Sharp LC-50UB30U. Completely dead, no power indicator.

[HAHOMETP]

Hello, folks.

The TV in question just died on its own. When plugged in with the rear panel off I can hear momentary hissing from the PSU. The LED indicator is off. No response to the POWER button.

PSU:
The fuse is good.
Varistors are good.
No bulging capacitors, nothing out of the ordinary.

CN102
Pins 1,2,3 (Standby) = 5v
No other voltages are present anywhere on the board.

My guess the MB is dead. Is there a safe way to connect CN102 Pin 1(+5Vsb) to either CN102 Pin 11 (Remote ON/OFF) or CN102 Pin 12 (Backlight ON/OFF) to verify my diagnosis?

Thx!

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

If you want to test the power supply board, you must remove the cables to the main board first.
You will then check the Voltage of the PD Remote on/off pin, BL_ON/OFF pin, Dim pins to see if they have built-in pull up-resistors or not, if they do then you should see >2V on those pins, if not you will need to connect the pull-up resistors.
You will use resistors (1K, for safety in case you jump the wrong pin) between 5VSTB and the REMOTE ON/OFF, BL-ON/OFF; you may need another resistors on the Dim pins if the backlights do not come on.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

If you want to test the power supply board, you must remove the cables to the main board first.
You will then check the Voltage of the PD Remote on/off pin, BL_ON/OFF pin, Dim pins to see if they have built-in pull up-resistors or not, if they do then you should see >2V on those pins, if not you will need to connect the pull-up resistors.
You will use resistors (1K, for safety in case you jump the wrong pin) between 5VSTB and the REMOTE ON/OFF, BL-ON/OFF; you may need another resistors on the Dim pins if the backlights do not come on.

Thank you for the reply.

Both cables from PSU to MB removed.

If REMOTE ON/OFF pulled up to 5v via a 1k resistor:

CN102 +12v jumps to +11v and slowly falls down, 5 seconds later the process repeats.
CN103 +12v jumps to +11v and slowly falls down, 5 seconds later the process repeats.

CN105 LED1 jumps to +2.7v and slowly falls down, 5 seconds later the process repeats.
CN105 Vout1 jumps to +57v and slowly falls down, 5 seconds later the process repeats.
CN105 LED2 jumps to +3.1v and slowly falls down, 5 seconds later the process repeats.
CN105 Vout2 jumps to +57v and slowly falls down, 5 seconds later the process repeats.

If BL-ON/OFF also pulled up to 5v via a 1k resistor:

same as above.

If Dimming1 and Dimmng2 also pulled up to 5v via 1k resistors:

CN102 +12v jumps to +10v and slowly falls down, 5 seconds later the process repeats.
CN103 +12v jumps to +10v and slowly falls down, 5 seconds later the process repeats.

CN105 LED1 jumps to +20v and slowly falls down, 5 seconds later the process repeats.
CN105 Vout1 jumps to +82v and slowly falls down, 5 seconds later the process repeats.
CN105 LED2 jumps to +20v and slowly falls down, 5 seconds later the process repeats.
CN105 Vout2 jumps to +82v and slowly falls down, 5 seconds later the process repeats.

During each of these 5 second pulses the backlight flashes several time like a stroboscope with a frequency of several hertz, and this frequency decreases during the pulse cycle.

To measure the above I had to break out my trusty 1958 analogue multimeter since the digital multimeter was going all over the scale during the pulse giving uncertain readings.

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

During the test, does the 5VSTB stay at 5V? If it does, then we have problem on the 12V power supply, it is bad or something causes it to go into shutdown/restart mode.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

During the test, does the 5VSTB stay at 5V? If it does, then we have problem on the 12V power supply, it is bad or something causes it to go into shutdown/restart mode.

Yes, the 5VSTB is rock solid to the 3rd place after the decimal.

So it's not the MB... Will be pulling the PSU off the chassis tomorrow and checking every p-n junction on the board.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Quickly tested near every semiconductor in situ (except a few SMD chips on the track side) - no shorts, all show the expected voltage drop in the forward direction.

Could it be that the backlight strips have a few burnt LEDs that trigger the shutdown like the old fluorescent tube monitors did when one tube was out?

OTOH the standby LED on the front never comes on when the TV is plugged in. So if the +12v part of the PSU is out then the MB never gets powered up thus the MB never tells the standby LED to come on. This probably indicates a defective PSU.

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

I forget to ask, when you test the power supply board by itself and use force on resistor. How low did the 12V go down to?

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

BTW, I need good clear high res straight shots of the whole bottom side of the power supply board so I can do circuits tracing.
I also think you may have problem with the main board since the standby power supply is OK, but right now we want to make sure the power supply board is OK.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

I forget to ask, when you test the power supply board by itself and use force on resistor. How low did the 12V go down to?

With all four ON/OFF CN102 pins pulled up to +5v the +12v pins jump to +9.8v, slowly fall to +6.4v and then jump to +9.8v again. These were taken with an analogue voltmeter.

BTW, I need good clear high res straight shots of the whole bottom side of the power supply board so I can do circuits tracing.
I also think you may have problem with the main board since the standby power supply is OK, but right now we want to make sure the power supply board is OK.

Will do so in a lightbox with an SLR camera when get back home from work later today.

