Samsung LA26R71BA cannot power up

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

ive just re read the whole thread and saw this budm

Quote,I cannot do any testing because the fuse FP801A keeps blowing. As soon as i switch on the power, the fuse will blow instantly.

This only started when i started to probe ICP801S. Now pin 6 and 7 are shorted. Is this normal,Quote

Maybe he shorted two pins whilst voltage testing?

also budm a question regarding dim bulb testers what wattage buld should we be using? also on a working psu with no shorts whats the correct bulb brigtness dim? completly off or does this vary with diffrent psus? thanks budm for any advice.

[budm]

Re: Samsung LA26R71BA cannot power up

75W to start with when the PS-ON is not applied.
In post 1 he indicate that he has power LED present, so the stby PS was OK and the PFC MOSFET was not shorted at that point and I guess that he shorted out the PFC IC.

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

yes i guess he shorted that ic too but would that damage the bridge rectifier?
in the uk 75 watt bulbs im not sure exist we usually have 60w or 100 w i tend to use a 60w on below 32in sets and 100w on larger sets i hope thats ok also ive noticed some times when powering a psu up with the dim bulb tester the bulb light bright then settles to just a dim glow only just on then the psu is usually ok if i have a short the bulb lights full brightness is that what we should be seeing? as i dont want to offer wrong advice to anyone.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

In an earlier post if I read them in order correctly, the PFC chip ICP801S, the fuse FP801S as well as the mosfet QP801S were changed.

After replacing the IC, mosfet and fuse the dim bulb tester was put in circuit and the bulb was lit about half brightness and then brighter when the P_ON was connected to Standby.

It was after removing the tester that the Varistor BT811S was damaged. The mains fuse did not blow, the bridge rectifier has shorts between pins however the mosfet QP801S, diodes DP901 & DP802 appears to be intact so does not look as though they were the cause of the problem.

I just wonder if the bridge rectifier sustained damage when the initial mosfet blew.

[john_doe]

Re: Samsung LA26R71BA cannot power up

DP804 (out of circuit) = 0.492vdc 1 way, the other way = OL
DZP801 (out of circuit) = 0.678vdc 1way, the other way = OL

Both are good.

I have ordered the bridge rectifier and the thermistor.

Are there anythings I should check while waiting for the parts to arrive? I hope I don't blow more parts.

A big Thank you to dick_barton, vinceroger69 and budm for all the help given.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

They are the only components confirmed so far as requiring replacement so that should be OK.

I think after reading a bit more about the bulb test, that when you initially switch on (without PS_ON jumped to Standby the bulb lights brightly as it starts to charges the 450V capacitor and as the voltage on the capacitor builds up the bulb begins to dim to almost off.
Then when P_ON is jumped to Standby the bulb should then become a lot brighter and remain steady at that brightness.

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

They are the only components confirmed so far as requiring replacement so that should be OK.

I think after reading a bit more about the bulb test, that when you initially switch on (without PS_ON jumped to Standby the bulb lights brightly as it starts to charges the 450V capacitor and as the voltage on the capacitor builds up the bulb begins to dim to almost off.
Then when P_ON is jumped to Standby the bulb should then become a lot brighter and remain steady at that brightness.

once the replacements parts are fitted we can then look at the original fault.
regarding the dim bulb tester thats exactly how i find it works some tvs will even power up and work with the dim bulb tester still in place regarding how bright the bulb is when someone else is testing i find comes with experience (is the bulb on dim/quarter brightnes/half brightness or even full) without seeing in person its hard to judge i guess on what people are seeing.

[john_doe]

Re: Samsung LA26R71BA cannot power up

I have replaced BD801S bridge rectifier and NT811S thermistor.

Beside these two, I have also replaced QP801S mosfet and ICP801S earlier.

With the dim bulb tester connected, when i power up, the bulb will light up with about half brightness for 2 secs and then it will be off. When i removed the dim bulb tester, there is no shorts.

Only the red light at the front of the TV is lighted. It is the same as the beginning of the problem.

With the main board disconnected, the voltage of the big filter cap is 334vdc.

How should i proceed as i am back to square one?

[john_doe]

Re: Samsung LA26R71BA cannot power up

Hi,

I am still only getting 5.2vdc standby and no PS_ON.

Any idea where should I check?

[budm]

Re: Samsung LA26R71BA cannot power up

PS-ON comes from the main board.

[john_doe]

Re: Samsung LA26R71BA cannot power up

hi budm,

Where should I start to check?

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

maybe we can force the ps to the on state so you can test other voltages i think its the 5v standby pin and the on/off pin? using a 1k resistor between the two pins but before you do this let someone else confirm these are the correct pins at least this way if the psu voltages come up and test ok we know it could be a mainboard fault then.


Edit just saw this thread can you run the tests like in this thread and let us know the results thanks
https://www.badcaps.net/forum/showth...ht=BN44-00192B

[dick_barton]

Re: Samsung LA26R71BA cannot power up

My thoughts on this.

I think there was an initial problem earlier when testing using the dim bulb tester since the bulb initially came on at about half brightness and then increased in brightness when PS_ON was taken to 5V.

With all the parts replaced, now when switched on it lights the bulb for about 2 secs which appears to be what was discussed earlier in that it lights, charges the capacitor and then dims when the capacitor is fully charged so applying 5V standby to PS_ON should bring the bulb back on.

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

Im still learning myself so as there are now no shorts and dim bulbtester etc goes out plus he has the 5v standby back we should be able to remove the bulb tester what i was thinking join 5v standby and ps on/off through a resistor and see if the other voltages now work ie 24 volts etc this will prove the pfc is now working too?

[dick_barton]

Re: Samsung LA26R71BA cannot power up

I'd agree with that. Can the bulb be left in place for this test?

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

im not sure on that, once i find i have no shorts and have standby voltage etc i remove it and power up and test as usuall.

[john_doe]

Re: Samsung LA26R71BA cannot power up

Hi vinceroger69 and dick_barton,

I want to play safe and connected the dim bulb tester.

With the 1k ohm resistor connect from standby to ps_on, the bulb glows half brightness continuously.

These arè the voltages at CNM801:
Pin1 = 4.2vdc
Pin7 = 9.6vdc
Pin8 = 9.6vdc
Pin13 = 5.4vdc
Pin14 = 5.4vdc
Pin15 = 5.4vdc
Pin16 = 5.4vdc
Pin19 = 9.6vdc
Pin21 = 9.6vdc
Pin22 = 9.6vdc

Thank you guys for helping me.

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

does the voltage on the mains filter capacitor go up? what is it now.
can you post a picture of the writing by CNM801 connector so we can see whats printed on the board

[john_doe]

Re: Samsung LA26R71BA cannot power up

Based on other thread, i checked CM808 out of circuit and it is 6nF. On the capacitor it is printed as 2J 183J SIC 460S. On the schematic, it is stated as 183 630v. The other thread said it is 18nF 630v.

What is the actual specs of this cap and could it be faulty?

[dick_barton]

Re: Samsung LA26R71BA cannot power up

does the voltage on the mains filter capacitor go up? what is it now.
can you post a picture of the writing by CNM801 connector so we can see whats printed on the board

It does look as though ICM801 circuit is working otherwise you would have no secondary voltages only the standby voltage, however please can you check what voltage you have on CNM802 pins 1 -5 (24V supply)