board compensates by upping the voltage to maintain the current. This of course would mean you have a bad strip in the TV.

And perhaps another explanation is that IC9151 from either being defective or not receiving the proper feedback is boosting the voltage to an uncontrolled elevated level.

Yes, I'm sure about that. But I still am not sure how having that R9165 circuit with missing components would affect this R9265 circuit, if at all?

Having that R9165 circuit with missing components should not affect the R9265 circuit, but it may cause an excess voltage as the boost voltage is adjusted upwards trying to increase current, which it can't because there are missing components. I was just asking to be sure about where you were measuring.

Anyways, with the instability and strangely high voltage, do you think I maybe I should just take the loss on this one and order a replacement power supply? It's only $43 vs the time I'd have to invest on continuing to debug this. Either way, I've learned a TON about how these LED driver circuits work, what to test, how to test, etc. just by going through this process with you. The time invested so far has been a valuable learning experience for me. So thanks so much for that.

And finally I would say yes to getting a new Power Supply. I am fairly experienced and I personally would not attempt to remove 20 pin IC's and reuse them unless replacements were not available. I believe the soldering process is stressful enough on a component, but the de-soldering process is even worse so I just wouldn't trust it. You said you weren't very experienced so kudos to you for even accomplishing the task with the etch / components still in tact.

I would also keep in mind if the issue with the 219V across the LED's is actually a bad strip then you may also be replacing those. Perhaps it would be a good idea to swap the wires around again like you did before and see if a different segment behaves normally when driven by the suspect Power Supply channel.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Which is "THE DIODE"? D9251? I assume bottom = anode & top = Cathode.

So just to be sure when you say bottom of the diode and top of the diode where are you probing - is that like Dxx51 anode / cathode? And just to be sure that is in respect to chassis ground?

Yes (D9521), yes (bottom=anode, top=cathode), and yes (wrt ground). Sorry for not being more specific, I'm brand new to this and didn't know which side was the anode vs cathode. I had to Google it

Is the voltage across Q9223 source resistor high like before (2.27v)

Interestingly, sometimes when I read it it is fluctuating between 2.5v-2.6v. Other times it is 0V. I do notice the LEDs flickering when this happens. More below...

If they were probed at the 801A and 801B connectors just like when you hook up the LED tester (R3+ to L3-) then that would be a more exact measurement of the voltage drop across the string. I am thinking anything over about 170 VDC I would shut it down.

Ok, I checked across R3+ to L3- and it sometimes it reads -2.5v, other times it reads 130v and then steadily rises to 219V. I notice the LEDs flickering, and the timing of the flickering seems to correspond to the change between the two voltage profiles I'm seeing. When I get the -2.5v reading across R3+ to L3-, the voltage across R9265 is 0v. When I get the higher voltage across R3+ to L3-, the voltage across R9265 is the 130V-219V reading.

Are we sure the 222V is not on the disabled string with the missing components?

Yes, I'm sure about that. But I still am not sure how having that R9165 circuit with missing components would affect this R9265 circuit, if at all?

Anyways, with the instability and strangely high voltage, do you think I maybe I should just take the loss on this one and order a replacement power supply? It's only $43 vs the time I'd have to invest on continuing to debug this. Either way, I've learned a TON about how these LED driver circuits work, what to test, how to test, etc. just by going through this process with you. The time invested so far has been a valuable learning experience for me. So thanks so much for that.

[RDC55]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

However, on all of the other circuits the voltage on the bottom of the diode is 116V and on the top of the diode is 160V, while on the affected circuit the voltage on the bottom of the diode is 116V but on the top of the diode is 222V, and the LEDs appear to be overdriven (too bright) on that strip (although I'm making that assessment from looking through the holes in the back of the TV because I don't have the panel disassembled).

So just to be sure when you say bottom of the diode and top of the diode where are you probing - is that like Dxx51 anode / cathode? And just to be sure that is in respect to chassis ground?

222V sounds scary, particularly if the LED's seem brighter. If they were probed at the 801A and 801B connectors just like when you hook up the LED tester (R3+ to L3-) then that would be a more exact measurement of the voltage drop across the string. I am thinking anything over about 170 VDC I would shut it down.

Are we sure the 222V is not on the disabled string with the missing components?

I suspect it would be brighter as a stand alone power supply since PWM would go to max drive. The measured voltage drop that you posted for the current limit resistors for the functioning strings indicates the LED's are just barely being driven. I believe that is somewhat normal particularly if there is no video then the PWM drive will adjust to minimum brightness.

If you don't have an oscilloscope to evaluate the PWM signal then perhaps swapping the suspect LED segment PWM input wire with another segment might be beneficial to rule out any issues with the main PWM drive signal for that segment.

