Electronics > Repair
Samsung PS51D8000FV power supply clicking
BoringName:
Hi,
This TV worked and then just went off one day and started clicking. Usually 5 or 6 times and then it would stop and the standby light would do 5 double flashes and repeat. If it was left off for a few hours or overnight it might work again in the morning but only for about 20 minutes. Now it doesn't work at all, just does the clicking.
The power supply board is a BN44-00446A
I found a couple of capacitors with high ESR and replaced them, that didn't fix it so I replaced a bunch of others. The only ones I haven't replaced are the high voltage (filter capacitors?). 450V 63 uF x 4 and 250v 180uF x 2 but they all tested good with an ESR meter and seem ok.
I unplugged all the boards from the power supply and shorted PS_ON to ground. It only clicks once and all the voltages seem to be ok. Standby at 5.21V along with all the other 5 volt points and 15.39V for all the 15 volt points. Rock steady with no fluctuations.
I bridged VS_ON to a 5.3 volt pin and this makes it do the clicking thing. From everything I have read this should make the board power up and supply VS and VA voltages. That's why I think the power supply is the issue with the TV. When it is doing the clicking thing, the VS and VA test point voltages seem about right. 205V and 58V. But they pulse on an off with the clicking.
I've tested all the resistors and diodes on the top of the board, they are ok. I haven't gone through the underside yet as there is heaps of surface mounted elements. I've looked over the board and can't see anything blown up or bad solder joints. I've tried looking for a short but some of the rails are connected to ground with a resistor so it's been tricky. A schematic would be great!! I refreshed the solder on all the transformers. I was hoping someone could point me in the right direction. It seems to work ok until it needs to supply the high voltages.
There is a replacement board at mxmee but once they tack on ridiculous postage charges and fees it will be over $200 and I would like to try and avoid that.
Thanks.
edit: Just found a service manual so I might be alright.
edit: Nope, service manual was pretty much useless.
BoringName:
Just trying to get my head around something.
There is a transformer at the bottom right of the board if you google BN44-00446A.
It has 5 pins on each side. Currently all 5 pins on the right are shorted to ground. 3 of them go to some rectifiers MBRF20100CT and SDURF1030CT. They both do the same thing but with different voltage ratings. Pins 1 and 3 are diodes to pin 2.
From top to bottom on the right of the transformer -
Pin 1 is on a small trace with just a diode on it connected to another trace. The transformer side of the diode is shorted to ground, the other isn't.
Pin 2 is on the same trace as pins 1 and 3 of SDURF1030CT. Pin 2 of SDURF1030CT is not shorted to ground. There is a capacitor between the trace and the trace pin 2 is connected to.
Pin 3 has a path to ground.
Pins 4 and 5 are on the same trace with pins 1 and 3 of MBRF20100CT. Pin 2 of MBRF20100CT is not shorted to ground. There is a capacitor between the trace and the trace pin 2 is connected to.
I can't see anything else on those traces that could be shorted to ground. The 2 capacitors seem ok but even if they weren't, they are not connected to ground.
So I guess the transformer is confusing me. It looks like there are 4 pairs of wires on each side which is 4 primary and 4 secondary windings right? Pins 2, 3 and 4 have two wires connected, other 2 pins only have one. I'm assuming there would probably be some continuity between some of those pins but it's confusing me.
Surely the whole right side of that transformer shouldn't be shorted to ground right?
Could it be possible that MBRF20100CT and/or SDURF10300CT are shorted to ground through the package?
BoringName:
Ok it seems I may have been misled by a video claiming you can just short VS_ON to a 5 volt line and the TV will give VS and VA voltages. I think it also needs a signal from the logic board so that could be causing the clicking because that signal is missing. One of the pins to the logic board is VS_Con so that's probably it. I already replaced the Xmain board so my focus is on the Y board now.
The TV does turn on with the main and logic boards connected but the logic board shows an error code with the LED due to the missing X and Y boards. I got sidetracked with the power supply clicking with the VS_ON bypass. Frustrating.
Anyway.... lets see how the Y board goes.
BoringName:
I'm focusing back on the power supply. I'm 99% certain it should power up with PS_ON jumpered to ground and VS_ON jumpered to a 5 volt supply.
While I couldn't find a schematic for BN44-00446A, I have found a schematic for BN44-00333A which is a very similar supply.
It has a PIC16F882 (ICQ802) microcontroller that seems to be the brain of the power supply. The schematic details what each pin is so I'm confident I should be able to test each pin and narrow down what part of the board is failing.
