Which part in the LCD TV power circuit gets short?

Thread Starter

XiQueming

Joined Nov 28, 2017
18
I have a Samsung TV LN46A650A1F, it Kept on/off many, many times, then, it went on. Now, I changed 7 capacitors in the power board, the TV keeps clicking as soon as I plug it into the power. The strange thing is the TV flash light is on but I can't detect the stand by voltage 5.1. After I unplug the TV from the power, I use a multiple meter to check the rectifier diode and discover there is a short circuit by pass the diode. Any idea to fix it would be greatly appreciated.
 

Thread Starter

XiQueming

Joined Nov 28, 2017
18
when measuring the diodes, did you allow time to discharge the caps?
Good hint, thanks
After more than 20 hours, I tested again, the rectifier diode still to be short.
I try to post the picture of the TV power board. may be you can see what is going on.
 

Thread Starter

XiQueming

Joined Nov 28, 2017
18
the arrow points the 7 capacitors have been replaced, I circled the rectifier diode, somehow, it gets short. it seems like I can't detect the stand by 5.1 voltage.
 
Last edited:

absf

Joined Dec 29, 2010
1,949
PWB.JPG
Did you change the 2 caps inside red lines? Where is the point you said to get the standby of 5.1V ?

Allen
 

Thread Starter

XiQueming

Joined Nov 28, 2017
18
I unplugged two connectors, from the big one, pin 3rd, got no voltage. However, as soon as I apply power, the TV start clicking and the red light flashing.
 

BR-549

Joined Sep 22, 2013
4,938
If those caps are good....recheck polarity.

Can you easily remove board to dis-connect one end of the diode to check? It does look suspicious from what I can see. The DS80*** labelled one.

You may also check diode with sig gen and scope....without removing.
 

Thread Starter

XiQueming

Joined Nov 28, 2017
18
If those caps are good....recheck polarity.

Can you easily remove board to dis-connect one end of the diode to check? It does look suspicious from what I can see. The DS80*** labelled one.

You may also check diode with sig gen and scope....without removing.
When the circuit unplug from power, the rectifier diode DS805 is short( I used multiple meter to check, 0 ohm)
I replace a new one, the symtum is the same.
 

BR-549

Joined Sep 22, 2013
4,938
We do have tv experts on here, I am not one of them. Using the image in post#5 as a reference......it appears the power comes in at the top right. Then across to the left with some fairly heavy filtering. This filtered AC bus then turns down the center of the board to the other separate power supplies. I would suggest following these voltages and check operation(rectifiers and caps voltages) of the other supplies.
 

JoeJester

Joined Apr 26, 2005
4,390
Lift one end of the diode and use the diode test function on your meter. Ensure you reverse the leads. Report both readings.

Can you remove the power board and take a picture of the reverse side. It will help to trace out the circuit.

Have you tested the devices on the heatsink .... the one below those two blown capacitors,
 

Thread Starter

XiQueming

Joined Nov 28, 2017
18
Lift one end of the diode and use the diode test function on your meter. Ensure you reverse the leads. Report both readings.

Can you remove the power board and take a picture of the reverse side. It will help to trace out the circuit.

Have you tested the devices on the heatsink .... the one below those two blown capacitors,
Great idea, I did measure both side of that diode with out lifting one end, the reading was 0.
There are two transistors on the heatsink, should I take them out to test them?
It beats me, the print circuit of reverse side looks very complicated.
I will post some pictures, report the reading later.
 
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