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Samsung 50-inch plasma Power Supply Board issue

DanieSpreeth

Jun 21, 2023
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Hey Guys,

Been struggling for a few days since my plasma tv started to refuse to turn on, just the all-known relay clicking sound after power-up...

Through disconnecting power to the other boards one-by-one, I managed to see that connecting the Y-Sus board is what's causing the startup to fail. At first obviously that would be my guess...

Then I checked the output voltages on the power supply to the Y-Sus board and this is what I found:

Vs - should be 205 according to label, when switching TV on, the meter reads about 230V and then immediately starts dropping down to about 15V or less like a capacitor discharging.

Va - should be 55, same happens as above only it starts at 80V then starts dropping.

All other voltages (12V, 5V, 3.3V) going to other boards seems to be ok.

Keeping in mind I would prefer to fix not replace, what would the next step be? Does the specific behavior of the voltage read-outs remind anyone of a previous issue - Im no expert, please help!?
 

Delta Prime

Jul 29, 2020
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Make model of your Samsung 50-inch plasma TV. Maybe I can find a schematic for you. If possible and accessible take photos of both sides of the suspected board but first.
Plug all the connectors back in because What I would first do is on the main power supply board.
I Look on the main power supply board on any plasma TV the connector branching out to the other boards.
There should be printed on the board itself SB (standby power) after all the clicking (relay chatter) is done on your relay check SB with your multimeter.If it is 0 volts then problem or (fault) is on main power supply board.
 

kellys_eye

Jun 25, 2010
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Depending on the PSU design it may be that the output requires a load in order to stabilised. 200-ish volts at 10mA (say) would require a load resistance of 20k ohms (and a power rating of 2 watts (use a 5W resistor to be safe). I'm not sure that 10mA is even enough..... so calculate a load resistance accordingly and then you might be able to measure the output.

You can also check for visible issues where 'burst' (or bulging) capacitors are the usual cause of such problems and simple replacement affords a fix.
 

DanieSpreeth

Jun 21, 2023
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Make model of your Samsung 50-inch plasma TV. Maybe I can find a schematic for you. If possible and accessible take photos of both sides of the suspected board but first.
Plug all the connectors back in because What I would first do is on the main power supply board.
I Look on the main power supply board on any plasma TV the connector branching out to the other boards.
There should be printed on the board itself SB (standby power) after all the clicking (relay chatter) is done on your relay check SB with your multimeter.If it is 0 volts then problem or (fault) is on main power supply board.
Hi,

Im having a hard time finding the "SB". Is this maybe the same as PS_ON or PS_OFF?

Wrt the make and model, it's a "Sansui" TV but all boards inside have the samsung labels. I hope this is the correct model number - "850HW - YB06" - does that look right?

Below is an image of the power board - the fuses I circled (and maybe this was more important to mention earlier) was blown when I started looking for this problem. I replaced it immediately but that made no difference nor did the fuse blow again - its still ok. Also attached an image of the whole TV...
 

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DanieSpreeth

Jun 21, 2023
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Depending on the PSU design it may be that the output requires a load in order to stabilised. 200-ish volts at 10mA (say) would require a load resistance of 20k ohms (and a power rating of 2 watts (use a 5W resistor to be safe). I'm not sure that 10mA is even enough..... so calculate a load resistance accordingly and then you might be able to measure the output.

You can also check for visible issues where 'burst' (or bulging) capacitors are the usual cause of such problems and simple replacement affords a fix.
Hey,

Did not find any "bulging" capacitors - should have mentioned that sooner too.

About the first paragraph in your reply - most of what you are saying I have no idea about - Im actually a software engineer - hehe...
 

kellys_eye

Jun 25, 2010
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About the first paragraph in your reply - most of what you are saying I have no idea about
No probs - we help where we can. In many Switched-mode power supplies (as yours is) the output voltage is monitored so it can be regulated by a feedback process and if the output specifications (current or voltage) fall outside a set parameter then the circuitry sees this as a (potential) fault and either limits the output or can even STOP the output.

The power supply thereby needs to 'see' something attached in order to reach an output. If you deliberately disconnect the output there is no load (seen as a fault) so the supply may switch off. Not all supplies do this but many do. Therefore, in order to test for an output, you need to have a KNOWN load attached and this known load needs to be approximately what it expects to see. I can't comment on what that is but many supplies just need a MINIMUM current to flow in order to deliver a regulated output hence the suggestion for creating a small current draw using a resistor.

If you are unfamiliar with power supplies then be aware that there are potentially FATAL voltages around and appropriate care needs to be taken.

