ZM821 in BN44-00440A power supply

[Chuper]

So I am working on a Samsung LN40D550K1FXZC that wasn't powering on.
SMPS BN44-00440A in it had UM801S and RM811 burned. After replacing those two it still didn't have output voltages. So I found a ZM821 between leads(pins) 3&5 and 6 of UM801S that read 20ohm in both directions. After removing it SMPS came back on and I have output. Across those two pads that ZM821 was soldered to is 0.9V. Under a microscope ZM821 marked with Z18. My guess that it is a 1.8V zener diode.
As far as I know schematic isn't available for this SMPS.
The question: is anybody knows for sure what that ZM821 is and can I run SMPS without it?
Pics by shopjimmy.com

[R_J]

Re: ZM821 in BN44-00440A power supply

Z18 is very likely an 18 volt zener

[budm]

Re: ZM821 in BN44-00440A power supply

That Zener is connected in parallel with the output (Emitter-Collector) side of the OPTOISOLATOR PC801S (used for feedback to regulate the output Voltage), you should check the resistance between the two pads where you remove the Zener ZM821, more likely the OPTO is also damaged when the SMPS IC (STRW6052??) blew up.
Most power supply do not install that Zener.

[Chuper]

Re: ZM821 in BN44-00440A power supply

Thanks budm! I checked the optoisolator and it’s ok.
Right now when everything is connected when I plug main AC tv standby led blinks 6 times and goes off and TV is on, I can see picture with the flashlight but no backlight. I can turn it off and on. A5V is 5V when TV is off and goes to 0V when I switch it on, B5V comes to 5V and B13V comes to 13V, Blu-ON comes to 5V and P_DIM comes to 0.8V from 0V respectively. So looks like main board sends necessary voltages. SEM 2016 got VCC of 13V. Am I right that it supposed to have some kind of AC on pins 12 and 13 to a little transformer TI803S? I got nothing on those pins. And I don’t have nothing on pin 1 and 3 of CN1801 of course. Where to go from here?

[R_J]

Re: ZM821 in BN44-00440A power supply

You would need a scope to see the drive signal on pins 12,13 of the sem2106, The signal is likely too low a level and too high frequency for a meter.
Do you get any voltage on cni801 pin1 or see a flash of the backlight when first turned on?

[budm]

Re: ZM821 in BN44-00440A power supply

What Voltage do you have on pin 10 ~ 12 of Connector CNI801?
Do you have 380 ~ 400VDC between the two legs on either one of the two main filter caps?

[Chuper]

Re: ZM821 in BN44-00440A power supply

What Voltage do you have on pin 10 ~ 12 of Connector CNI801?
Do you have 380 ~ 400VDC between the two legs on either one of the two main filter caps?

CNI801 when TV is on
pin 10: 12.04V
pin 11: 12.64V
pin 12: 11.21V
main filter caps got 392.6V

[Chuper]

Re: ZM821 in BN44-00440A power supply

You would need a scope to see the drive signal on pins 12,13 of the sem2106, The signal is likely too low a level and too high frequency for a meter.
Do you get any voltage on cni801 pin1 or see a flash of the backlight when first turned on?

no flash of backlight and about 3VAC on pin 1

[R_J]

Re: ZM821 in BN44-00440A power supply

Does this tv have a seperate backlight inverter? does it look like this? Maybe post a picture of the rear of the tv with the back off
I will guess that the circuit is working

[budm]

Re: ZM821 in BN44-00440A power supply

It has inverter transformers board, the drive circuit (half-bridge) for the primary of the inverter transformers is on the power supply board.
I expect the backlights should at least flash on for seconds if the drive Voltage (over 200VAC (high frequency) was present.
So CI826 capacitance tested OK?

[Chuper]

Re: ZM821 in BN44-00440A power supply

It has inverter transformers board, the drive circuit (half-bridge) for the primary of the inverter transformers is on the power supply board.
I expect the backlights should at least flash on for seconds if the drive Voltage (over 200VAC (high frequency) was present.
So CI826 capacitance tested OK?

Yes, CI826 tested good. The marking on the SMPS said that I should have 280VAC on pin 1 of CNI801.

[budm]

Re: ZM821 in BN44-00440A power supply

280VAC at around 50 ~ 100KHz in which your meter does not have the bandwidth to be able to read AC frequency that high.
Since you do not even see the backlights flash on for seconds at all, then something in the drive circuit is not working. Need to see what IC it uses to drive the two MOSFETs for that inverter driver circuits.

[budm]

Re: ZM821 in BN44-00440A power supply

There is repair kit for the BN44-00440A
http://www.shopjimmy.com/samsung-bn4...repair-kit.htm

I did not look in details to see which section of the power supply the kit is made for.

[R_J]

Re: ZM821 in BN44-00440A power supply

I would think that if your meter is measuring some ac (3 volts) on pin 1 of CNI801, that the drive circuit is working. You could try using a diode, with the anode to pin one and cathode to your meter probe and see what dc you measure.

[budm]

Re: ZM821 in BN44-00440A power supply

The MOSFETs (MI801S, 802S) are driven by the isolation transformer TI803S, you may be able to check the Voltages on the primary winding of the transformer which is in the cold side of the circuit, you should also check Voltages of that IC ??801 (SEM2016?).