Sharp LC-47SB57UT will not power on

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I don't know what it means, or if I'm even testing it correctly, but with all four legs soldered in circuit, relative to ground, IC904 shows
5V at pin 1
4.22V at pin 2
0V at pins 3 and 4

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Measured relative to negative lead of big cap?

No, let's try again
13.59v at the collector.

[tom66]

Re: Sharp LC-47SB57UT will not power on

Vcc is good then.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

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

These are also showing
13.59v
12.91v

I'm not sure what happened that they changed. My other meter bit the dust last night and I'm using a cheaper meter right now.

[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

I must have measured wrong. I'm seeing 2.2V at pin 4 of IC901 now. Everything else is the same.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Any ideas why the voltage at those two big caps and BD901 is so low?

[budm]

Re: Sharp LC-47SB57UT will not power on

OK, we do have VCC at the Collector of that switched transistor but 0V at the Emiiter which is used to run the PFC IC by supplying the VCC to pin 7 of the PFC IC, so I want make sure that is going on.
By the way when you read the voltage at the Emitter, you did turn on the power switch, right? And you do see >2.5vdc at the PS_ON.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

No I didn't hit the power button. I don't know how I can hold the probes on those tiny little pins on the IC and hit the button at the same time.

[budm]

Re: Sharp LC-47SB57UT will not power on

All you have to do is to turn on the TV, verify that you do have PS_ON, the check the DC voltage on the Emitter of the transistor, I see why you are see 0V at the Emitter since you are not turning on the TV.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

The tv will not turn on at all. There are no indicators lit, there are no relays clicking, there are no transformers humming, there is nothing but standby power. With the unit plugged in and the psu connected to the main board, I have 1.4V at ps_on. With the psu disconnected from the main board, I have 0V at ps_on. With everything hooked up correctly, pushing the power button does nothing whatsoever.

[budm]

Re: Sharp LC-47SB57UT will not power on

OK, that 1.4v for PS_ON is too low to turn on the transistor. So the problem is on the main board. We will need the diagram of the main board to trace out that power on signals and voltages on the main board.

[budm]

Re: Sharp LC-47SB57UT will not power on

I would like you try again using 1k resistor between the ps_on pin and the 5v, but you will have to make sure the connector on to the main board is not connected. This should cause the voltage on the 450vdc cap to go up over 300vdc.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

I've already tried forcing the psu on. It doesn't want to come on. With a resistor between ps_on and 5v, ps_on fluctuates from 2.5v to 3.5v and back. I documented it all on page 1 & 2.

[budm]

Re: Sharp LC-47SB57UT will not power on

At 2.5~3.5V is enough to turn on that transistor in the PFC. If you monitor the voltage on the 450v cap, do you see it go up and down.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

With a resistor from 5v to ps_on and another from 5v to on/off I still have 168v at c907, c908, and bd901. All 3 legs of q906 fluctuate from ~7.5v - ~9.5v, as does pin 7 on IC901.

[tom66]

Re: Sharp LC-47SB57UT will not power on

Since PSON is fluctuating I guessed 5VSTBY was too. Is it?

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Yeah

[tom66]

Re: Sharp LC-47SB57UT will not power on

So this is where the problem is. We need to figure out why. Trace the standby power supply. Sometimes there are other outputs, such as 12V. If one of these has a shorted diode it can cause the symptoms you are seeing.

[johnboy1313]

Re: Sharp LC-47SB57UT will not power on

Since PSON is fluctuating I guessed 5VSTBY was too. Is it?

Just so I'm sure we're on the same page, 5VSTBY is the voltage coming from the 5V rails on the PSU, right?

[tom66]

Re: Sharp LC-47SB57UT will not power on

Yes, when in standby. There may or may not be a separate 5V rail which comes up when the TV is on.