Samsung LN32B360 no standby voltage

[martino]

Hello,
I'm in the process of forcing myself to learn more and more on these type of powerboards...
Could use a little help on this one trying to understand this part of the circuit.
I'm guessing it's the ac detect circuit?
Anyway's,here's what I did so far.:
There is no 5 volt standby.
I have 167 volt DC on startup cap CP815.
Traced the 167 volt all the way through the transformer and ended on the negative side at UM801s (see pic of schematic)
Looking at that circuit,I'd say it supposed to give the transformer a high frequency pulse(negative) once it approved of certain voltages.It also checks feedback through an opto coupler of the 5volt standby line?
Anyway's,pic 2 shows pinout of UM801s which is a ICE3BR0665J,
pin 1=BA (in the dark on what it does)
pin 2=FB (feedback through opto coupler from 5volt standby circuit?)
pin 3=Cs (has resistor of 0.5 ohm to ground,resistor set voltage checkpoint?)
pin 4&5=drain,going to negative side of transformer coil)
pin 6= NC (not connected to anything)
pin 7=Vcc (I measure 15.4 volts,IC spec says between 10 and 25v)
pin 8=GND.

I'm thinking this IC is not functioning as I do get Vcc on it,but I'm in the dark on how to test this IC. I did measure resistor that is connected to Cs and it's in spec(0.5ohm 1 watt)

Can somebody shed some light on this for me? Am I correct in my thinking?

board is :BN44-00289B
Found schematic on Tom's site,but have no clue how to include in this post..

[budm]

Re: Samsung LN32B360 no standby voltage

What DC voltage (@ the two legs) do you have at that start up cap CM804 47uF 50VDC? Once I have that info then I can explain how the circuit functions.

[martino]

Re: Samsung LN32B360 no standby voltage

Interesting.....zero volts(between legs)

[budm]

Re: Samsung LN32B360 no standby voltage

OK, I just want to make sure the IC is not running, if you do have voltage on that cap, it will mean that the IC is running. So right now the IC is getting enough startup voltage (18VDC) from the D pin, if you look at the internal diagram, it is connected to the STARTUP CELL to give the IC a kick start.
Here is an exert about Power management:
The Undervoltage Lockout monitors the external supply voltage VVCC. When the SMPS is plugged to the main line the internal Startup Cell is biased and starts to charge the external capacitor CVCC which is connected to the VCC pin. This VCC charge current is controlled to 0.9mA by the Startup Cell. When the VVCCexceeds the on-threshold VCCon=18V the bias circuit are switched on. Then the Startup Cell is switched off by the Undervoltage Lockout and therefore no power losses present due to the connection of the Startup Cell to the Drain voltage. To avoid uncontrolled ringing at switch-on, a hysteresis start up voltage is implemented. The switch-off of the controller can only take place when VVCC falls below 10.5V after normal operation was entered. The maximum current consumption before the controller is activated is about 150μA.When VVCC falls below the off-threshold VCCoff=10.5V, the bias circuit is switched off and the soft start counter is reset. Thus it is ensured that at every startup cycle the soft start starts at zero.The internal bias circuit is switched off if Auto Restart Mode is entered. The current consumption is then reduced to 150μA. Once the malfunction condition is removed, this block will then turn back on. The recovery from Auto Restart Mode does not require re-cycling the AC line. When Active Burst Mode is entered, the internal Bias is switched off most of the time but the Voltage Reference is kept alive in order to reduce the current consumption below 450μA.

VCC Turn-On Threshold typ is 18VDC. right now you only have 15V. So it can be that the cap at the VCC has leakage resistance that will not allow the VCC to get to 18V for starting.
Take the resistance read on GND PIN 8 against all other pins and report the readings.
If I remember correctly, the common problem is due to leaky Zener diodes ZD903, 904.

[martino]

Re: Samsung LN32B360 no standby voltage

DMM max is 2000kohm...
pin 8 versus:
1= OL
2=1856Kohm
3=0.5 ohm
4&5=500Kohm and climbing(reverse leads,goes down,cap charging?)
6=OL
7=50Kohm climbing also.

Excellent read!! Will have to read that a few times to get a good grasp on it ha! Does make sense.....

[budm]

Re: Samsung LN32B360 no standby voltage

VCC is 50K is high which is good since the internal start up cell can provide 0.9mA of charging current, but some how it is only reaching 15V. I still suspect cap CM808 (22uF) at the VCC pin which may be leaky but you meter is not putting out anything close to 15V to cause it to show the leakage resistance, try changing that cap. Also try lifting one leg of the ZD903 or 904 and check the resistance both way with the meter. Something is loading down that VCC pin, unless the internal current source inside the IC had failed.

[martino]

Re: Samsung LN32B360 no standby voltage

Also try lifting one leg of the ZD903 or 904

Do you mean ZD803 and 804 perhaps? I can't seem to find a 900 zener diode.
The 803 and 804 are in the Vcc area next to CM808..

[budm]

Re: Samsung LN32B360 no standby voltage

Sorry, I mean ZD803, 804.

[martino]

Re: Samsung LN32B360 no standby voltage

cm808=23.7uF .66 ohm(ESR meter)
ZD803=1N4742A=0.749v/OL (DMM for OL set at 2000Kohm/0.749 in diode mode)
ZD804=1N4740A=0.594v/384Kohm(measured same as above)

Safe to say ZD804 is bad? I pulled up specs on diode but cant make sense of it(see pic,both are on there)

[budm]

Re: Samsung LN32B360 no standby voltage

What you can do right now is to do a quick power up to see if the 5V is working with the zener not installed, just do quick power apply.

[martino]

Re: Samsung LN32B360 no standby voltage

CM808,ZD803 and 804 out of circuit and I get 2.91volt.

[martino]

Re: Samsung LN32B360 no standby voltage

I re installed CM808,diodes 803 and 804 still out and sure enough:The 5volt standby is 5.1volt!!!
budm,you are the master!!!!!!
Now on to source the diodes,is there a reason why they use two different zeners in series?

[budm]

Re: Samsung LN32B360 no standby voltage

I do not know why, 1N4740A, 4742A are just typical Zener diodes and should be easy to get. So what is the VCC reading now?

[martino]

Re: Samsung LN32B360 no standby voltage

I completely forgot to check Vcc I found the diodes,installed them and got busy checking the output voltages on psu,then test TV and zipped up cover.
(I like to run them with cover on for better airflow over everything)
I run them for 3 days straight(and turn off/on cycles) before they leave,will take cover back off and measure Vcc etc,I'm curious now too lol..
I don't get too many with a 0volt problem like this,mostly all cold side problems or other board issues.
It intrigues me how these psu's work,want to educate myself more on the works of em
Big thanks,budm,that was excellent help