Sharp LC-47SB57UT will not power on

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Are the standby circuit and the PFC circuit tied together? If so, why do you suppose I have full 5V at the 5V pins but only partial votage at BD901? If not, what would bring that 168V at BD901 up to the 320V that the troubleshooting guide suggests?

[tom66]

Re: Sharp LC-47SB57UT will not power on

The PFC circuit is separate from the standby supply, but sometimes it is powered by it. It boosts the voltage across the main cap from 168V to 390V or so. However, this only happens when the TV is turned on, as boosting the bulk voltage while it is in standby wastes a lot of power. This also won't affect the voltage across the bridge rectifier.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

The PFC circuit is separate from the standby supply, but sometimes it is powered by it. It boosts the voltage across the main cap from 168V to 390V or so. However, this only happens when the TV is turned on, as boosting the bulk voltage while it is in standby wastes a lot of power. This also won't affect the voltage across the bridge rectifier.

The trouble shooting guide says that BD901 should be at 320V when the tv is off and "about 400V" when the tv is turned on.

[budm]

Re: Sharp LC-47SB57UT will not power on

320V (230V x 1.414) for 230V line AC, 170V (120V x 1.414) for US 120V.
As I explain how the PFC works, that PS_ON is used to turn on the PFC circuits if you look at the diagram of the PFC, that is why I keep asking for you to find out what DC do you get at the 450v cap when you use that 1K resistor, You can use 2 batteries in series to get 3v to feed that PS_ON PIN, and if the voltage on those two caps still does not jump up to 380~400V , then the PFC is not working.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

320V (230V x 1.414) for 230V line AC, 170V (120V x 1.414) for US 120V.
As I explain how the PFC works, that PS_ON is used to turn on the PFC circuits if you look at the diagram of the PFC, that is why I keep asking for you to find out what DC do you get at the 450v cap when you use that 1K resistor, You can use 2 batteries in series to get 3v to feed that PS_ON PIN, and if the voltage on those two caps still does not jump up to 380~400V , then the PFC is not working.

+ of batteries in series to PS_ON and - to chasis ground?

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

With + of the batteries on pson and - on ground I measured 3.11v at pson referenced to the ground pin on the psu. When plugged in I measured 170v across c907, c908, and bd901

[budm]

Re: Sharp LC-47SB57UT will not power on

OK, now you will have to check the DC voltage on that Emitter and at the Collector of the transistor that supplies the VCC to the PFC IC, they should have about 12~15vdc, you will have to use the neagtive leg of the large filter cap as the ground ref for this measurement, if you do have the voltage, then the PFC IC is not running.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

With the psu plugged in and 3v applied via batteries to ps_on, the voltage at ps_on and 5vstby both fluctuate from ~2.5-3.5v when referenced to ground. All three pins of q906 and pin7 of ic901 fluctuate from ~7.5-9.5v when referenced to the - side of c907.

[budm]

Re: Sharp LC-47SB57UT will not power on

This is really confusing that the PS_ON is from external 3V power source that feed just one transistor Base drive, I cannot see why it will be fluctuating at all, are only getting 7.5~9.5V on the transistor which being fed by the startup caps voltage that you were getting about15~16v in post 63.
At 7.5~9.5V is not enough for the PFC IC to run, the PFC IC will not run if the VCC is less than 11V, min voltage to run is 11.8V.
So we are back at the 5VSTBY power supply again. I have to think what else that can cause the outputs (5V output and the AUX output for running the 5V SMPS IC and for running the PFC IC) to be low and fluctuating. I have to think more on how to isolate the problem.

[tom66]

Re: Sharp LC-47SB57UT will not power on

I am thinking perhaps another tap from the 5VSTBY supply that is shorted or has a shorted output diode.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I went ahead and ordered a used psu. It came in the mail today. The tv still won't turn on but I can force this psu and the backlights on. It looks like I may be dealing with a bad main board too. Any ideas about trying to troubleshoot this main board?

[tom66]

Re: Sharp LC-47SB57UT will not power on

Held down Ch- and Vol- while powering up? This can overcome some errors temporarily.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

No luck.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Used PSU and a brand new main. Not cheap at all. Lets just say there won't be much profit with this one.

[tom66]

Re: Sharp LC-47SB57UT will not power on

Great, looks nice. Thanks for repair info.