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

The +12V is generated by the same largest transformer that also generate the 57V to feed the two backlights boost converter LED power supplies, that is why you see the backlights flashing, that power supply section is not stable, so try it again without having the LED backlights connected and see if the 12V is steady.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

The +12V is generated by the same largest transformer that also generate the 57V to feed the two backlights boost converter LED power supplies, that is why you see the backlights flashing, that power supply section is not stable, so try it again without having the LED backlights connected and see if the 12V is steady.

With the backlight disconnected the +12v pins still jump to +9.8v and slowly drop to +6.4v in about 15 seconds. Attached are the pictures of both sides of the board, top and bottom.

I am beginning to suspect a cracked transformer Ferrite core.

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

I really have to trace out the circuit where Transformer T150 is to see what is driving it, etc. so it may take me awhile to figure out what is going on, I am really busy right now with so many threads.
I download all your pictures so I will have look through them.

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Thank you, budm!

[R_J]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Can you provide the number for IC150? Also what is the voltage across C120a or C120b, This voltage should be around 160vdc in standby and increase to around 380vdc when turned on, (this is the pfc voltage) If it is not coming up or changing it might be a simple as a bad C155

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Can you provide the number for IC150? Also what is the voltage across C120a or C120b, This voltage should be around 160vdc in standby and increase to around 380vdc when turned on, (this is the pfc voltage) If it is not coming up or changing it might be a simple as a bad C155

IC150: SSC9522S

C120a and C120b:
STANDBY: +164v
REMOTE ON: jumps to +340v, then drops down to +220v in about 13 seconds, then jumps to +340v and so on.

C155: 10uF x 50v, 105°C, Rubycon, 9.3uF actual as measured out of the board.
Replaced anyway just in case with a 10uF x 50v, 125°C, GXE, 11.1uF actual as measured out of the board.

No difference.

[budm]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

So somehow the PFC Boost circuit is not staying on steady.

[R_J]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Have you tried disconnecting CN103 to see if the 12 volts stays up?

The pfc voltage does seem low, it is usually around 385vdc, when it is turned on it should stay steady, It looks like the pfc ic is ic120 (post its number also) C124 is be the vcc supply filter for the ic so it could be low causing the ic to shut down and restart. The VCC supply for the pfc and main ic150 comes from the standby circuit (C603) and is switched by a small transistor, likely by Q601 which is both a switch and a regulator, ZD604 and D605 should be connected to Q601 base and they set the VCC voltage on the transistors emitter, usually around 15~16 volts. It looks like PC601 is the optocoupler that turns on Q601.
C603 should have around 18~20v and connects to Q601 collector. R613 connects to one pin of pc601 and the other pin connects to Q601 base. So check that C603 voltage is good and steady, then check Q601 base and emitter voltages

[HAHOMETP]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Have you tried disconnecting CN103 to see if the 12 volts stays up?

The pfc voltage does seem low, it is usually around 385vdc, when it is turned on it should stay steady, It looks like the pfc ic is ic120 (post its number also) C124 is be the vcc supply filter for the ic so it could be low causing the ic to shut down and restart. The VCC supply for the pfc and main ic150 comes from the standby circuit (C603) and is switched by a small transistor, likely by Q601 which is both a switch and a regulator, ZD604 and D605 should be connected to Q601 base and they set the VCC voltage on the transistors emitter, usually around 15~16 volts. It looks like PC601 is the optocoupler that turns on Q601.
C603 should have around 18~20v and connects to Q601 collector. R613 connects to one pin of pc601 and the other pin connects to Q601 base. So check that C603 voltage is good and steady, then check Q601 base and emitter voltages

Yes, CN103 was disconnected.

PFC voltage was taken with the old analogue multimeter with the range set to 1,000v DC. If taken with the digital multimeter with MAX HOLD activated the PFC voltage first jumped to +390v; in subsequent cycles it showed +340v, +380v, +374v, +347v.

IC120: 1608B, like you said is a PFC Controller.

C603 voltage across its terminals is fluctuating between +11.62v and +11.74v, so yes, it is steady.

Q601 collector, base and emitter voltages are all zero relative to the chassis.

Q601 collector is the same as C603 <+> so it is @ +11.7v relative to C603 <->, steady.
Q601 base @ +11.6v relative to C603 <->, steady.
Q601 emitter @ +10.9v relative to C603 <->, steady.

[R_J]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

Remember ALL voltages on the primary or HOT side use hot ground not chassis ground.
Those voltages seem low, the ncp1608b startup threshold is 11 volts, this supply would come from Q601 emitter.
It could be that C603 is weak, Can you make out the markings on ZD604?

Even the startup VCC for the main ic ssc9522s is 11.8 so the voltages are just enough to start but likely not enough to maintain operation. IE if the vcc drops to 9.8v the ic will shut down, this voltage can drop and your meter would not pick that up.

[R_J]

Re: Sharp LC-50UB30U. Completely dead, no power indicator.

You may also want to check that D121A, D121B, D120A & D120B are ok and not shorted. Sometimes if one of the diodes short the pfc will not operate to full boosted voltage, this is not likely the issue but worth checking.
ZD604 marking would help as that will confirm the vcc operating voltage.