LC-37D43U Sharp LCD, No power No standby lights

[Glitcher]

Hey I need a little help with this Sharp 37" LCD TV. I opened up the TV, there are no bad caps that I see anywhere. I checked the 5V output and there isn't any. I tried probing around the board and I can't seem to find any voltages anywhere, on the secondary (If i did it right, black probe on ground and red probe on the board)

There are two large caps on the power supply and I checked the AC voltage on both and they measure about 169V, so I think the 120VAC is going into the board fine.

What am I missing?

and thanks for the help.

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

I've highlighted some resistors.

The two smaller ones: Check the resistance and see that it is under 100 ohms.
The larger one: Check the resistance, and also with the power supply powered (mains connected) check the voltage on both ends, red probe on each end, black probe on primary ground.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

Thanks for the fast reply. on the first post I made an error, the voltage on those two biggest caps is actually oscillating between 162.0V-162.8V (not 169V)
I measured the two smaller resistors (in circuit) and they both measure 0.4 ohms.
The larger resistor measures 2.5 ohms. Measuring the voltage on both ends of the resistor I get about 162.0V-162.8V DC on both sides.

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

162V is still fine.
Those resistors are fine too.

See image:-
3 pin device = Should be a diode. Check for shorts between pins 1, 2, and 3. (1->2, 2->3, 1->3) and read off device code.
7 pin device = Read off device code.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

the 3 pin device I'm measuring: (in circuit)
pin 1-2: 215 ohms, and 1450 ohms in the other direction
pin 2-3: 215 ohms, and 1450 ohms in the other direction
pin 1-3: 0.4 ohms in both directions

Its labeled:

STPS2045CT
7SACS VW
PHL 708

For the 7 pin device, there seems to be a metal piece on top of it?

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

The 3 pin device is good.
7 pin device is heatsinked.
Check the 5VS -- please make certain you have not got 5V standby.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

Is it these pins? Whats the best way to measure these pins? I have the board out of the chassis right now, does everything have to be plugged in to order to get the readings?

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

Probably not...
Just check along the connectors going to the main board from the PSU. Each pin. If you find 5V, the PSU is giving a standby output.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

Ok so, I have the PSU board out and with only the main AC plugged into it. I'm measuring all the pins showed, with black probe on the metal piece where the screw hole is.

It all reads 0V except for the pin labeled BU5V, which measures oscillating between 0.29V to 0.33V

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

Ah, the low oscillating voltage tells me one of three things
- either there is a short on the main board; or
- the PSU has a bad startup capacitor; or
- there's a short on the PSU somewhere

Can you source a 22 ohm, >=2W resistor?

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

I manage to find a 22 ohm resistor, but i'm not sure of the wattage.

Which would be the start-up caps on the psu?

And how did you know there a short on the main board? When I tested the voltages it wasn't connected to the main board but only connected with the AC main.

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

Well, looks like 1 watt maybe... as long as you don't run it for too long should be fine. (May overheat and burn if left for too long -- max 5 minutes.)

Put it across the BU5V output with the main board disconnected.

If the 5V is then steady: Main board
If not: PSU

I do not yet know where the fault is until you do these tests.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

I'm confused so where exactly do I put the resistor?

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

From BU5V to GND.
Main board must be disconnected.
Then, while doing this, measure BU5V.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

Ok so hopefully I did this right. I soldered one end to the BU5V and the other to the screw hole as seen in the photo.

I disconnected the two cables shown that go to the main board.

Measuring with black probe on ground and red probe on the BU5V I'm now getting a measurement of oscillating 0V to 0.09V.

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

Hmm, seems to indicate a problem with the PSU. Can you try a 100 ohm resistor too? Just to be certain...

[selldoor]

Re: LC-37D43U Sharp LCD, No power No standby lights

Measuring with black probe on ground and red probe on the BU5V I'm now getting a measurement of oscillating 0V to 0.09V.

That IS with the mains power ON?

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

With the main 120V AC connected yes.

[Glitcher]

Re: LC-37D43U Sharp LCD, No power No standby lights

Ok, I just put in the 100 ohm resistor, now i'm getting a measurement of oscillating 0V to about 0.16V

I was just searching on ebay for the PSU, there arent much at all, but there are tons of repair services for this particular board and TV. Guessing this might be a common problem?
I found this one:
http://www.coppelltvrepair.com/p/102...r-sharp-lcd-tv

Stating:
"The most common failure in that board results in a completely dead TV - no front LED, no response to buttons, nothing."

I just wish they would say which part is bad lol.

[tom66]

Re: LC-37D43U Sharp LCD, No power No standby lights

OK, the first place I would look is at the startup capacitor. This will be a small electrolytic capacitor next to the 7 pin IC.