Samsung backlight issue

[huffton]

I have a Samsung TV, model LE26B450C4W. I believe it is a backlight issue because I have tried shining a light at the display and could see a picture when it was all connected up. I can't see the menu now I have disconnected it, but maybe that is because I am not getting the right angle. The LED on the remote sensor behaves as I would expect though.

The issue was intermittent for a while, the TV would either work or not but now it seems fairly consistently dead. I measured the resistance of the secondary windings of the inverters, and found the following resistances:

978 Ohm
976 Ohm
969 Ohm
968 Ohm

I assume the issue is on the power supply/inverter board, so am posting a picture of that. There are two rows of connectors, but only the bottom (Closest to the board edge) are used. Voltages compared to the case metal are (Measured in DC mode in the 20V range) from the other board as it is easier:

0v
0.29v
2.18v
5.13V
13.03v
13.03v
0v
5.15v
5.15v
0v
0v
13.07v
0v
5.19v
0.74v

I assume the voltage round the inverters is expected to be AC. All measurements on the capacitor lets are 0v in DC and AC mode. Also the primary is 0v in DC and AC mode.

I In standby they are all 0v (Well less than 0.1v anyway) apart from the second from the end which is 5.24v.

I don't think the inverter itself is the problem as it doesn't seem to be powered. It does seem to be getting a signal to tell it to switch on though. The capacitors all look OK. I notice the board looks like it has got hot, though this is around the components with large heat sinks, so I don't know if that is an issue or not.

Any ideas? What should I look at next?

[R_J]

Re: Samsung backlight issue

when you first turn the tv on, do you get a short flash of the backlight?

[nomoresonys]

Re: Samsung backlight issue

Test connector from mainboard to powerboard on powerboard side with all boards hooked up, test with tv plugged in but off then hit power, report both conditions here, give pin number, pin description/name and condition on and off. You have some voltages but you need to label them and measure on and off.

[huffton]

Re: Samsung backlight issue

when you first turn the tv on, do you get a short flash of the backlight?

Not so far as I can see.

Test connector from mainboard to powerboard on powerboard side with all boards hooked up, test with tv plugged in but off then hit power, report both conditions here, give pin number, pin description/name and condition on and off. You have some voltages but you need to label them and measure on and off.

I believe I did this, though on the mainboard end of the cable, because it is difficult to get to the pins on the power board side. The measurements are left to right, and the labels are in the picture. I will reformat them below in the hope that helps.

Pin  Voltage on Voltage Standby
NC     0    0
DET_5V   0.29   0
E-PWM    2.18   0
Inv_ON/OFF 5.13   0
13V(M)   13.03   0
13V(M)   13.03   0
GND     0    0
5. 3v    5.15   0
5. 3v    5.15   0
GND     0    0
GND     0    0
13V(S)   13.07  0
GND     0    0
STBY5.3V  5.19   5.24
PWR-ON.OFF 0.74   0

[nomoresonys]

Re: Samsung backlight issue

Nope, wouldn't bother looking at the picture then click to magnify then look back at your non labeled readings, WOULD YOU???

[R_J]

Re: Samsung backlight issue

Check the voltage on BD901 (should be around 380v) and then check the voltage on QI821 drain pin

[huffton]

Re: Samsung backlight issue

Check the voltage on BD901 (should be around 380v) and then check the voltage on QI821 drain pin

Thank you for your help.

BD901 is at 116v DC to ground, which isn't what you expected. Does this mean the problem is before the inverter?. QI821, I am not sure which is the drain pin, so with the writing facing me, and working left to right, I have (Again measuring in DC):

• -117.5
• 117.5
• -118.2

I did download the schematic but don't really know enough to understand it properly.

[R_J]

Re: Samsung backlight issue

I guess i should have said you need to use HOT ground, That is Either the [-] leg of the main bridge rectifier or the [-] of the main filter cap CP815
When measuring voltages on the primary side of the power supply, use P-GND for your negative meter lead, When measuring voltages in the secondary, use S-GND (chassis ground)

[huffton]

Re: Samsung backlight issue

When measuring voltages on the primary side of the power supply, use P-GND for your negative meter lead

OK, Thanks for your patience!

BD901 - 340v

All pins on QI821 are 0v. I don't understand how this fits in with my measurements yesterday, but I checked a number of times.

[R_J]

Re: Samsung backlight issue

That voltage on QI821 would be 0 if that circuit is not working and QI820 is not shorted.
Before you measured from cold ground to the hot side, checking anything on the hot side like that usually gives you a false reading.
Now that you know where hot ground is, locate UI801 and see if you have VCC on pin 16, This is the VDD voltage likely 10~12 volts

[huffton]

Re: Samsung backlight issue

locate UI801 and see if you have VCC on pin 16, This is the VDD voltage likely 10~12 volts

Thanks again.

Looking at the schematic, I think I need to measure to S-GND which is the chassis? This chip is on the solder side, so I needed to unplug the board from the backlight connectors and flip it over to access the chip. It is still connected to the mainboard. I believe the dot is next to pin 1, so pin 16 is opposite pin 1. Voltage to chassis is 9.7v. I also checked to pin 6, again 9.7v.

Voltage to P-GND is 0v, but if I understand correctly these circuits are isolated so that is meaningless.

[R_J]

Re: Samsung backlight issue

Check UI801 pin 12, this should be ~5 volts (to S-GND)

[huffton]

Re: Samsung backlight issue

OK, so pin 12 of UI801 is 0v.

I'm trying to think what is supposed to bring the voltage up, and wondered if it is ON_OFF. I decided to try checking PC803S, and this is 0v apart from pin 4 which is 5.22v

Then I checked round ub802 to chase INV_UVL0, and all pins are 0v. I have a strong feeling I am barking up the wrong tree here though.

[R_J]

Re: Samsung backlight issue

UI801 is the ic that drives QI806 and QI807, they drive QI802,3,4,5 that drives the DT801s gate drive transformer that then drives the output fets QI820/821
If UI801 is not on there will be no backlight
These power supplies are quite reliable as far as the backlight circuit goes, it might be that the ccfls are the problem

[huffton]

Re: Samsung backlight issue

Thanks very much for your help!

Is it worth trying to source replacement CCFLs? Are there any good suppliers in the UK? Or should I just accept that after 10 years it is probably time to get a new TV set?