Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

[Pauldesign]

Hi All,

I've this Samsung 2493HM screen with PSU: BN44-00195A for a while now.

Well, i've read all the threads / responses on this model/PSU inside out and more than twice -- so, i've done all the necessary recommendations but to no avail.

The blue light is ON and the standby voltage (5V2) is OK as well as the PS_ON (3V3). However, there is no power to the inverter -- there is no 24V and 5V3.

Using equivalent PSU BN44-00192B schematic, I found the transistor QB802 faulty (i de-solder and measure the B-E to be 30 ohms either way which i've replaced with equivalent 2SD526). The collector now has ~ 18V (which was ~10v5 before) but the Base after the 2K2 resistor drops to ~3.5V and off course the emitter to ~3V, which seems to be the issue now -- i've re-tested the replacement NPN transistor out of circuit and seems OK).

ICB801S (STR-A6159) works well -- its Vcc measures ~16v5 and the drive voltage to PC801S (PS2561) transistor section measures about 19V at the input and Output; but drops to ~3v3 just after 2k2 resistor to QB802 base?

I've further isolated the QB802 and PC801S circuitry from the rest, by disconnecting the M_Vcc output to the other circuitry and still having the same issue -- the M_Vcc voltage (from the Emitter of QB802); still reads ~3.3V, the Base voltage reads ~ 3v7 and the Collector ~ 18V.

My question is, what voltage must M_Vcc be or what must the voltage to the Base and from the Emitter of transistor QB802 be? Which in my case, it's around 3v3, low enough to turn ICP801S (TDA4863) ON, as well as ICM801 (MC33067) -- since the Vcc (M_Vcc) to respective chips reads ~3v3 and Pin 10 (the fault pin) of ICM801 reads ~0.25V.

I've tested all the diodes/resistors and everything seems OK as well disabling the Over Voltage Protection circuitry around QT851 but to no avail.

Any more hints on why M_Vcc voltage is about 3v5 and what must be the right value?

Attached are the bottom and top pics of the PSU.

Thanks

[budm]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

There are 15V Zener diode and regular diode connected in series and then connected between the Base and GND of Q801 (802 in BN44 xx192 diagram). Are they OK?
Also check the resistance on the Emitter (output) and GND of that LINEAR regulator circuit to make sure it is not the loads that are causing the problem and took out the transistor in the first place.

[Pauldesign]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

Thanks budm for taking your time to assist.

There is no Q801/2 on the board/schematic -- i guess you're referring to QB801/2, just to avoid confusion.

Well, using the equivalent PSU BN44-00192B schematic as reference:

Since there is a 15V Zener diode (BZX84C15) at QB802 base, it means the base voltage can't exceed 15V, which means i can assumed a required base voltage of 10 to 14V.

Therefore, if the base voltage shouldn't exceed 15V, then the voltage at the emitter can't be more than 15V as well, since there'll be some volt drop across the B-E diode -- implying M_Vcc is expected to be around 14V -- I'll appreciate if someone can confirm?

By the way, i forgot to mention that DB804 (LMBD2838) was faulty /burnt (opened in all directions) but interestingly the series BZX84C15 tested Ok -- which shouldn't be the case, considering the circumstances. I couldn't find a replacement LMBD2838, thus i used but a normal 4148 signal diode -- which you can see it on the power board bottom side photo. Could this be an issue?

Also, when i was trying to confine the issue around QB802, i did remove RB805 (tested OK: 7K5) and measure the output resistance to be ~320 Ohms with and without RB805 (7K5) -- for a supposed 14V (M_Vcc) at the emitter, 320 Ohms doesn't seems to be a big load, perhaps an output resistance below 100 ohms could cause loading issues.


Below are my plan of actions / next likely options:

- Remove QB802 as you suggested and measure the voltages again.
- Temporally remove DP804 and use at BD804 (since they are the same) and rather use normal 4148 signal diode at DP804, just to check if it does the trick.
- Bypass the QB802 and PC801S OFF/ON circuitry and regulate the ~18V voltage from DB803 (across CB806) to ~15V -- to be now use as M_Vcc and see if it helps.
- Re-check ZBD805 (BZX84C15) properly and simply just replace it even if it tested OK.
- Modify RB808 (2k2) to less until QB802 turns On -- which i doubt?

