Samsung LN40B500P3F with an odd problem

[baitism]

I bought this from a guy who tried to fix it by ordering a new PS/inverter board from Hong Kong. The board they sent him was all effed up so he gave up.

Here is the weird thing. When I plug in the power supply it seems to run alright. Some of the voltages are a little low, though. However, when I plug it into the tv, the backlight comes on just for a split second and then goes off; the tv is always unresponsive. I also noticed that the pins that attach to the cable that runs to the main board don't received voltage when it is plugged into the tv, but do when it is not plugged in.

I am kind of at a crossroads for what to do. The four main capacitors that usually fail all test well.





The sloppy soldering job was the sellers...

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Update: I put this on the back burner for awhile, but I am back to it with some new info.

The power supply tests out fine by itself. However, when it is plugged in the LCD panel it does not. What could possibly cause that? A short somewhere or is there a component farther down the line malfunctioning? Thanks.

[tom66]

Re: Samsung LN40B500P3F with an odd problem

Check the voltage across the large 450V capacitors both in standby and when powered up.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Alright Tom. I will try that. I am using this thread, too https://www.badcaps.net/forum/showthread.php?t=19681 because it seems like it could be a similar problem.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

So I am getting ~66 volts on the two large ones. Here is the odd part. For the first time ever, I plugged it in and the backlight stayed on (I had it disconnected from the main board), but I moved it a couple inches and it went back off. So, the back light functionally works.

[budm]

Re: Samsung LN40B500P3F with an odd problem

What are you using for the ground point when you measure that voltage with your meter?
"I plugged it in and the backlight stayed on (I had it disconnected from the main board)" That is how you verify if the power supply/inverter board is working or not on this chassis by disconnecting the wires going to the main board. So it looks like the main board is not sending the BL_ON to turn on the backlights. The backlights should come on and stay on when the TV is plugged in, if it goes off then it is not working.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Metal workbench I have. I got the backlight to come on briefly again. Transformer pulling to much voltage, perhaps?

[tom66]

Re: Samsung LN40B500P3F with an odd problem

Measure across them: red one end, black the other. Only need to measure one, they are in parallel.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Measure across them: red one end, black the other. Only need to measure one, they are in parallel.

I just did that. Is it abnormal for the volts to jump around a lot. As in, I couldn't get a consistent reading.

[budm]

Re: Samsung LN40B500P3F with an odd problem

Batteries are good for the meter?
By the way, when work in the primary side of the circuit, do not use Safety ground/ or chassis as the Ground ref point, the ground ref point for the primary circuit is the negative leg of the large filter cap.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

161. I was on AC. Will all these be DC?

[tom66]

Re: Samsung LN40B500P3F with an odd problem

DC for those caps. 161V on DC?

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Yeah. I just redid it and got 163v DC. Also, in the limited time I had to test them before the backlight went off, I checked all the transformers and one of them isn't giving me a consistent reading. Those should be checked on AC, correct?

On a side note, is there a diagram or instruction sheet that tells someone fairly new to troubleshooting this stuff where to test for AC and where for DC? Thanks.

[budm]

Re: Samsung LN40B500P3F with an odd problem

The voltage on the caps should be over 300VDC when powered up.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Update! With the backlight on, the caps read ~390VDC. When the backlight turns off it drops to ~160VDC.

I have noticed the transformer shown below seems to have inconsistent voltage readings. Could that be causing the panel to shut down?

[baitism]

Re: Samsung LN40B500P3F with an odd problem

To further expand. From top to bottom, the transformers read:

With the backlight on: 2VAC, 2VAC, 4VAC, Inconsistent reading

After the backlight shuts off they all read ~2VAC.

So is the culprit the 3rd or 4th transformer, or none of the above. I feel this is where the problem lies, but I am no expert.

[budm]

Re: Samsung LN40B500P3F with an odd problem

You cannot use the typical low voltage AC meter to read the high voltage output of the inverter transformers, it puts out up to 2000VAC or more at start up and drop down to 800~1000V when they stay on, the frequency is about 70~100KHz which way too high for your meter to read. I hope your meter input circuits did not get damaged when you try to read the output of the inverter transformers.
You need to check the resistance of the high voltage winding of the transformers, no power to the TV of course.

[tom66]

Re: Samsung LN40B500P3F with an odd problem

You shouldn't test such a transformer with a DMM while energised. Your meter may have been damaged by the high voltage.

[baitism]

Re: Samsung LN40B500P3F with an odd problem

Where are the windings? Thanks.

[tom66]

Re: Samsung LN40B500P3F with an odd problem

On the bottom of the board, clockwise, the first two are probably one secondary, second two other secondary and the final two on the other side will be the primary.