Samsung LN52B550K1F no backlight has picture has sound

[henry21347]

Hi I have a samsung LN52B550K1F LCD TV. There is no backlight but I can see a picture under a flash light. The sound is playing as normal. I have attached the pictures of the boards.

With my laptop connected to HDMI4 port, I tested the voltages of the connectors:


Power board (BN44-00267B) to main board (BN97-03035M) connector (CN801):
5V_DET 0.1V
E-PWM 2.77
I-PWM 0
INV-ON 0.05
13V 12.81
5.1V 5.15
SB 5.2V
PS 1.22
SYNC 1.74

Power board to LED driver board (SSB520H18V01):
VCC 12.81
FB1 0
FB20
GND0
LD9.3

Power board main capacitor: 60-64V~
Unwelded caps:
2H684k: 650n
470nk: 450n

Voltage of T-con fuse: 12.8V

[KYBOSH]

Re: Samsung LN52B550K1F no backlight has picture has sound

Can you us a little history of the TV?
Did you aquire it in this state or have you been using it normally until these symptoms developed?


Here is the service manual for your TV.
[Mods pardon me if this is not allowed]
https://www.manualslib.com/manual/24...1f.html#manual

Once you download it you can jump straight to pg 169 to follow the troubleshooting section.
Let us know how it goes!

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

looks like you are not getting the proper inverter on signal from mainboard, my guess is mainboard fault.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

try unhook cabe from power board to mainboard, plug power cord in, see if backlights come on and stay on, if they do, that usually means that backlights and powerboard are ok, I always make all unhooks and rehooks with tv unplugged from power to avoid any errant surges etc.

[henry21347]

Re: Samsung LN52B550K1F no backlight has picture has sound

Can you us a little history of the TV?
Did you aquire it in this state or have you been using it normally until these symptoms developed?


Here is the service manual for your TV.
[Mods pardon me if this is not allowed]
https://www.manualslib.com/manual/24...1f.html#manual

Once you download it you can jump straight to pg 169 to follow the troubleshooting section.
Let us know how it goes!

looks like you are not getting the proper inverter on signal from mainboard, my guess is mainboard fault.

try unhook cabe from power board to mainboard, plug power cord in, see if backlights come on and stay on, if they do, that usually means that backlights and powerboard are ok, I always make all unhooks and rehooks with tv unplugged from power to avoid any errant surges etc.

@KYBOSH
I picked it up from others in this state. It's free.

Following the troubleshooting part, I started with "No power" section:
backlight off, power LED flash 5 times after down.
BD102:5.1V
BD104:12V
BD108:13V
BD106:5.1V
R102:3.3V
R171,R173,C122,R527: all good
LVDS pin1-5:12V
T-con fuse:12V
power is supplied.
Then it tells me to go to No video-HDMI part but I have no wavevision to debug this part. I do have an image on screen under flash light.

@nomoresonys
The inverter_on signal is nagative because DET_5V is negative. DET_5V is negative because FB1 FB2 feedback is negative. I tried connecting INV_ON to 5V but still no backlight.
@nomoresonys
With CN801 disconnected, the backlight is still off. But I have another good TV (Samsung LN40A500T1F) which backlight is also down with main board disconnected to power board.

@all
The HDMI1, HDMI2 and HDMI3 are not working. I think it's because of plugging cables too hard. After I had it only HMDI4 worked and it's located on the edge of the main board.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

It helps when I read the whole post, lol, so the main caps have 64 volts and you can see an image with a flashlight? what voltage do you get on main caps with inverter unhooked from powerboard?

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

guess its possible it's going into protection? maybe ohm out the little transformers on the inverter, they should be very close to each other in ohms, can also check the resistors and diodes of inverter.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

I would check the voltage at ps-on and inv-on, check it just like this, tv off but plugged in[standby] put neg. probe on metal chassis ground, pos. probe on the top of connector contacting the metal part of wire terminal, make note of voltage, leave it in place power up tv, make note of voltage, do the same to both ps-on and inv-on, both should change from 0 to 5 volts or 5 volts to 0, and hold steady in properly working tv.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

can also test the powerboard, UNHOOK cable from powerboard to mainboard, could need up to four approx. 1k resistors, with resistors jumper ps-on to sb-5.2, inv-on to sb-5.2, then plug in power cord, see if backlights come on and stay on, if not may need to add the other resistors from e-pwm to sb-5.2 and or 1-pwm to sb-5.2, and retry see if backlights come on and voltages are as expected. Of course make all the jumps with tv unplugged from power, then plug it in after jumps are made, so you don't get any unwanted accidental contacts.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

also check cable from powerboard to mainboard, they may get faulty with age, had one a while ago had a burned wire that wasn't real obvious, look it over, see if there's any brown marks on either end, can also check each wire for continuity.

