Samsung - no standby - already replaced all caps but still

**CapLeaker** · 01-25-2016, 05:57 AM

Re: Samsung - no standby - already replaced all caps but still

SBRF 20100CT
That is a dual schottky diode, not a PNP transistor. If you get weird measurements because of the circuit itself, always take it out and retest out of circuit.

When you test a diode, you should see a voltage forward drop in one direction and nothing when you reverse the legs.

Let's take the SBRF 20100CT.
Put the negative lead of the DMM on the middle leg and the positive lead of the DMM to either side of the diode. You should see a voltage foreward drop. If you reverse the leads by using the positive lead on the middle leg with the negative lead on either side of the diode, you should see nothing. If you see a short, it is bad.

**Joseph cauchi** · 01-25-2016, 07:12 AM

Re: Samsung - no standby - already replaced all caps but still

Mario as leaker said those are shottky diode to test with multimeter take the middle pin as negative and put the red lead to one of the outside pins and you should have areading if you turn the leads you get OL then you do the same on the other side.
Check like diode.
call me if you need help 99897236

**budm** · 01-25-2016, 10:47 AM

Re: Samsung - no standby - already replaced all caps but still

'The problem seems to be the power supply. It has no standby power. I changed all the 6 low voltage caps and the bridge rectifier.... they were all good but just in case. There is no DC power on any pin which connects to the Main Board. The main capacitor charges to around 680 volts. A good look at the board shows no signs of damage. If it helps, the TV worked fine till one day I did not switch on, ie: the damage happened whilst switching it on.'
I think we need to make sure your meter is good first, are you sure you are getting 680V between the two legs of the main filter cap? The cap is only rated at 450VDC, right?

**CapLeaker** · 01-25-2016, 03:14 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by budm

'The problem seems to be the power supply. It has no standby power. I changed all the 6 low voltage caps and the bridge rectifier.... they were all good but just in case. There is no DC power on any pin which connects to the Main Board. The main capacitor charges to around 680 volts. A good look at the board shows no signs of damage. If it helps, the TV worked fine till one day I did not switch on, ie: the damage happened whilst switching it on.'
I think we need to make sure your meter is good first, are you sure you are getting 680V between the two legs of the main filter cap? The cap is only rated at 450VDC, right?

Must be a typo. Probably 380VDC.

**budm** · 01-25-2016, 03:48 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by CapLeaker

Must be a typo. Probably 380VDC.

Well, he cannot get PFC Boosted Voltage if the standby power supply is not working since the PFC IC and the other SMPS IC get the switched DC generated by the AUX winding of the standby power supply.

**sciclunam** · 01-25-2016, 05:38 PM

Re: Samsung - no standby - already replaced all caps but still

Thanks all for the help. Yes the main capacitor is being charged at around 680v. I tried with 2 different dmms. Unfortunately i cannot see its rating as the side where the voltage is printed on it is glued to the board. Will chech the SBRF 20100C and post back. Thankd

**sciclunam** · 01-25-2016, 05:50 PM

Re: Samsung - no standby - already replaced all caps but still

Also these 2 components that are before the main capacitor... shoul d these be tested as diodes... attached. The resistance in front of them has been removed as it seemed slightly deformed from the top - white ceramic 5w R12k. According to the tech shop that is only 0.12 ohms and he is sure its not the problem. See photo.

Attached Files

**budm** · 01-25-2016, 05:54 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by sciclunam

Thanks all for the help. Yes the main capacitor is being charged at around 680v. I tried with 2 different dmms. Unfortunately i cannot see its rating as the side where the voltage is printed on it is glued to the board. Will chech the SBRF 20100C and post back. Thankd

That is impossible to have 680VDC between the two legs of that main filter cap, the cap would have blown up like a bomb.
What setting did you use on your meter?
Pictures provided by shopjimmy.
BN44-00208A, and B pictures.

Attached Files

**budm** · 01-25-2016, 05:55 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by sciclunam

Also these 2 components that are before the main capacitor... shoul d these be tested as diodes... attached. The resistance in front of them has been removed as it seemed slightly deformed from the top - white ceramic 5w R12k. According to the tech shop that is only 0.12 ohms and he is sure its not the problem. See photo.

QP801 is the MOSFET, DP 805 is the Diode, these two components are for the PFC Voltage booster, you do no need to go that far yet, we need to see about 330VDC between the two legs for the main filter cap first before going any further, wrong reading will not help us troubleshooting the problems.

**sciclunam** · 01-25-2016, 06:37 PM

Re: Samsung - no standby - already replaced all caps but still

Thanks. I think a mistake I did when taking the reading was that the dmn was on AC! Will resolder the resistance and check on dc. Sorry but this is only the second time in my life trying to troubleshoot a circuit board.

Thanks to this forum and to you all I am encouraging myself to keep going.

**CapLeaker** · 01-25-2016, 06:41 PM

Re: Samsung - no standby - already replaced all caps but still

oh boy... 680VDC on the main filter cap? Geez, man... that's a hard pill to swallow... If that is the case, that capacitor should be going off like a space shuttle on NASA's launch pad. I hope that either the battery is almost dead or your not using the both leads on that capacitor.
Now that is going to be interesting...

