SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

[Locutor13]

The TV and keeps clicking on-off on power cycling, sometimes it does the whole power cycle and I can see backlight turning on before it turns back off but most of the times it stop before the red led blinks for 3 times. When it finally randomly turns-on (after several power cycles) the TV works fine.

So far I have verified that all the electrolytic caps on the PSU are ok and that there is no apparent bad solder joints. Also I was able to update the firmware but it made no difference.

I was wondering if this may be caused by a bad tantalum SMD caps on the main board as happen with other TV (LCDs or Plasma) that were manufactured in year 2008.

Any help will be highly appreciated since right now I have no idea what to do next.

I am attaching pictures and specs diagrams of the PSU.

[budm]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

Like the cap C102 in the LN5xx series:
https://www.badcaps.net/forum/showth...t=25810&page=7
https://www.badcaps.net/forum/showth...LN46A500T1FXZA

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

Thanks Budm,The main board the LNxxA750 is a little different from the series A5xx, but it still has what it looks like tantalum SMD caps that might be causing the problem.
Unfortunately the specs diagrams for the main board I got from elektrotanya are too big to be uploaded and I am not sure how to determine if this is the problem. Any ideas? Thanks.

[budm]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

Did you try heating those suspect caps with hair dryer to see if the TV will function normally?

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

I tried the hair dryer and it didn't make a difference so I went back to the PSU and found that when I plug-in the TV to wall after it clicks the STB voltage is only of 2.6V instead of 5.1V and when it turns-off it goes to 0 volts and then back to 2.6V in cycles.

My guess is that when it turns-on the STB goes to the 5.1V but I would have to leave it to rest for a while to get it turn-on back again and verified that reading.

I tried the hair dryer also with the caps of the STB circuit and made no difference.

[budm]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

STB5.1v PIN 3 of the connector CN801 goes down to 2.6V?

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

On PIN 3 of connector CN801 I am getting only 2.6V or 0V. Never 5.1V

[Terryfictv]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

For a test, unplug all the connectors on the power board
except the ac in and ground wire....plugin and recheck standby
voltage....change ?

[budm]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

On PIN 3 of connector CN801 I am getting only 2.6V or 0V. Never 5.1V

Are you sure you are measuring pin 3 sinc epin 3 is the always on stndby power supply, for it to go to 0V means it went dead. May be you are on PS-ON pin.

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

Sorry, I got confused with the CN801 connector's labels on the power supply so I followed the labels in the main board where every pin is individually described and found the following:
I get a steady 5.1 STB V while the TV is power cycling or when it is in STB mode.
On the pins where I should get 13V when the TV won't turn-on it I get a max of 2.6V if the TV turns-on then I get 12V.
SW-PWR: I get a max 4.3V.

Also I took the voltages of the connector CNI 803 and found the following voltages while the TV was power cycling (not turned-on):
DET_5V= It goes from 0 to 5.1V
PWM=0.0 or 0.1
ON/Off=0.0 or 0.1V

Might a problem with the PSU?

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

I tested the PSU as shown on the shopjimmy’s video: www.youtube.com/watch?v=n54IurMvr6o and I got the backlight right away and all the voltages.

So I am going back to troubleshoot the main board and I’ll start testing the tantalum caps. I have found one that looks like it is somehow the equivalent of the C102 47UF 10V of the main boards of the Samsungs A5XX, it is label as C132 33UF 10V. I am planning to take it out of circuit and test it.

Can a 33UF cap be replaced with a 44UF tantalum cap?

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

The 33UF tested ok but I found another possible cause of the problem, the connector of cable CN801 that connects the power supply to the main has 3 burnt female pins jackets (see picture).

This is a 24 2X12 double pin cable and the burnt pins are the ones that conduct the 12v which I don' get when the TV won't turn-on.

Where and how I can find a replacement for this cable?

[Locutor13]

Re: SAMSUNG LCD LN52A750 POWERS ON & OFF MAIN BOAD or PSU??

Since I was unable to find a replacement cable; I cleaned the male connectors of the main and the PSU boards and the female connectors on both sides of the cable with a ``contact and head cleaner`` and turned the cable around. I connected the burnt side of the cable to the main board and the other side to PSU and got the problem solved.