[henry21347]

Re: Samsung LN52B550K1F no backlight has picture has sound

I would check the voltage at ps-on and inv-on, check it just like this, tv off but plugged in[standby] put neg. probe on metal chassis ground, pos. probe on the top of connector contacting the metal part of wire terminal, make note of voltage, leave it in place power up tv, make note of voltage, do the same to both ps-on and inv-on, both should change from 0 to 5 volts or 5 volts to 0, and hold steady in properly working tv.

Both main caps read 60-64V~ with inverter unhooked from power board. But there is a surge of volts at the begining exceeding the multimeter (>750~).
I can see an image like this with a flashlight put in the holes from the back.
All little transformers on the inverter board read about the same ohm (output 17 ohm between pin2 and pin1/3, input 0 ohm between pin1 and pin4, 0ohm between pin2 and pin3, open between pin 1/4 and pin 2/3). The little transformers model is 488R B8CU1.
I don't know which one is power_on actually, but I suppose 'PS' is power_on. In stand_by mode (power connected but shutdown), all pins of CN801 is 0V. After I turned it on and heard the start up music, I measured CN801 on the POWER BOARD:
PS 1.02V
SYNC 1.6V
SB 5.1V
INV_on 0.02V
Can anyone tells me what the pins mean? I uploaded an image of the CN801 connector.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

is that capacitor right next to the connector domed/bulged on top at all? looks like it.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

Both main caps read 60-64V~ with inverter unhooked from power board. But there is a surge of volts at the begining exceeding the multimeter (>750~).
I can see an image like this with a flashlight put in the holes from the back.
All little transformers on the inverter board read about the same ohm (output 17 ohm between pin2 and pin1/3, input 0 ohm between pin1 and pin4, 0ohm between pin2 and pin3, open between pin 1/4 and pin 2/3). The little transformers model is 488R B8CU1.
I don't know which one is power_on actually, but I suppose 'PS' is power_on. In stand_by mode (power connected but shutdown), all pins of CN801 is 0V. After I turned it on and heard the start up music, I measured CN801 on the POWER BOARD:
PS 1.02V
SYNC 1.6V
SB 5.1V
INV_on 0.02V
Can anyone tells me what the pins mean? I uploaded an image of the CN801 connector.

read Budms' most excellent guide here, explains it all, read the basic tv and monitor troubleshooting guides pdf: https://www.badcaps.net/forum/showthread.php?t=25810

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

quote "Both main caps read 60-64V~ with inverter unhooked from power board. But there is a surge of volts at the begining exceeding the multimeter (>750~)" what type of multimeter are you using, it was set to dcv, 400 or 600? if it isn't auto-ranging.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

maybe test the main cap/s on the good tv if you have easy access like this one, should be around 400 volts dc when powered on, something seems quite off with the readings.

[henry21347]

Re: Samsung LN52B550K1F no backlight has picture has sound

quote "Both main caps read 60-64V~ with inverter unhooked from power board. But there is a surge of volts at the begining exceeding the multimeter (>750~)" what type of multimeter are you using, it was set to dcv, 400 or 600? if it isn't auto-ranging.

Both main caps read 64-64V AC and 319/-55 V DC against ground and 0V AC and 390 V DC from themselves. This is about the same as the good TV.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

ok, I kinda thought you were measuring in ac instead of dc, the 390 is all we needed, so thats good, means the powerboard is working ok, at least up to that point.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

test fuse on inverter board, make sure it shows continuity/is not blown.

[nomoresonys]

Re: Samsung LN52B550K1F no backlight has picture has sound

test voltage at cnl801, meter set to dc 20, neg. probe on chassis metal back.