[budm]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

The picture in my post 2 is from BN44-00195A board, Q801 is on my board, the diagram has it as Q802.
The 15V + 1 forward bias diode is series is to set the output (at the Emitter) Voltage to be at 15V, it is just a simple Transistor/Zener linear regulator circuit, what DCV you have between Base and GROUND will be what you will get at the Emitter minus the VBe drops of about 0.6V that is why there is a diode in series with the 15V Zener so the Voltage at the Base and GND will be at 15.6V so the out put will be = 15.6 - 0.6 = 15V.
The Zener and the diode in series is being bias by the resistor that gets the DCV through the OTPO to set the Voltage to 15.6V.
So you need to check the OPTO, the bias resistor (2.2K, the 7.5K is just the light load), and also check the load resistance (you may have bad PFC IC, or the SMPS IC for the 12V/24V power supply which may have cause the Transistor to go bad in the first place).

'i did remove RB805 (tested OK: 7K5) and measure the output resistance to be ~320 Ohms with and without RB805 (7K5)' 320 Ohms is not normal. So you may have bad IC.

[Pauldesign]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

Thanks Budm for assisting.

I replaced the 15V zener (BZX84C15) with an equivalent as well as the series diode, which seems to do the trick, although the output voltage (M_Vcc) at the emitter is now ~14V; which is not exactly 15V but i supposed should suffice for now, considering the wide tolerable input voltage range.

Well, the problem now is the drive voltage from ICP801S (TDA4863G) into QP801S (i supposed yours is Q801S) seems to drop to about 2V and i can hear some ON and OFF switching noises around inductor LP801S or perhaps L801S in your case. I disconnected the gate resistor (RP817= 47 Ohms on my board) and measure the voltage to be about stable 11v7 but drops to fluctuating 2V and the noise heard, once connected back to the MOSFET QP801S (FCPF11N60) gate.

I measured the voltages on ICM801S (MC33067) which shows 14V on Vcc Pin 15. The rest pins besides ground, fluctuates around ~1 to 2V. The fault Pin 10 is around 1.2V indicating there is an issue, which from the looks of things seems like ICM801S is trying to start but shutdowns repetitively.

I've tested for un-intentional shorts / opens on suspected components (diodes/IC/transistors/ protection resistors) and everything seems OK. So, either ICM801S is faulty or there is a short somewhere in the PFC or 24V/5V line; which i'll try to isolate by disconnecting PFC and M_Vcc supplies to certain sections of the circuit and re-test again.

[budm]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

You have BN44-00195A which is what I have, your designators on your board is the same as my board but they do not match the diagram.
At this point you need to get the PFC section working first and I believe you have bad PFC IC.
You do have 14VDC or so on the VCC pin of the TDA4683 chip, right?

[Pauldesign]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

Thanks Budm for your continuous and quick responses.

Yes, we do have the same boards and I can measure stable 14v on ICM801S Vcc pin. Unfortunately my current toy multimeter can only measure up to 300v and complains if more which can give me an idea if PFC is OK, although not ideal. I'll probe more and update you.

[Pauldesign]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

Well, from probing more with ICM801S disconnected from the circuit by cutting off PFC and M_Vcc supplies to that sections of the circuitry, i realized the following:

- The voltage across the big cap exceeds 300V, although i still need to measure the exact value.
- When diode DP802 is removed, the big cap voltage reads ~300V as well as the drain voltage and the repetitive ON/OFF switching click noise stops and furthermore, there is stable ~11V3 drive voltage from Pin 7 of ICP801S (TDA4863) to the gate of QP802 (FQPF11N60).
-DP802 tested OK in and out of circuit but the same fault arises when connected. The diodes as well other components after the big cap CP803 (with reference to PSU BN44-00192B schematic) seems Ok as well -- which i thought could be causing the problem.

@Budm, do you still think the fault is due to a faulty ICM801, even when the supplies (PFC and M_Vcc) to ICM801 / the entire section on page two of the schematic are disconnected?

[budm]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

The dcv between the legs of the main filter cap when the PFC is not working will be = LINE Voltage x 1.414, so if you AC is 230V, then the DCV should be around 325VDC, and if the PFC is working then the Voltage will be close to 400VDC.

[Pauldesign]

Re: Samsung 2493HM (PSU:BN44-00195A): No 24V 5.4V Drama

Thanks Budm.

I'll do more measurements and with better test equipment and update you.

Is just that the PFC IC isn't readily available in my area and is a bit pricey; otherwise, i could've done a replacement and see the outcome, although the current process is a good learning curves which is the right way to go first.