Samsung 740B plus won´t start

[rogfanther]

Hello,

Got two of these that won´t power up.
PSU is IP-35155A, inverter transformer is TM-0917.

First one got bad caps in the 5V line C109 and C110 ( two 1000 uF x 10V, Samwha XC ) . Swapped them and the one 820 uF x 25V near them. ( C113 , also Samwha XC ).

Well, now the psu gives correct voltages in the CN2 connector, with and without the logic board connected.

Then the other : none of the caps were bulged. Just to be sure, I swapped most of the caps ( C190 and C110 , 1000uF x 10V, C113, C112 and C316, 820uF x 25V ). All were Samwha XC.

Connecting just the psu gets the voltages in the CN2 connector ok. Starting it up with the logic board connected gives just about 1V in the 5V line, and 3.something in the 15V line.

Have swapped C107 ( 47uF x 50V SamYoung LXV) and the controller ( FSDM0465R ) to no avail. Have also tried with both logic boards, it made no difference.

Now for the interesting part :

If I connect just the IP board, turn it on, measure the voltages to see it is working, turn it off. Wait for the big cap to discharge, then connect the logic board and turn the thing on, it will not start ( Voltages between pin 2 - 3 of the D0465 stay around 9.7 - 10.3V ) . Output voltages in 1V for 5V and 3.2 for 15V.

But if I connect the logic board before the big cap has time to discharge fully, and turn power again fast, the smps will start, voltage between pin 2-3 will be around 12.4V, and output voltages are ok ( 5.1V and 14.7 - 14.9V ) .

As I have the two boards , and one seem to work, I´ve tried :

Swapping the NTC TH101 ( 5 ohms, SCK-053 ). No difference.
Swapping the big cap ( Samwha WL 100uF x 400V ) . No difference. By the way, in the other board it is a SamYoung KMF 100uF x 400V.

Any suggestion of other things to troubleshoot ?

[selldoor]

Re: Samsung 740B plus won´t start

Sorry lost me ( easy done) What is an IP board? Can you recap - when you say one seems to work, either it does or it doesnt. If you have one set of boards that work then
great - you will have something to compare.

Get the Power supply that doesnt work and swap it for the board that does work then
give us a commentary from plug in to power on to stable fault. What does the power led do what does the panel do and are there any noises.

EDIT - just read it again - you dont have set that works - do you.

It will be a lot simpler if you decide which pair of boards you think are the best and JUST
work on them for a start otherwise it is too complicated as we will not be able to remember
what has been done on which board.

[rogfanther]

Re: Samsung 740B plus won´t start

Hi, IP is the code of the psu+inverter board ( IP-35155A ) .

I mean it works with respect to the voltages ( the boards are with me at work, and the lcd panel is at home, so will only know if it shows image tonight ) . Before recapping, it would not give correct voltages when connected to the logic board.

The situation now : I have one PSU board that powers up ok, with or without the logic board connected. This is the one I consider the best then. Will test it with the lcd panel later.

And the other board will only start up ok if connected to power by itself, then disconnect power and reconnect logic board + power fast enough for the primary cap not to discharge. Then it can start and gives ok voltages.

[selldoor]

Re: Samsung 740B plus won´t start

Ok probably best to wait and see
Just to get it out of the way,whilst you indicate the inverter transformers are TM-0917 you do not say if you have tested them. Can you test the secondary windings- meter on ohms 2000 test pins 5>6 7>8 and post results as follows

pins 5>6 Ohms xxxx
Pins 7>8 Ohms xxxx

[rogfanther]

Re: Samsung 740B plus won´t start

Yeah, 1030 ohms one, 1032 ohms the other.

[retiredcaps]

Re: Samsung 740B plus won´t start

Any suggestion of other things to troubleshoot ?

Does the power LED come on at all with the "good" PSU board?

If not, that suggests your problem lies with the main/logic board.

There should be some voltage regulators on that board. See if they are all within spec. A photo of it would help also.

[rogfanther]

Re: Samsung 740B plus won´t start

Here are the photos of the main/logic board.

Made a slight detour when going to lunch and went home. The good PSU board, along with what I will call the goog Main board, work with the panel. Power LED on, message of Analog / Digital on the screen. Disappear with vga cable connected. Power button works to turn the display on and off. This psu still has a little hiss, that I assume is caused by the caps I didn´t swap yet. When my order arrives ,will recap it all with Panasonic FM.

The regulators in both main boards are within spec ( 1117-18 gives 1.82, 1117-33 gives 3.42 ).

Don´t know if I explained ok : the "phenomenon" is that, if I plug the bad PSU board with the main board, and turn on power, it seems not able to start, and Vcc to the pwm chip stays lower than the minimum.

But if I connect just the bad psu board, turn on the power, after about 2 seconds it will stabilize power . Then if, while the main cap is istill charged, I turn off the power, connect the main board, and turn power again, the psu will start and stabilize the outputs. Vcc in the pwm will be ok, as also the voltages in the main board regulators.

[budm]

Re: Samsung 740B plus won´t start

3.3V is more than 2% off, it is off by 3.6%.

[rogfanther]

Re: Samsung 740B plus won´t start

Just measured them again. 1.81V and 3.32V. I don´t believe the problem to be in the main board, because it happens with both of them connected to the "bad" psu. Both main boards in the "good" psu give correct readings.
I have changed a zener ( 47V ) and another diode ( 1N4007 ) , but no difference was made.

