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Old 05-28-2020, 05:27 PM   #1
bellyacres
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Default Samsung 245BW BN44-0019A

Hello all,
First time post here. I've searched here and used up my know-how. I'm at a loss on how to continue with this. I really don't want to blown another pair of mosfets!

http://monitor.espec.ws//files/bn44-00192b_195.pdf
Is the schematic that I've been using, not an exact match, but I believe that it's close enough for this board as a guide.

Here is what was found on initial inspection.

CM809 47pf obviously blown open on initial examination
QM801, QM802 FQPF5N50C shorted
RM801 0.22 1W FR open
RB801, RB802 100K 2W open
Checked ICM801, ICP801S, ICB801S for shorts, all appear to be fine.
CM808 appears to be okay

Changed all the above bad components, power on bang! PooF goes the
magic smoke. QM801, QM802, RM801 all toast.

All other parts that were changed are okay. Obviously I've still got a nice short somewhere...

Any help or pointers in the right direction would be appreciated.
Attached Images
File Type: jpg bn44-00195a-front.jpg (792.3 KB, 11 views)
File Type: jpg bn44-00195a-back.jpg (1,022.6 KB, 9 views)
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Old 05-28-2020, 05:45 PM   #2
R_J
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Default Re: Samsung 245BW BN44-0019A

Are these component numbers from the board or the schematic? If they arefrom the schematic, RB802 is 47Ω not 100k. with RM801 removed and the board turned OFF, what is the voltage across the main cap CP803, Then turn the power supply board ON and re check the voltage. It should be 160vdc OFF and aprox 385vdc ON
Then check all the secondary diodes DM851~DM853. If they are ok I suspect all the gate drive zeners & resistors are bad. When the fets short they supply 160~385vdc to the gate circuit, there is no way those poor little zeners survive.

Last edited by R_J; 05-28-2020 at 05:52 PM..
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Old 05-28-2020, 05:52 PM   #3
bellyacres
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Default Re: Samsung 245BW BN44-0019A

Those numbers are from the board. I'm going to double check, but those resistor values were both 100K by the color codes if memory serves. They were open though. They had the infamous glue on them.

As to the voltage on CP803 I've never seen above 160 vdc. If I understand the circuit right the mosfets should be handling the PFC to get the voltage to the 385 vdc.

There is the rub, if I put in the mosfets they go bang! Along with RM801...

Edit due to missing part of last post.

I'll check those diodes and see what I find.

Last edited by bellyacres; 05-28-2020 at 05:54 PM..
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Old 05-28-2020, 05:58 PM   #4
R_J
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Default Re: Samsung 245BW BN44-0019A

You can operate the board without RM801. This power supply is in 3 sections, the standby supply, the PFC supply and then the main supply. The pfc ic and mosfet are ICP801s and QP801s
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Old 05-28-2020, 06:17 PM   #5
bellyacres
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Default Re: Samsung 245BW BN44-0019A

DM851 and DM852 show forward drop of .199 DM853 .176, in circuit.

If they are ok I suspect all the gate drive zeners & resistors are bad. When the fets short they supply 160~385vdc to the gate circuit, there is no way those poor little zeners survive.

The gate zeners are DZM80[3456] and the surrounding resistors?
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Old 05-28-2020, 06:40 PM   #6
R_J
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Default Re: Samsung 245BW BN44-0019A

Quote:
The gate zeners are DZM80[3456] and the surrounding resistors?
Yes those are the components, the resistors look ok in your picture but check them anyway, also check the transformer to make sure the two secondary windings are not open.
The diodes are likely schottky diodes so they will check around .2xx

Last edited by R_J; 05-28-2020 at 06:42 PM..
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Old 05-28-2020, 06:45 PM   #7
bellyacres
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Default Re: Samsung 245BW BN44-0019A

Quote:
Originally Posted by R_J View Post
Yes those are the components, the resistors look ok in your picture but check them anyway
The diodes are likely schottky diodes so they will check around .2xx
Yes, the diodes all test right around there, no shorts. The 473 resistors are reading nearly open, the 220 are bang on.
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Old 05-28-2020, 07:12 PM   #8
R_J
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Default Re: Samsung 245BW BN44-0019A

the 473 should read 47k but if you look at the circuit you will see they are basically in parallel with the transformer and the RM811/RM813 resistor. so it would seem, either the resistor or the transformer is open.

Last edited by R_J; 05-28-2020 at 07:19 PM..
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Old 05-28-2020, 07:33 PM   #9
bellyacres
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Default Re: Samsung 245BW BN44-0019A

Quote:
Originally Posted by R_J View Post
the 473 should read 47k but if you look at the circuit you will see they are basically in parallel with the transformer and the RM811/RM813 resistor. so it would seem, either the resistor or the transformer is open.
Right, my bad. Just checked again I'm getting .022 across both RM812/RM814. That was the reading I got on both RM811/RM813. I'd say RM812/RM814 are both open, if the transformer was open I wouldn't see the .022 from RM811/RM813...

I'll have to see what I can scavenge up tomorrow to replace those two. My eyes are done for now and the boards that were handy don't have any 473.
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Old 05-28-2020, 07:45 PM   #10
R_J
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Default Re: Samsung 245BW BN44-0019A

It is likely easiest to unsolder the transformer secondaries from the board to check the smd components
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Old 05-31-2020, 06:56 PM   #11
bellyacres
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Default Re: Samsung 245BW BN44-0019A

Okay
I've got 385 Vdc across CP801.
RM801 and QM801/QM802 are still out of the circuit.

TM802 was pulled and had some issues due to glue. One secondary was reading high, cleaned up more, the winding wire came free due to corrosion. That is resolved. No shorts or opens on TM802. All the diodes and resistors are good in that area.

I'm at a standstill as I don't want to blow another set of mosfets...

Still staring at the schematic, but I'm missing something obvious I'm sure!
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Old 06-01-2020, 11:45 AM   #12
bellyacres
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Default Re: Samsung 245BW BN44-0019A

Fixed!

CM809 was obviously bad upon opening this thing up. The trace on the bottom of the board was gone. I had jumpered it to fix that. Upon closer inspection it looks like one of the jumpers in that area was able to arc over and cause the short. I re-worked that with some insulated wire and a bit of heatshrink. Also cleaned up the carbonized area as much as I could. It would probably be a 12mm hole to get it all gone. Replaced RM801 and QM801/QM802 and all is well again.

Thanks for the help and getting me looking at the right area of the schematic. Still don't have a great grasp of this circuit. Ever learning...
Attached Images
File Type: jpg cap-before.jpg (205.5 KB, 11 views)
File Type: jpg cap-rework.jpg (504.0 KB, 10 views)

Last edited by bellyacres; 06-01-2020 at 11:47 AM..
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