Samsung LCD TV Power Supply BN44-00438B

[gruv2ths]

All,
Been troubleshooting a Samsung 32" LCD TV.
Initially i noticed that the red LED was flashing twice and would not turn on.
Now there is no LED at all.

Observations so far:

-5V standby is 4V, 13V output is 10.5V (both off equally - 20%)
-If i disconnect the 5V standby from the PS(actually 4V), and inject my own 5V the led in the front now gives me the 2 blinks again and the PS_ON pulses momentarily enabling the PS including the power factor controller and it comes up at 400V.
-If i put a scope probe on the 5V/5V standby it is bouncing around, and so is the feedback from the optocoupler. But the converter regulates at 4V.
-I replaced the pwm/integrated FET IC thinking that in internal ref was bad, no joy, standby is still 4V.

Does anybody have experience on how opto's fail? Would their current gain change as a failure mode, or do they normally just go open?

Thanks!

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

It may have bad startup cap. Post good clear pictures of the board, both sides?

[gruv2ths]

Re: Samsung LCD TV Power Supply BN44-00438B

Would a bad start up cap change the regulation point? Is it soft-start?

Thanks!

[gruv2ths]

Re: Samsung LCD TV Power Supply BN44-00438B

OK so I replaced the opto and no dice. The standby output is still 4V and the 13V is approx. 10V.
So I have replaced the PWM/FET IC, opto and the electrolytic on the output.
Could it be that the xformer has a shorted winding or 2?

Attached is a pic of the Output converters drain waveform.
(looks like a flyback converter min duty cycle? Am i right about that?)

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

The voltage are steady at 4V and 10V and not fluctuating? If that is the case, it can be the Vref Diode such as TL431 which is commonly used in the SMPS, it is connected to the LED side of the Opto that i used for regulation.
What is the P/N of that ICM801?

[gruv2ths]

Re: Samsung LCD TV Power Supply BN44-00438B

On the DMM the 4V and the 11V are stable. On the scope it is bouncing around a little. Not sure how obvious it is from the scope pic above, but the converter is in burst mode, it is not constantly switching.

So is the zener diode a small plastic package, 3 pin divice?
The opto that i changes is circled in the pic below, am i right in assuming that the zener in question is in the circle?
Thanks!

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

Yes, it will be 3-pin device.
This typical usage of the TL431 and the Opto used in SMPS.
You should get the P/N of that multi-pin of the SMPS IC mounted on the heatsink, it will have application notes on how it is used, most manufacturers will just copy the design.
https://cdn.badcaps-static.com/pdfs/...9ac3a965b0.pdf

[gruv2ths]

Re: Samsung LCD TV Power Supply BN44-00438B

Thanks for the links, some great info..
So the zener in question has 2.4V on it's ref pin, and 2.5V on the Cathode. Seems like with Vout being 20% low the ref pin should be lower than 2.4V? Based on the design guide and data sheet, it should be 2.5 when it is right at 5Vout. Am I correct in my understanding??

Seems like the feedback network is making the converter think that 4Vout is correct.

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

"Seems like the feedback network is making the converter think that 4Vout is correct." That is what it looks it is acting that way. I would try replacing it.

[gruv2ths]

Re: Samsung LCD TV Power Supply BN44-00438B

replacing the TL431?

[tom66]

Re: Samsung LCD TV Power Supply BN44-00438B

If not that, it could be a leaky ceramic cap in the compensation network, or a failed/cracked resistor.

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

i want to order a new power supply but first must ensure that it is indeed the power supply has failed
Symptons no back light tv working fine (via flash light)

cn801 voltages
HIGH FET 0
LOW FET 0
OVP 0
CNT_PRT 13V
LD 11.6V

VOLTAGE AT BIG CAP 160DC?V

I have voltages at both CN802 and CN801
available with CN801 or CN802 connected or not
if necessary
What can I do to make sure it is Powe supply

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

Decided to post voltages to facilitate help with connectors in and out
These are the voltages at the power supply read at CN801


ALL IN CN802out CN801out ALL OUT
HIGH FET 0 0 0 0
LOW FET 0 0 0 0
OVP 0 0 0 0
CNT_PRT 13v 13v 0 0
VCC 13v 13v 13v 13v
LD 11.6v 11.6v 0 0

voltage at big cap 160vdc

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

The Voltage on the big cap should jump up to >360VDC when the PS_ON signal is applied, without that boosted Voltage the main supply will not work.
Please list ALL the Voltage reading with its name of each pin of CN802 when the TV is not turned on then with the TV power switch is activated.

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

allin no802 no801 ALLOut
1 B13V 13 13 13 13.1
2 P_DIM 2 5 2.1 5.1
3 B13V 13 13 13 13.1
4 NC NC NC NC NC
5 B13V 13 13 13 13.1
6 BLU_ON 5.2 1.3 5.1 1.3
7 B13V 13 13 13 13.1
8,10 9 GND 0 0 0 0
11,13 B5V 5.1 5 5.2 5.2
12 A5V 5.1 5 5.2 5.2
14 PS_ON 3.5 3.2 3.5 3.2

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

Sorry when I transferred numbers no longer aligned
Thanks for your prompt reply
Will check voltages with tv on and tb off
Is that what is needed

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

"no801 ALLOut" what does it mean ALL OUT?
'Will check voltages with tv on and tb off
Is that what is needed" Yes, and all the cables are connected.

So far it PS_ON is present but you still do not have PFC boosted Voltage on the main cap, so the PFC circuit will have to be looked at next.
I will need really good clear pictures of the whole bottom side of the board, and then in four sections because I will have to trace out the connections on the bottom side of the board.
ICP801 looks to be the PFC Controller IC, can you read the numbers on the top of the IC? We will need to get the spec sheet to get the pin out of the IC.
NOTES: I assuming this is ACTIVE HIGH signal.

Check resistor RP825, it should be <1 Ohm.

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

to budm I cannot get over the quick replies to my problems. I really just needed to know if it was definitely the power board to replace (bn4400438b) but if you think it can be repaired well hurrah. The TV is a 32 in LCD Samsung LN32D430G

[liomendonca]

Re: Samsung LCD TV Power Supply BN44-00438B

Allout means CN801 and CN802 not plugged in

[budm]

Re: Samsung LCD TV Power Supply BN44-00438B

I cannot tell until you have all the boards connected back together and check the PS_ON Voltage when the TV is off and when POWER switch is activated, this PS_ON should change state when TV goes from off to on, and from on to off, after that we need to find out if the Voltage on the big cap goes up to >360V, if it does not then when the PS_ON is active, then we know you do at least have power supply problem, but it does not mean you do not have the problem in other boards, you have to do step by step first.
There are signals and Voltages that must be present at the correct time and correct voltages.
You should read basic TV troubleshooting guides first. It comes down to this: does the power supply have standby voltage to run the main board, does the main board send out the PS_ON signal (If not, then the main board needs to be checked) to turn on the PFC Voltage booster which the other section of the power supply circuits need to run, so if the PS_ON is present but the PFC Voltage booster does not go up to > 360V then you have power supply problem.