[retiredcaps]

Re: Samsung 740B plus won´t start

Don´t know if I explained ok : the "phenomenon" is that, if I plug the bad PSU board with the main board, and turn on power, it seems not able to start, and Vcc to the pwm chip stays lower than the minimum.

C107 is your startup cap. On the bad PSU, measure the DC voltage across it with the big cap charged and uncharged states.

But if I connect just the bad psu board, turn on the power, after about 2 seconds it will stabilize power . Then if, while the main cap is istill charged, I turn off the power, connect the main board, and turn power again, the psu will start and stabilize the outputs. Vcc in the pwm will be ok, as also the voltages in the main board regulators.

Unplug lcd monitor. Wait until big cap is fully discharged. Now hookup multimeter aligator clips to the big cap to measure DC voltage. Now hit power switch. What is voltage across the big cap?

[rogfanther]

Re: Samsung 740B plus won´t start

On both PSUs, with the main board disconnected, the voltage in the cap stabilizes at 10.9-11V.

With the main board connected, in the good PSU the voltage is 12.7V.
With the main board connected to the bad PSU ( big cap uncharged ) , the voltage fluctuates betwen 9.3 - 10.4V.
With the main board connected to the bad PSU after the big cap has had time to charge alone, the voltage stabilizes at 12.4V

With the main board connected, the big cap measures 175.2V
With the main board disconnected, the big cap measures 175.2 V
With the main board reconnected before the big cap has time to recharge, it measures 174.3 V

Voltage measured in the outlet is 127.3 VAC

[retiredcaps]

Re: Samsung 740B plus won´t start

With the main board connected to the bad PSU ( big cap uncharged ) , the voltage fluctuates betwen 9.3 - 10.4V.

Try replacing C107 on the bad PSU and retest. Little caps can often be bad without any signs of physical bloating.

[rogfanther]

Re: Samsung 740B plus won´t start

I have already replaced it, after seeing it mentioned in a bunch of threads here. Will try to find another of the same value ( 47 uF x 50V ) tomorrow for a new try. I had searched for one with low esr, but from the datasheet it seems the original one would be considerably high ( 0.45 ohms ) . Will try to borrow a suitable one from one of the PSUs around here or, last chance, try swapping with the cap from the good, working board.

[retiredcaps]

Re: Samsung 740B plus won´t start

I have already replaced it, after seeing it mentioned in a bunch of threads here.

Sorry, when 2 monitors are involved, even if they are the same, it is still hard for us to straight what was done to what. If you already changed it, leave it alone for now.

Check all the diodes. Namely, ZD101, D102, ZD102, D103, D104, D105, D106, D107, and D108. Report all your readings.

[rogfanther]

Re: Samsung 740B plus won´t start

   
      good PSU    bad PSU
ZD101   1940 / 599	1947 / 586
 D102   open / 473	open / 470
ZD102   1793 / 515	1794 / 515
 D103	  793 / 515	1793 / 515
 D104	 1739 / 142	1741 / 145
 D105	     NOT POPULATED
 D106	 1740 / 142	1746 / 142
 D107   800 / 243   757 / 243
 D108	  801 / 243	 758 / 243

Measured with all connected to the boards.

I will buy the replacements for ZD101, ZD102 and D102 / D103, and replace them just in case. Then I will dessolder the secondary diodes and take new measurements.

[selldoor]

Re: Samsung 740B plus won´t start

Whoa - not sure you need all those replacements why do you think zd 102 is bad?
reading is virtually identical?
Perhaps retest lifting one leg

[rogfanther]

Re: Samsung 740B plus won´t start

I don´t mean to replace all of them ( well, not yet anyway ) .
Just imagining that maybe one of those zeners is failing when under stress. I believe I have some in my parts box.

Board has a small cap, 1 uF x 50V, between ground and TL431, that I haven´t replace yet. Could it be the culprit ?

[rogfanther]

Re: Samsung 740B plus won´t start

Well, lifted one leg from a diode in each pair. Measures still the same between them, Will search for that 1uF cap and try it.

[retiredcaps]

Re: Samsung 740B plus won´t start

I will buy the replacements for ZD101, ZD102 and D102 / D103, and replace them just in case.

As selldoor mentioned, you don't have to replace them as they all test good assuming D103 is really 1793 and not 793 (typo).

[rogfanther]

Re: Samsung 740B plus won´t start

Oh, it is 1973. I must have deleted the "1" when trying to align correctly the columns.

The idea of replacing them was in case they were failing under load...

Swapped the 1uF 50V cap and same symptons remain.

Interesting point :
Decided to ressolder all the secondary and pwm joints in case of a hidden cold solder joint. After that ( with the board still a little warm ) when powering it up, voltages were almost ok ( 5V was 4V, 15V was 12V ) . Thinking it was a cold joint , I let it cool during lunch time, and tried again. Back to not functioning.

Then tried heating the secondary caps with hot air. No difference.
Tried to ressolder the pwm section joints, in case there was a really stubborn one. Power it up, almost right voltages ( 4V and 12V ) .

Thinking about what I could be heating that was making a difference, I considered the optocoupler was something I had not replaced. So I made the test : power the boards up. Voltages too low ( 1V in the 5V line, 3.2V in the 15V line ) . Then I hold the solderin iron near the optocoupler while measuring voltages. After a little while, they climbed up to the correct values.

Now, is it more probable to be just the optocoupler, or some other part would be too weak to make it work, and when I heat the opto it somehow conducts easier ?