[R_J]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Which is "THE DIODE"? D9251? I assume bottom = anode & top = Cathode. Vdv is common to all for circuits, The high voltage may be due to lack of feedback, The circuit assumes the current through the leds is too low so it increases the voltage to compensate. Is the voltage across Q9223 source resistor high like before (2.27v)

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

So no voltage across R9265 suggests an open in the LED circuit, or a problem on the low side as R_J suggested earlier. You already verified boost voltage is working on that section, but perhaps rechecking boost since you swapped IC's might be in order.

Are you working with the TV open and just the Power Supply connected - no main?

If open, does that suspect circuit flash the LED's and subsequently produce a momentary voltage across R9265 when first powered up?

Was testing with PSU & main connected.
Re-checked boost voltage and it still jumps up to 170V right after I apply power. But the LEDs don't come on when it's at that voltage. A moment later the voltage drops to 128V on the bad circuit and 160V on the good circuits, and that's when the LEDs on the good circuits turn on for the first time (then they turn off, then turn back on again as is evidently the normal boot cycle). The LEDs on the bad circuit never come on.

If working with the main connected try it with just the power supply by removing the cable between the main and power supply - perhaps there is an issue with the PWM drive signal.

THIS is interesting. Without the main board connected, the LEDs on that circuit DO turn on. However, on all of the other circuits the voltage on the bottom of the diode is 116V and on the top of the diode is 160V, while on the affected circuit the voltage on the bottom of the diode is 116V but on the top of the diode is 222V, and the LEDs appear to be overdriven (too bright) on that strip (although I'm making that assessment from looking through the holes in the back of the TV because I don't have the panel disassembled).

Does that give you any clues?

[RDC55]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

So no voltage across R9265 suggests an open in the LED circuit, or a problem on the low side as R_J suggested earlier. You already verified boost voltage is working on that section, but perhaps rechecking boost since you swapped IC's might be in order.

Are you working with the TV open and just the Power Supply connected - no main?

If open, does that suspect circuit flash the LED's and subsequently produce a momentary voltage across R9265 when first powered up?

If working with the main connected try it with just the power supply by removing the cable between the main and power supply - perhaps there is an issue with the PWM drive signal.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The LEDs are not always correct, you can check the tester-led, you need to measure the current consumption on each group (channel) Here is a service manual that can help you figure it out.

Thanks for the reply. I have already verified that the LED strips are ok earlier in the thread. I attached these segments of the LED strips to a different driver circuit, and they worked fine, so the LED strips are not the problem here.

[lotas]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Service BN44-00428B

[lotas]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The LEDs are not always correct, you can check the tester-led, you need to measure the current consumption on each group (channel) Here is a service manual that can help you figure it out.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The problem is that when Q9253 shorted and R9265 went open, the voltage on Q9253 source would have been quite high and likely damaged the FBN2 input of IC9151, that's why R9264 also went open. So I suspect the ic is damaged.
On an equivalent ic the voltage range on that pin is 0.3 ~ 5.5v volts (5.5v max)

Ok, I swapped IC9151 with the known working one from the other circuit. No change (the other circuits still work, the non-working circuit still does not work and has the same voltage readings). So, IC9151 was ok.

What next?

[R_J]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The problem is that when Q9253 shorted and R9265 went open, the voltage on Q9253 source would have been quite high and likely damaged the FBN2 input of IC9151, that's why R9264 also went open. So I suspect the ic is damaged.
On an equivalent ic the voltage range on that pin is 0.3 ~ 5.5v volts (5.5v max)

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

...

Update: R9264 was also blown out, so I went ahead and swapped R9263, R9264, R9265, and Q9253 out with the corresponding components from one of the working LED driver circuits so we can continue the debug of this non-working circuit (and I will replace the components in the known working circuit later once I receive the parts in the mail).

Now the voltages in this non-working circuit read as follows:

• R9165 - this is the circuit that I cannibalized, so it's disabled.
• Voltage drop across R9265=0V, R9635=45.7mV, R9645=46.1mV
• If I read the voltages at the bottom and top of each resistor as compared to ground, I see:
  • R9265 -5.2mV (bottom), -5.2mV (top)
  • R9365 -5.1mV, 40.4mV
  • R9465 -5.1mV, 41.2mV

NOTE: Since I took R9263, R9264, R9265, and Q9253 from a working circuit, we can be certain that they are all known good components.


What to check next? Thanks again for your help!
P.S. One other thought I just had - will taking those components from the R9165 circuit affect the ability of the R9265 circuit to function? (in other words, should I have taken them from the R9365 circuit instead)?

