Samsung LA40C530F1R Backlight goes off/ 5v detect drops

[Grunder]

Hi guys,

This Samsung looses its back lights about a minute after switch on.

Voltages back lights on. (CN802 psu to main cpu photo3)

1. B13V 12.2V
2. 5V_DET 3.4V
3. B13V 12.2V
4. BLU_ON 5.2V
5. B13V 12.2V
6. E-PWM 3.8V
7. GND 0.0V
8. H_SYNC 1.8V
9. GND 0.0V
10. GND 0.0V
11. B5V 5.2V
12. GND 0.0V
13. B5V 5.2V
14. B5V 5.2V
15. B13V 12.2V
16. A5V 5.2V
17. B13V 12.2V
18. PS_ON 0.7V

Voltages Back light gone off

2. 5v_det 0.1v is the only voltage that changes (photos 1 & 2)

all other voltages remain the same.

If I unplug cn802 to cpu pcb the 5V_DET voltage rises to 5 volt on the power supply side, and the backlights come on.

Guessing the inverter needs this 5V_DET voltage to be 3.4v or more for the backlights/ inverter to power up?

I have tried another PSU & Main CPU pcb with the identical results.

This confuses me,.... where does the CPU pcb get the info from to switch the 5V_DET from 3.4v down to 0.1V?

Both CPU pcb's have the same fault? or a faulty lamp triggering the 5V_DET?

If someone could please help

[budm]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

I cannot clearly read the power supply/inverter board P/N, BN44-003??
Pictures are very poor.

[tvtimmy]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Is this it? Shopjimmy has a repair kit for $22.

[Grunder]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

I cannot clearly read the power supply/inverter board P/N, BN44-003??
Pictures are very poor.

Hi Budm, sorry for the poor pictures,will try in future for better pics. The psu/inverter pcb is BN44-00340. Thanks for the the links & photos. I now have tested three main cpu pcb's & two psu pcbs giving the same fault symptom.

I have repaired a few of these pcbs in the past (common is c1831/ spike suppressor going short blowing fuse & the two fets m1802/1 driving eht tx's going short circuit)

The date stamp on the tv says 2010, so I guess after seven years the lamps will be pretty worn. I guess I have to open the panel/ lamp house and see if I can identify the faulty lamp/s

[Grunder]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Is this it? Shopjimmy has a repair kit for $22.

Hi tvtimmy, thanks, nice clear picture of the psu pcb. I wonder what the kit includes? is the kit for the specific problem that I have? As I said to Budm in the post above that it starts looking like lamp failure. will open panel and let you guys know what I find.

[vinceroger69]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

certainly worth stripping the tv down and inspecting the ccfl s first before wasting money on a repair kit, ive had a couple of old ccfl sets recently with bad tubes but usually show the 2 second to black fault, a minute running seem a odd amount of time but if you can power up the set without the lcd attached and watch the tubes you may see one failing etc causing your fault.

[tvtimmy]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

The kit doesnt say where the parts go, unless you buy it.

[Grunder]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi Vince, thanks, quite right, one of the lamps turned out to be faulty. The other lamps also look blackened at the tips. Can the lamps be bought loose? I took one out of a scrapped panel and managed to get the lamps running. (photos)

Thanks again input from Budm & tvtimmy as well

[vinceroger69]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Glad you managed too get the tv working, lamps can be brought from the likes of aliexpress.com, i used to collect free to collect smashed screen tvs and strip for parts and keep all the ccfl eefl tubes,and use in sets to repair them but nowadays in the uk most people want led sets and dont want the older ccfl eefl type sets so ive stopped collecting them now.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi All.
My question is wrt LA40C530F1R. When switching power on at the power socket the standby light starts blinking 6 times then it goes off for approx 5 sec then it repeats the process every time without the display turning on. I have done the following troubleshooting:
Disconnect the main board from the supply board then I've noticed the leds are coming on.

Next I measured the voltges on CN802 and I compared my measurements with the (Gruders see the previous post) and the only difference is my DET_5V Pin 2 = 0.02V and H_SYNC Pin 8 = 0V this does look right? Please advice.

[nomoresonys]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Seems like mainboard issue.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi, nomoresonys
Thanks for the response....
DET should read in the range 5V but im getting 0.02V
Is this voltage supplied by the main board and not the power board?

[budm]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

5V_DET is the error signal to tell the Main board if B phase drive for the backlights inverter circuit is working or not, good = >3V (typ. 5V).
See schematic of BN44-00340B page 2 that I marked the signals pins.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi budm,

Thanks for the reply.....

The following I am observing........

When Main Board is disconnected from PS Board the backlights go on but as soon as I am adding main board the lights go off.

In terms of measurements when Mainboard is disconnected I do get 5.28V @ DET on PS board but as soon as I connect Main board Voltage drops to 0.02V.

Can I suspect Main board defective?

Also, H_SYNC no voltage...

I've also noticed on the back of the Main PCB is a number of flux residue spots. Will clean this up and check again will keep you posted.

[budm]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

"When Main Board is disconnected from PS Board the backlights go on " That's good.
Check the resistance of that 5VDET pin on the main board side against GDN to see if it shows low resistance or not.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Ok I will check the resistance and get back to you. At work now so will check this evening and get back to you. Thanks!

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi there.
I have measured the resistance on pin 2 and I got 11.47kohm. I need to inform you that I tried cleaning the Main board With alcohol wipes due to flux but it was difficult to remove . I howewer tried switching on tv again and guess what tv switched on for a minute and then cut out again. I would like to give the pcb a proper clean with brush and alcohol and try it again. please let me know what you thinking. much appreciated.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Apologies, the resistance I measured was on the PS side. I will measure the main board side today and get back to you.

[EDE]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

Hi Budm.
If for instance the resistamce is low on main board side for 5VDET will this mean its a problem on main board?

[budm]

Re: Samsung LA40C530F1R Backlight goes off/ 5v detect drops

The resistance on the main board has to low enough to load down the 5V_DET line. How low is it?