SAMSUNG LE23R82B - Power supply troubleshooting

[FrozenHaxor]

Hello, I have recently attempted to fix my friend's TV. It's rather old but surprisingly the caps seem to be good, from the information I have managed to find at this site and others, I've got the PSU schematics (BN44-00158A, MODEL: SMA23-P, Rev 1.0). Symptom was PSU only delivering 5V standby voltage and nothing else, it would not power up the 12V nor the 24V rail at all. I have found that RM801 was burned and open circuit, DM804 was shorted out and RM814 had very high fluctuating resistance ranging in MOhms.

Since all those components were around ICM801S (F9222L hybrid), I considered that hybrid faulty (after replacing all the components but the hybrid, the RM801 would burn instantly after switching the PSU on). Then I have also replaced the hybrid, but it's still burning through the RM801 resistor.

Since I was unable to get the direct replacement for DM804 which was D1S4 Schottky diode, I have replaced it with 1N5819 since the parameters were close enough.

I have tested around some random components and found no faults really. I have access to an oscilloscope, DMM and a desoldering station so testing components out of circuit is easy.

Could you guys point me in the right direction? I'm stuck on the diagnosis with this one... I did search this forum throughly, but the information was insufficient.

Thanks in advance!

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

"DM804 was shorted out" Does it test out as shorted off the board, it is connected in parallel with resistor RM803 0.39 Ohms resistor so when you test the diode it will show conduction in both direction due to the resistor connected in parallel.
So RM803 is also OK or it burnt open circuit?
Check cap CM802 off the board to see if it shows leakage resistance.
Check the resistance between the RM801's leg that is connected to the F9222 and the primary circuit GND to see if it shows low resistance reading.

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

I test components out of circuit, that diode was conducting both ways at low voltage.

RM803 is OK, just tested it again, out of circuit. (Tests spot on 0,39 Ohm)
I have desoldered RM801 and checked that leg to primary ground (negative of main 380V cap), shown 1,7 MOhm in one direction and open circuit in another, so no shorts there.

CM802 tests 13,6nF, when charged to 24V and the power supply is cut off, it almost immediately drops down to 0V. Tests open circuit (ohms).
The rating on that cap is unclear, if I were to replace it, I don't even know what to look for.

Thanks for your help so far.

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

You have shorts circuit some where that caused the fusible resistor to blow. The power MOSFETs are inside the F9222L which are connected in series to form half bridge drive for the power transformer, you my want to check the resistance between MOSFET #1 PIN 4 (Source 1) and PIN 19 (Drain 1), and then between PIN 19 (Source 2) and PIN 23 ( Drain 2).

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Pin 4 to pin 19 has 0,84MOhm in one direction and open circuit in another
Pin 19 to pin 23 has open circuit in one direction, and 0,86MOhm in another.

This is a brand new chip, I just bought it.

Old chip tests short circuit in all directions between pin 4 and 19 as well as pin 19 and 23.

Thankfully I killed the power when the fusible resistor started smoking so I guess I did not kill the new hybrid yet.

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

"This is a brand new chip, I just bought it." One of the people I help got one from EABY and turns out to be bad so it is best to test it out.
So it does not have shorted out MOSFETs, those are the only two component that capable of drawing too much current to blow the fusible resistor. It is also strange that the Source resistor did not blow open circuit but the Diode connected in parallel with it shorted out.
Have to do more thinking here. I also do not see any snubber caps for the MOAFETs either.
Can I see the pictures of your board, both top and bottom sides?

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Here you go, sorry for the bad lighting, did my best. If you need me to shoot a certain detail again, please tell.

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

So, any thoughts on this? Maybe I could test the transformer? Or maybe there could be something wrong with the secondary side drawing too much current?

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Bump. Anyone knows? I really don't want to give up on this!

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Interesting, I have turned on the PSU without any load, just jumped the PS_ON to 5.2V standby rail and it doesn't blow the resistor now, the voltages are also present. What could cause this? Is something else possibly shorted out in this TV overloading the PSU or shorting it out completely?

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Okay, the problem is that I have replaced the RM801 0.22k 1W "FUSIBLE RESISTOR" with just a standard 1W 0.22k resistor. When I put dummy load on the supply, this resistor gets extremely hot and starts smoking quickly. How do I substitute this resistor for something else? I can't seem to get a hold of a replacement, if that's even a problem.

This resistor is the only thing getting hot there, nothing else gets even warm but this resistor just wants to burst into flames after 10 seconds of 3 amp load.


There is a 14V drop across that resistor with 3A load on the 5.3V rail.

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Where did you connect the 3A load and what kind of load is it and its resistance value? at since per post 1 "Since all those components were around ICM801S (F9222L hybrid), I considered that hybrid faulty (after replacing all the components but the hybrid, the RM801 would burn instantly after switching the PSU on). Then I have also replaced the hybrid, but it's still burning through the RM801 resistor."
I would remove the hybrid to see if it will still burns up the resistor, i think the hybrid is not switching on and off at high frequency as it should but it stuck fully on so it blows the fusible resistor.

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

It must be switching because the voltages are coming on, before they wouldn't. The load is a 12V automotive halogen light bulb rated at 55W.

The resistor doesn't heat up when there is no load connected, but the hybrid is working and the transformer is oscillating. It's just that when I start to load down the PSU, that resistor heats up really badly really quickly.

I have soldered a wire to a big trace carrying 5.3V to the pin header. I connected it up there since the 5.3V output is rated at 4A.

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Wait a second, the old resistor has different color coding on it, could the schematic be wrong?


Upper resistor is 220Ohm as on the schematic.

Lower one is the old desoldered burned up resistor.

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Attachments are dicky for me, won't load at all. Here's a direct photo in higher res:




The upper resistor is the new one that smokes, lower one is the old open circuit.

The color bands tell me it's supposed to be 0.22Ohm resistor. NO WONDER IT BLOWS!


Bloody schematic! Never trusting again!

[flocko]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

The schematic does shows it as 1W0.22 ohm ? I have replaced these because of the brown glue that is used on the board ,on later boards it is replaced with a wire link that also shorts out with the glue

[FrozenHaxor]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

So now ya clever ha? Schematic says 1W0.22 k, at first glance it's totally 220Ohm. For what they put the "k" there, I don't know.

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

It is 0.22 Ohms 1W, 10% (K = 10%).
55W lamp may cause way too much current surge due to the cold resistance of the lamp filament, the cold resistance can be 15 times lower than the hot resistance so the surge will be really big(> 10 times).

12V 55W lamp will draw about 4.6A, the resistance will be about 2.6 Ohms when HOT, but the cold resistance will be 2.6/15 = 0.175 Ohms, which means the surge current the power supply will see will be about 68A!

This is a small power supply.

[dssence]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

Hey friends , I just came in here, cause I'm fixing this psu, from a friend of mine. I cleaned all the glue which was oxidising many components, a transistor near the main cap, QB802 transistor full of this sticky shit.
I realised RM801 has been heated up and it showed around megohm value, on the schematic says it should be 220ohms , so I replaced just to test with 2 50ohms in parallel , but as soon as I plug everything it starts to heat up in flames.
What could be posibly wrong, should I use some freeze spray to check for the shorte component on either side of RM801? can I inject some voltage?

thanks!

[budm]

Re: SAMSUNG LE23R82B - Power supply troubleshooting

RM801 is 0.22 Ohms 1W.