The capacitor 'bulge' issue may even be on other boards - have a good look - but sometimes it's not as readily visible and an actual capacitor (ESR meter) test is required.
 

Delta Prime

Jul 29, 2020
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Well I know my crystal ball is working I knew he was going to say that. quite frankly I can't see the designators on the silk screen it's out of focus. But I concur with @kellys_eye
 

DanieSpreeth

Jun 21, 2023
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There should be printed on the board itself SB (standby power) after all the clicking (relay chatter) is done on your relay check SB with your multimeter.If it is 0 volts then problem or (fault) is on main power supply board.

Hi,

I finally found the SB pin - was on the "other side" of the PS_ON pin I thought it might be - it reads just a tad under 5V after clicking is done and also when tv is switched off. while switched ON before & during clicking its less.

Does that mean something?
 

Delta Prime

Jul 29, 2020
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I haven't forgot about you sorry I'm at lunch right now you have to take better photos I need to see the component designators on the PCB board itself now this photo is not your unit it is reference only but you can see the similarities I'll get back to you in a while five volts is good should be steady 5 volts make no measurements for the relay is chattering and be careful it'll blow you across the room while probing!!!
photo_1687377676769.png
 

DanieSpreeth

Jun 21, 2023
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Hey,

Apologies first for the quality of the pics - unfortunately my phone's lens is cracked and that makes it unable to focus. these images are the best I can do right now.

Below are 2 images, one of the main board and other the power supply. On the main board where I circled in green, that was where I found the SB label (guess the main boards are more "standard"). The blue circle on the power board - the two pins far left is VS_ON and pin 2 is PS_ON. I found the exact same voltage reading there on nr 2 PS_ON.

The red and green circles on the power board is where I found the Va (red) and Vs (green) test points. Also in the blue circle were the other smaller DC voltages.

Then on the Main Board, circled in red I noticed that capacitor the have a VERY SLIGHT bump. I doubt its higher than 1mm. Testing does not show that board to be defective - and since I dont have any capacitors is it worth the effort to get some and replace that?

Oh, yeah, I know what its like to get shocked by 220V (feeling if there's a lamp in a light behind the tv once and since the lamp was taken out and I already had it switched on - duh - I managed to touch both pins on the inside of the "naked light" - that was rough - that was many years ago tho)

Im being as careful as I can be - not to say that is careful ENOUGH - guess time will tell - lol.
 

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kellys_eye

Jun 25, 2010
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just the all-known relay clicking sound after power-up...
The clicking is indicative of the protection circuitry kicking in and quite possibly as a result of a fault elsewhere in the set and not directly related to the PSU. That the fuse didn't re-blow is a good sign and firms up on the aforesaid.

Remove off-board power 'out' connectors one at a time until the relay chatter stops (at switch-on). This might point to where the real fault is but unless we can determine which supplies go where then removing one supply might stop another working - which is why the resistor load test is often the way to go.
 

Delta Prime

Jul 29, 2020
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Through disconnecting power to the other boards one-by-one, I managed to see that connecting the Y-Sus board is what's causing the startup to fail
Post number One. :eek:
Take some photos of that board both sides please thank you.
 

DanieSpreeth

Jun 21, 2023
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The clicking is indicative of the protection circuitry kicking in and quite possibly as a result of a fault elsewhere in the set and not directly related to the PSU. That the fuse didn't re-blow is a good sign and firms up on the aforesaid.

Remove off-board power 'out' connectors one at a time until the relay chatter stops (at switch-on). This might point to where the real fault is but unless we can determine which supplies go where then removing one supply might stop another working - which is why the resistor load test is often the way to go.
Please see above - this process indicates its the Y-Sus board. But I could not confirm as yet if the Y-sus is getting the right power - if not then the symptoms might show also that its y-sus even though its psu - anyway thats how my brain works - the resistor load test - will that test for this? please explain how to do this?
 

kellys_eye

Jun 25, 2010
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please explain how to do this?
Remove the DC connector to the suspect board. Connect a resistor across the DC lead that causes a 'reasonable' current to be drawn (dunno - 10mA?, 50mA?) using the formula R (ohms) = volts/current.

So if it was 100V and you wanted to load it to 50mA then R = 100/0.05 = 2000 ohms. Wattage needs to be current squared x resistance so 0.05 x 2000 = 5 watts (use 10W to be safe).

With the 2000 ohm resistor connected across the 100V wires see if the PSU operates correctly. If it does then the Y-sus board is dud and in need of repair/replacement. Might be easier to get a new Y-sus board and swap it out anyway.
 
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