Samsung LA26R71BA cannot power up

[john_doe]

Re: Samsung LA26R71BA cannot power up

I have removed ICP801S and the pin 6 and 7 are not shorted.

But when I checked the locations on the board where pin 6 and 7 were, there is a short. Does this mean ICP801S is good?

Do I need to check DZP801 out of circuit?

[john_doe]

Re: Samsung LA26R71BA cannot power up

I lifted DZP801 pin 3 off the board. With the multimeter on diode measurement, the voltage across pin 1 and 3 is 0.687v in 1 direction. The voltage at other direction is OL. DZP801 seems to be working too.

So what caused the short-circuit and blowing the fuse?

[budm]

Re: Samsung LA26R71BA cannot power up

Check the resistance between the Source and Drain pin of the PFC Power MOSFET QP801S to see what you get.

[john_doe]

Re: Samsung LA26R71BA cannot power up

The resistance between the Source and Drain pin of the PFC Power MOSFET QP801S is 0 ohm.

[budm]

Re: Samsung LA26R71BA cannot power up

Well, it is either the MOSFET or the snubber cap CP802 is shorted out (it is connected in parallel with the Source and Drain pin of the QP801S.
So would lift one leg of cap CP802 off the board to see if the shorts is still present between the S and D of the MOSFET or not, if it is still showing as shorts, then remove the MOSFET and then test it off the board. See diagram for details.

[john_doe]

Re: Samsung LA26R71BA cannot power up

I removed QP801S and tested it. Yes it is shorted.

Will order new MOSFET and test again.

Thanks budm, dick_barton and vinceroger69!

Will be back when the new MOSFET arrived.

[budm]

Re: Samsung LA26R71BA cannot power up

The PFC IC may be bad or damaged, I would replace the IC at the same time otherwise you may damage your new MOSFET.

[john_doe]

Re: Samsung LA26R71BA cannot power up

Hi budm,

Is the PFC IC ICP801S?

[budm]

Re: Samsung LA26R71BA cannot power up

Yes, the 'P' in the IC designator: ICPXXX the 'P' = PFC circuit section.

[john_doe]

Re: Samsung LA26R71BA cannot power up

I have replaced the mosfet and PFC IC. Is there any final check before I power up?l

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

maybe build yourself a dim bulb tester this may save your new parts from blowing again.
I have enclosed a picture of a basic dim bulb tester circuit.

[john_doe]

Re: Samsung LA26R71BA cannot power up

What wattage should I use for the bulb?

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

i usually use either a 60 watt or a 100 watt.

[john_doe]

Re: Samsung LA26R71BA cannot power up

I connected the TV to a dim bulb tester of 60 watt and disconnected to the main board.

The bulb lighted up with about half brightness. There is 5.2 vdc standby and all other pins at CNM801 are 0 vdc or almost 0 vdc. It seems to be the same when I first started out.

The voltage at the big filter cap is 323 vdc.

Any suggestions?

[john_doe]

Re: Samsung LA26R71BA cannot power up

When i connect a 1k resistor from PS_ON to standby, the bulb lighted up very brightly.

[Packard93]

Re: Samsung LA26R71BA cannot power up

if u jump 5vsb with pson other voltages should be active if not power supply issue.

[Packard93]

Re: Samsung LA26R71BA cannot power up

Use a multimeter not a bulb . Make sure u set to cold or hot depending on side your testing on inverter . Cold will be dc and hot side will be ac . Read your board should be on there .

[Packard93]

Re: Samsung LA26R71BA cannot power up

You dont find voltage have u checked for fuses.

[Packard93]

Re: Samsung LA26R71BA cannot power up

You can blow things up if ur bulb uses to much current.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

If the bulb light more brightly when you connected PS_ON then I would say that your PFC circuit is now working.
Check the voltage again. Without PS_ON voltage at large capacitor = 323V and with PS_Connected voltage at large capacitor = ???