Led tv power supply no standby voltage

Thread Starter

KN4RUI

Joined Apr 3, 2019
6
Here it is..my nemesis....
1. no stb voltage......short somewhere.....
2. V810 needs a voltage on base to turn on..right? Allowing opto diode to shine.....passing 12v collector to emitter....right?
3. Why does V820 bring 16V from collector to emitter..already at 153V AC?
4. why does vz812 have both AC & DC even with C824 and C826?
5. note the different voltages on R827 at different power ups....?
6. Why couldn’t I have picked an easy hobby!
 

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AlbertHall

Joined Jun 4, 2014
9,022
Repairing SMPS safely is one of the most difficult things.
Note that the ground symbol on the left hand side of your diagram is different to the one on the right side. They are at different voltages and likely the right hand side one is connected in some way to the AC supply - CAUTION NEEDED!
 

MisterBill2

Joined Jan 23, 2018
4,859
Here it is..my nemesis....
1. no stb voltage......short somewhere.....
2. V810 needs a voltage on base to turn on..right? Allowing opto diode to shine.....passing 12v collector to emitter....right?
3. Why does V820 bring 16V from collector to emitter..already at 153V AC?
4. why does vz812 have both AC & DC even with C824 and C826?
5. note the different voltages on R827 at different power ups....?
6. Why couldn’t I have picked an easy hobby!
The lack of a voltage is more probably due to an open circuit rather than a short circuit., if there is no smoke.
 

MisterBill2

Joined Jan 23, 2018
4,859
Does the TS mean that there is no voltage at that point tagged as "STB"? Or is the complaint that there is no regulated output? Once again, as in many other threads, much information is missing.
A positive voltage at the base of transistor V810 should switch it on, causing current to flow through the LED in the opto-isolator, N881.
If there is already 153 volts AC at the point marked as "VCC" then the problem is not with the circuit shown so far.
SO now my questions: are the points marked 12 V and 20V at those voltages? AND where is the negative meter connection made to? We would guess the point shown as a common "ground", but given the marking of 153Vac I don't think so.
AND nobody should offer guesses until the TS responds. No use in making guesses without more data.
 

Tonyr1084

Joined Sep 24, 2015
4,036
Divide and conquer. First thing I'd try - and it's safe to do so - is to short the collector to the emitter on V810. It's nothing more than a switch. If doing so turns the TV on then the problem is on the right side. If not - it's either the opto or something on the left side. One thing confusing me is the 16 / 20 volts before V820 and the 153V Vcc. Doesn't seem to me like that would do much of anything when V820 switches on. But I'm also thinking the 20V is needed to turn on V820. If you're reading 16 volts, maybe the zener diode is pulling the voltage (current) down and not allowing V820 to turn on. Of course, I'm no expert. All I can do is raise questions.

And I definitely agree with @AlbertHall you're dealing with high voltages. "CAUTION NEEDED" is worth repeating.
 

MisterBill2

Joined Jan 23, 2018
4,859
STILL, nobody should offer guesses until the TS responds. No use in making guesses without more data. way to many questions and no answers yet. I certainly DO NOT re.commend bypassing switching transistors that appear to have wrong voltages on their terminals.
 
Thank you for helping me.
Right of the N881 opto, I agreed with the 12v on schematic. I had 0 volts at STBY. V810 not activated. Update and lesson learned. My current limiter on my Variac was flashing regular over current on the lamps. I reasoned I had a short when the controller sensed the over current..shutting the circuit down. Not so.....I removed the current limiting lamps and now have STBY at a proud 3.3V. Evidently there wasnt enough current flowing to START the circuit. Well that was three days wasted.
Left of N881 I measure 16V instead of schematic 20V. And yes, I must have had the grounds mixed when measuring, and i will recheck them. This section now providing STBY with the proper power up without the limiter.
I have moved over to the SW pin (switch on?) on the power board, as there is no voltage here. There is no voltage SW at the main board pin either. I assume that this voltage originates at the control board and is sent to the power board to activate the high voltage for the LEDs. I would inject this voltage to the SW pin directly to the power board if I had benefit of schematic but wont without knowing the voltages needed.....visual of main board looks good.
I still think power board is suspect.
Todays question: I have measured the voltages at V805 as in table at right with multimeter. V805 looks suspect....but these smd transistors were all measured in circuit. Is that completely worthless and should I need to remove them for usefull accuracy?IMG_1446.jpgIMG_1505.PNGIMG_1353.jpgIMG_1530.jpg
 

MisterBill2

Joined Jan 23, 2018
4,859
Thank you for helping me.
Right of the N881 opto, I agreed with the 12v on schematic. I had 0 volts at STBY. V810 not activated. Update and lesson learned. My current limiter on my Variac was flashing regular over current on the lamps. I reasoned I had a short when the controller sensed the over current..shutting the circuit down. Not so.....I removed the current limiting lamps and now have STBY at a proud 3.3V. Evidently there wasnt enough current flowing to START the circuit. Well that was three days wasted.
Left of N881 I measure 16V instead of schematic 20V. And yes, I must have had the grounds mixed when measuring, and i will recheck them. This section now providing STBY with the proper power up without the limiter.
I have moved over to the SW pin (switch on?) on the power board, as there is no voltage here. There is no voltage SW at the main board pin either. I assume that this voltage originates at the control board and is sent to the power board to activate the high voltage for the LEDs. I would inject this voltage to the SW pin directly to the power board if I had benefit of schematic but wont without knowing the voltages needed.....visual of main board looks good.
I still think power board is suspect.
Todays question: I have measured the voltages at V805 as in table at right with multimeter. V805 looks suspect....but these smd transistors were all measured in circuit. Is that completely worthless and should I need to remove them for usefull accuracy?View attachment 196827View attachment 196828View attachment 196829ion
Looking at very small parts of a circuit is OK AFTER the problem has been isolated to that part of the circuit.t It is NOT an efficient way to to diagnose a system problem because it does not allow understanding how the system functions.
AND on the left side of this small circuit we once again see a resistor connection tied back on itself. That damages the belief that the circuit may actually exist as drawn.
 
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