**CapLeaker** · 01-25-2016, 06:42 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by budm

Well, he cannot get PFC Boosted Voltage if the standby power supply is not working since the PFC IC and the other SMPS IC get the switched DC generated by the AUX winding of the standby power supply.

Didn't think on it that it was in STBY.

**CapLeaker** · 01-25-2016, 06:43 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by sciclunam

Thanks. I think a mistake I did when taking the reading was that the dmn was on AC! Will resolder the resistance and check on dc. Sorry but this is only the second time in my life trying to troubleshoot a circuit board.

Thanks to this forum and to you all I am encouraging myself to keep going.

What meter do you use?

**sciclunam** · 01-26-2016, 12:51 PM

Re: Samsung - no standby - already replaced all caps but still

Hi All. Thanks all for your great help.

First of all I was mistaken with regards to the reading across the main capacitor. It is 305v.
I have taken several readings starting from the AC entry and have attached photos of each. What I find strange is that I got no reading from LP810 (power supply) neither at the input nor at the output side! (maybe I am checking the pins the wrong way?)

The above readings were tested with all the boards disconnected. I have checked the output pins that go to the main board and there is no voltage on none of the pins!

Also what I find strange is that when checking the fuse to ground I only get around 4V and it is rated 250V!

Please see attached photos... and thanks once again!

Attached Files

**budm** · 01-26-2016, 01:17 PM

Re: Samsung - no standby - already replaced all caps but still

First of all, when you make the ACV Primary reading in the HOT (Primary) side of the circuit, you DO NOT use chassis GND and the ref point for the meter, when check the ACV feeding the board you measure the ACV between LINE (HOT/PHASE) and Neutral which it should be around 230VAC for most Euro countries.
Then when you make DCV measurement in the HOT (primary side), you will use the Negative leg of the main filter cap as the GND ref for the meter.
When making the DCV reading on the cold side (secondary side) you can use the chassis as the GND ref if the board is still mounted to the chassis ( the GND of the secondary side of the circuit is Grounded to the chassis through the board mounting screws).

So what is the ACV feeding the board, 230VAC? if that the case then the DCV between the two legs of the main filter cap should be around 325VDC (230VAC x 1.414) when TV is OFF.
Bottom line, you need to know which GND ref to use when you are making the Voltage measurement. Wrong readings will not help us troubleshooting the problems.

**sciclunam** · 01-26-2016, 02:47 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by budm

First of all, when you make the ACV Primary reading in the HOT (Primary) side of the circuit, you DO NOT use chassis GND and the ref point for the meter, when check the ACV feeding the board you measure the ACV between LINE (HOT/PHASE) and Neutral which it should be around 230VAC for most Euro countries.
Then when you make DCV measurement in the HOT (primary side), you will use the Negative leg of the main filter cap as the GND ref for the meter.
When making the DCV reading on the cold side (secondary side) you can use the chassis as the GND ref if the board is still mounted to the chassis ( the GND of the secondary side of the circuit is Grounded to the chassis through the board mounting screws).

So what is the ACV feeding the board, 230VAC? if that the case then the DCV between the two legs of the main filter cap should be around 325VDC (230VAC x 1.414) when TV is OFF.

Bottom line, you need to know which GND ref to use when you are making the Voltage measurement. Wrong readings will not help us troubleshooting the problems.

Thanks BUDM for the explanation.

The input side is 230VAC when checking CN801, ie: where the AC wire plugs on the board. The Main Capacitor is reading 305V but the be honest I am sure it was 330V before I saw a spark coming out from the Bridge Rectifier whilst checking some components and the fuse blew (I guess I touched something by mistake). I will replace the Bridge Rectifier BD8016S it as it is giving 305V. Shouldn't this give 330V when checking its + and - pins?

So I guess all the 305V in the previous photos are all low and should read 330V right?

How should I measure the output power of LP801 (trasformer?) attached

Also is my diagram of Hot and Cold divisions good? attached

How should I measure the main fuse (attached)

Thanks for your time and excuse me for my ignorance. Sometimes I feel it would be best to throw away the whole TV but this Power Supply has become a personal challenge now!

Attached Files

**budm** · 01-26-2016, 02:49 PM

Re: Samsung - no standby - already replaced all caps but still

You should see 325VDC between the two legs of the main filter cap.
The Main filter cap is connected AFTER the PFC diodes so you will not see 325VDC between the +/- legs of the Bridge rectifier.
So are you sure now that you do not have STANDBY DCV since you did not use the meter correctly before?

**sciclunam** · 01-26-2016, 02:53 PM

Re: Samsung - no standby - already replaced all caps but still

So if I get 325VDC at the capacitor it would mean that all components before it (Bridge Rectifier, PFC diodes, etc) are good? I will check it shortly.

**budm** · 01-26-2016, 02:55 PM

Re: Samsung - no standby - already replaced all caps but still

Originally posted by sciclunam

So if I get 325VDC at the capacitor it would mean that all components before it (Bridge Rectifier, PFC diodes, etc) are good? I will check it shortly.

Yes, then we can verify if the standby power supply is getting the 325VDC feeding it next. But verify to make sure you really do not have standby Voltage on the cold side since you did not set the meter properly the first time.

Samsung - no standby - already replaced all caps but still

Samsung - no standby - already replaced all caps but still

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