Sharp LC-47SB57UT will not power on

[tom66]

Re: Sharp LC-47SB57UT will not power on

Regarding your problem, can you check the voltage across those 100uF/10uF startup caps?

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

will do

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

100uf 50v = 16.2v
10uf 100v = 15.5v

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Can I use a computer psu to apply the 12v/24v to the appropriate pins?

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I've been doing some reading and it looks like using a computer PSU is out of the question. I wish i could identify the fault with this one.

[tom66]

Re: Sharp LC-47SB57UT will not power on

Connected to one of the caps there should be a resistor. Does this resistor test good? (Should be <100 ohms.)

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I'm not really sure which resistor is the one you are referencing. The only one that measures less than 100 ohms is JR905 (if that is a resistor) which measures 00.8 ohms when the DMM is set to 200 ohms.

*Edit* C927 and C928 are the caps I replaced, if you can't tell.

[tom66]

Re: Sharp LC-47SB57UT will not power on

That should measure zero, it is good.
How is D910?

I am fairly sure it is the startup section or a shorted output.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

The 0.79v on the 12v rail is odd. It could be a short on the mainoard.

If it is still like that when the PSU is powered on, while disconnected from the chassis, then there is an issue with the PSU.

That should measure zero, it is good.
How is D910?

I am fairly sure it is the startup section or a shorted output.

Do you think ben was on to something? Maybe the .79V on the 12V rail while in standby is due to a short on the output.

[budm]

Re: Sharp LC-47SB57UT will not power on

OK, I went back and read the voltage readings, you are getting all the correct signals (BL_ON: ON/OFF, and PS_ON) from the main board, but the 12v shows almost 0 and no 24VDC either which means that the transformer for the 12v/24v (both of these voltages are generated by the same transformer).
So you need to check the DC voltage on that large 450VDC caps C907, C908 on the two legs of the cap, it should have about 380~390VDC on it when you hit the power switch on the TV.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I will check them although nothing happens when I hit the power switch. I'm used to hearing components on power supplies make noises when plugged in and again when powered on. This one does neither. According to the trouble shooting guide, BD901 should have 320V at the output prior to power on and 400V after. I found BD901, it's a D10XB60 rectifier. I'm just not sure which leg is the output.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I checked BD901, C907, and C908. When checking across + and - of the rectifier, and both caps, I get 168V on all three. When checking each of the four legs on the rectifier against ground individually, starting at + and working my way up, I get
7.5V
0.0V
0.0V
2.0V

The trouble shooting guide says when following "NO" to whether or not BD901 has 320V,
"Does the PFC circuit operate normally? (L904, Q904, D902, IC904 and etc. And, the circuit around the protection circuit etc. is checked.)"

I'm still getting up to speed on the diagram. This will probably take me a while.

[budm]

Re: Sharp LC-47SB57UT will not power on

First, when you work on the hot side of the power supply, you need to use the negative leg of that 450V cap as the ground ref point, not the chassis of the TV, and be real careful since the hot side is deadly so do not touch any thing in the hot section with your body.
OK, the PFC Voltage boosting circuit is not working.
As you can see, the PS_ON (>2.5v) signal is fed into R953 which turn on Q908 which causes the LED in the OPTOISOLATOR IC904 to be on which turns on the Phototransistor on the receiving side of the IC904, this will turn on switched transistor Q906 which allows the VCC (generated by the AUX winding of the 5VSTBY transformer, this VCC is also used for running the 5VSTBY SMPS IC) at the Collector of Q906 to appear at the Emitter of the Q906 (there will be small voltage drop acoss Emitter/Collector). This voltage is used to run the PFC Voltage Booster IC901. So that is how the PFC is turn on. The 12v/24v required that boosted voltage (320V) to operate.
C909 is the filter cap for the PFC VCC pin7, so check the DC voltage on that cap.
Report the DC voltage reading of the PFC IC pin, use Pin1 for ground.
Then take the DC resistance reads between the ground pin PIN1 of the PFC and all other pins of the IC and report each reading.
Also check C918? (cannot read the print) 1uf/25V hwich is connected to pin4 of the PFC IC.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

First, when you work on the hot side of the power supply, you need to use the negative leg of that 450V cap as the ground ref point, not the chassis of the TV, and be real careful since the hot side is deadly so do not touch any thing in the hot section with your body.
OK, the PFC Voltage boosting circuit is not working.
As you can see, the PS_ON (>2.5v) signal is fed into R953 which turn on Q908 which causes the LED in the OPTOISOLATOR IC904 to be on which turns on the Phototransistor on the receiving side of the IC904, this will turn on switched transistor Q906 which allows the VCC (generated by the AUX winding of the 5VSTBY transformer, this VCC is also used for running the 5VSTBY SMPS IC) at the Collector of Q906 to appear at the Emitter of the Q906 (there will be small voltage drop acoss Emitter/Collector). This voltage is used to run the PFC Voltage Booster IC901. So that is how the PFC is turn on. The 12v/24v required that boosted voltage (320V) to operate.

Thanks for explaining that.

C909 is the filter cap for the PFC VCC pin7, so check the DC voltage on that cap.
Report the DC voltage reading of the PFC IC pin, use Pin1 for ground.

Which pin is the PFC IC pin? Pin 7?

Then take the DC resistance reads between the ground pin PIN1 of the PFC and all other pins of the IC and report each reading.
Also check C918? (cannot read the print) 1uf/25V hwich is connected to pin4 of the PFC IC.

That's C911. The writing is burried under the writing for R977, 100K OHM 1/8W.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

At 20k ohms on the meter,
Pin 1 - 8 = 9.98
1 - 7 = overload
1 - 6 = 11.07
1 - 5 = overload
1 - 4 = overload
1 - 3 = 0.23
1 - 2 = overload

Voltage across C909 = 0.1v

Voltage at every pin on IC901 is 0.1V except pin 6 which is 1.3V

[budm]

Re: Sharp LC-47SB57UT will not power on

What DC voltage you get at the Collector (VCC) of Q906? This VCC is from what you read earlier in post 63.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I believe I have identified the collector. According to the schematic, R921 and the collector of Q906 are connected, right?

[budm]

Re: Sharp LC-47SB57UT will not power on

yes.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

0.0v at the collector

[tom66]

Re: Sharp LC-47SB57UT will not power on

Measured relative to negative lead of big cap?