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The voltage going high would mean the boost circuit is working, so the fault would seem to be in the led current monitoring.
It looks like current is monitored across R9265 (for dimch2) R9264 would supply the feedback voltage for that channel back to IC9151S.
Try and check the voltage across R9265, R9165, R9365, R9465. One of these should be different and may be what is shutting down the circuit.

IC9151S pin 19 drives (pwm) Q9253 gate, this fet connects the return of the led string to ground through R9265. If this mosfet is shorted or not being tuned on, the current through R9265 will be wrong and trigger a shutdown.

Well, I messed up and accidentally shorted that R9265 resistor and blew it. So, I'm going to give you the post-catastrophe numbers below. I rechecked the resistors and voltages around the diodes and those are still ok, so nothing I posted before has changed...

• Voltage drop across R9165=45.4mV, R9265=2.3V, R9635=45.5mV, R9645=45.2mV
• If I read the voltages at the bottom and top of each resistor as compared to ground, I see:
  • R9165 -8.7mV (bottom), 36.7mV (top)
  • R9265 -8.6mV, 2.27V
  • R9365 -8.0mV, -37.4mV
  • R9465 -7.5mV, 37.4mV
• I checked R9264 and it reads 0.469kΩ. I checked R9464 for comparison, and it also reads 0.469kΩ. So R9264 seems ok.
• I checked the resistors around the Q9253 gate. The bottom resistors checked out ok (they were all 10Ω), but the upper resistors read as follows: R9163=1kΩ, R9263=110kΩ, R9363=1kΩ, R9464=1kΩ.
• I also checked the Q9253 for shorts and the bottom pin is shorted with the middle pin, so it is blown.

So it looks like I need to replace R9265, R9263, and Q9253 at a minimum. Not sure if that will fix the problem or just get me back to where I was before I accidentally shorted the board. *sigh* Anyways, it's going to take a while for me to get parts, but I'll update again once that's done.

[R_J]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

The voltage going high would mean the boost circuit is working, so the fault would seem to be in the led current monitoring.
It looks like current is monitored across R9265 (for dimch2) R9264 would supply the feedback voltage for that channel back to IC9151S.
Try and check the voltage across R9265, R9165, R9365, R9465. One of these should be different and may be what is shutting down the circuit.

IC9151S pin 19 drives (pwm) Q9253 gate, this fet connects the return of the led string to ground through R9265. If this mosfet is shorted or not being tuned on, the current through R9265 will be wrong and trigger a shutdown.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Have you checked if D9251 is ok and not shorted? Also check R9253 (0.22Ω) to see if it is ok and not open.
Monitor the voltage on D9251 cathode to see if it ever gets above 126v, If it does spike to 160v and drop, the problem is likely not here but on the return side of the led's

Both D9251 and R9251 check out ok (note: my multimeter only goes down to 0.1Ω, and it showed 0.3-0.4Ω for R9251, but I checked the other R9x51 resistors and they all read the same).

I monitored D9251 voltage when I powered on the TV. My multimeter sampling rate isn't the best, but I tried it a few times and did see it go up to 160-170V momentarily and then drop back to 126.8V and stay there.
For reference, the other LED driver circuits would jump up to ~160V, then drop back to 126.8V, then go back up to ~160V and stay there (the normal boot sequence of the TV has the LEDs light up, then go dark, then light up and stay on, so this is normal).

Next steps? Thanks for the help!

[R_J]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Have you checked if D9251 is ok and not shorted? Also check R9253 (0.22Ω) to see if it is ok and not open.
Monitor the voltage on D9251 cathode to see if it ever gets above 126v, If it does spike to 160v and drop, the problem is likely not here but on the return side of the led's

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

Make sense? If it's still confusing let me know and I can draw a picture of it.

I made a little diagram to show the "R1+ to L1-" circuit, in case the text was still confusing.

[Daveleone]

Re: UN55D6900 BN44-00428B partial led driver circuit failure

I would not rule out bad LEDs. Your LED tester only puts out 10-20mA and the circuit likely runs at several hundred mA. I have replaced LEDs on a few models with symptoms of bad backlights but all the LEDs tested good with a tester (LG UH series with logo flash & Sony with 6 blinks). I've done so many that I typically don't even test any more and just replace them all with new.

Ok, the suspect LED driver circuit or LED segments were in the "R3+ to L3-" circuit. So, I swapped the wires from the "R3+ to L3-" circuit with the wires from the "R7+ to L7-" circuit. When I did that, LEDs previously attached to the "R3+ to L3-" circuit lit up properly, and the LEDs previously attached to the "R7+ to L7-" circuit did not. In other words, the problem is a faulty LED driver circuit, not the LEDs themselves.

Would appreciate your help with next steps to diagnose the faulty component(s) of that LED driver circuit. What should I test / how to test it?

Thanks!