BoringName:
I realise I'm talking to myself at this point but I'll keep going for posterity.
So, even though BN44-00333A has a very similar layout, it uses different microcontrollers. Referencing some other schematics and through some tracing I've worked out the pinouts.
Probably goes without saying that some of these might be incorrect, it's just my best guess with my limited experience.
MCV14A - This one seems to do most of the work.
Pin - Description
1: VDD - powers the chip. 5.2 volts. This is constant even when the relay is clicking.
2: VS_ON - Supplied by the logic board, I currently have it jumpered to one of the 5 volt rails.
3: AC_DET - While the board is powered it shows 1.5-2.0mv but once the power is switched off it sits at 4.35V, supplied by capacitors I assume. Sits at zero without VS_ON jumpered.
4: PS_ON - Normally connected to the mainboard. I currently have it jumpered to ground.
5: PFC_OK - Is what it's supposed to be, on this board it just goes to a 1k ohm resistor and a spot for an LED that is not present. This sits at 4.38V while the relays are clicking and without VS_ON jumpered
6: Main_On - Pulses between 0 and 9 volts while the relay is clicking. Seems a little high. It goes to an optocoupler linked to the hot side, pretty sure this is what turns on VS and VA voltages.
7: B15V_ON - Forgot to measure this one....
8: B5V_ON - pulses between 4.6v and 9 volts while the relay is clicking. 4.87v constant without VS_ON jumpered.
9: Relay_ON - Pulses between 4.7 and 8.7 volts while the relay is clicking. 4.7v constant without VS_ON Jumpered
10: A5V_ON - 4.91V this stays constant the whole time.
11: D15V_DET - Sits at 2.7V
12: VS_DET - Fluctuates between 3.7V-3.8V while the relay is clicking. The Capacitors probably smoothing that one out.
13: A5V_DET - 2.59 volts.
14: GND
2A20118B - Manages the PFC.
Pin - Description.
1: ZCDB - Goes to pin 1 of first PFC via a 31k ohm resistor. Sits at 0.552v
2. ZCDA - Goes to pin 1 of the second PFC via a 31k ohm resistor. Sits at 0.465v
3: VINAC - Goes to pin 2 of the first PFC via a bunch of resistors. Sits at 4.66v
4: No idea - Goes to a grounded capacitor. Sits at 4.99v doesn't fluctuate at all with the clicking.
5: No idea - Goes to a grounded capacitor via a 82k ohm resistor. Sits at 4.93v
6: No connection
7: No idea - Goes to ground via a 32k ohm resistor. Sits at 1.64v
8: No idea - Goes to a grounded capacitor. Sits at 3mv
9: AGND
10: No idea - Goes to a capacitor that goes to ground via a 12k ohm resistor. Also a second grounded capacitor and a 1M ohm resistor to ground. Sits at 4.3v doesn't fluctuate.
11: VSENSE - Not sure if this is HVSENS or VSENSE. Sits at 2v doesn't fluctuate. Capacitors keep it constant.
12: HVSENS - Not sure if this is HVSENS or VSENSE. Sits at 2v doesn't fluctuate. Capacitors keep it constant.
13: No idea - Goes to a grounded capacitor. Rises to 400mv, goes OL then back to zero and repeats. Same with VS_ON Jumpered or not.
14: No connection
15: No idea - Goes to ground via a 220k ohm resistor
16: No idea - Goes to ground via a 220k ohm resistor
17: GDB - Sits at 0.9mv
18: PGND
19: GDA - Sits at 0.6mv
20: VCC - I think this supplies power to the chip and is also the collector for the transistors where GDA and GDB connect to the base. Sits at 16.38v doesn't fluctuate
From what I can see, MCV14A is the only thing that controls the relay so when it's clicking on and off, that is this chip doing it. Given that the chip never loses power and AC_DET/PFC_OK behave the same way regardless if VS_ON is jumpered, I'm assuming this chip is either broken or performing a reset due to VS_DET not hitting a threshold or another chip cutting the power ie) the DET pins drops to zero, it's going to be 2A20118B or the PWM chip (SQT8011K).
I have found instructions on how to bypass MCV14A but I think I will leave that as a last resort and keep checking 2A20118B/SQT8011K first.
edit: Added voltages for the 2A20118B chip. That's the voltage they sit on without VS_ON jumpered. Unless it's stated they don't fluctuate, when VS_ON is jumpered they generally fluctuate a bit but are around the listed voltage.
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