AOC TV LE22H138 Power cycle when trying to turn it on

[vitim]

I have a 21" AOC TV that is not turning on, the standby led (orange) is ON but when trying to turn it on, the led goes on blue for a second and it shuts off and goes back to stand by (orange).

There's a whine noise coming from the transformer.

I couldn't notice anything obvious in the visual inspection, while I know it is probably something wrong with the power supply, I don't know where to start.

I don't have schematics, but the circuit is fairly simple as it is a single sided board. I included the best pictures I could take of the top and bottom, and a combined top/bottom image.

From the quite loud noise coming from the transformer I suspect that it is not switching as fast as it should be, due to some dry cap, but it is just a guess.

Any suggestion of where should I start?

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

You cab=n start by reporting the DC Voltage of the pins of the connector on the power supply board that goes to the main board. You must report the pin name and follow by the Voltage reading for each pin. Black probe of the meter on the chassis.
For example:
+5.2V = xx V

[vitim]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

I will get a meter, and report the voltages back.

Until then, I recorded a video about the high pitched noise I mentioned.

https://youtu.be/if9sb0EM0zI

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

Can we see straight shot pictures of the whole bottom side of the power supply board?
Never mind, the second pictures of the bottom side is fine for me to see the traces..

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

Does it still squeal without the backlights LED connector in place?

[vitim]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

Can we see straight shot pictures of the whole bottom side of the power supply board?

Please check the pictures 2 and 3 attached on the first post, I don't think I can get better pictures than that.

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

I was looking at number two picture with that see through of the top side showing but then I look at number 3 picture and that one is OK for me to see the traces.

[vitim]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

Does it still squeal without the backlights LED connector in place?

Removing the backlight connector makes no difference. (still squeaking, tv doesn't start either)
Removing the main board makes it quieter, but it stills audible.

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

It is either the main board is drawing too much current or problem in the power supply with failed parts so it cannot handle load.
Unplug the TV from the outlet and use hair dryer to heat up the entire board for about 3 minutes and the try again to see if the squeal is still there.
I suspect bad cap C928 in the hot side of the circuit.
Also look real close at the legs of the main filter caps C904, C905 to see if there is some kind of corrosion on the legs of the caps.

[vitim]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

I will replace C928, but the main filter caps c904 and c905 just after the rectifier are holding charge for quite a while, after plugging the mains, so they are not completely dead, although they can be out of spec, I don't think I would blame then just yet. Or do you think they can still be the cause? Or ripple on the primary is that critical? I'm just speculating here, I don't really know

I thought about the main board, but the PSU is squealing by itself with nothing connected on it, so definitely theres something wrong with it. Although the main board can be the cause the PSU gone wrong in the first place.

[budm]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

"I thought about the main board, but the PSU is squealing by itself with nothing connected on it, " When I watch your video, I did not hear the squeal until you put the main board connector in place.

[vitim]

Re: AOC TV LE22H138 Power cycle when trying to turn it on

It gets louder, when I plug the main board, but it still there, and I don't mean the usual coil wine that stuff usually does. And when it was OK it didn't squeal anyway.