Samsung LN40A530P1F backlights go off

[CDGraves]

I have a very strange problem with a Samsung LN40A530P1F (LN40A530P1FXZA).
The problem: In the settings, the back light level can be adjusted from 0 to 10. 10 is the highest. As expected, the backlights get dimmer as the number is adjusted down. When going down from 5 to 4, the backlights go off. The lights stay off even when the backlight level is adjusted back up. They stay off until the power is cycled.
Also, when "Mode" is changed from "dynamic" to "standard", the backlights goes out.
There is a signal from the main board to the power supply/inverter board called PWR_DIM. I measured the voltage of PWR_DIM at different backlight levels:

Level - Voltage
10 - 4.74
9 - 4.55
8 - 4.42
7 - 4.11
6 - 3.73
5 - 3.21
4 - 0.80
3 - 0.48
2 - 0.26
1 - 0.12
0 - 0.09

There is a big voltage change between level 5 and 4, where the lights go off.
I assume the voltage from the inverter to the backlights gets too low to keep the CCFLs on. Once the CCFLs go off, they stay off. They will not restart until the power is cycled.
I assume the main board is putting out an incorrect voltage on the PWR_DIM line. Possibly a Digital to Analog error.
The main board is Pearl BN41-00975C (BN96-08758B). There are a few for sale on eBay. Around $100. It would be great if I could get this down to a component level. I am not even sure the main board is defective.

Some history: I acquired this TV from a local e-cycle place. It had a color problem that I fixed with a new T-Con board. I stumbled across this second problem when adjusting the TV. I have checked the P/S board with an ESR meter. I did not any bad caps.
I could live with the problem, since I know never to adjust backlight level or change mode from "dynamic" to "standard". But I would like to sell this. I can not sell it with this problem.

Questions:
Has anybody seen this problem?
Is there a fix?
Short of buying another main board, What else should I do to trouble shoot?
Suggestions?

[CDGraves]

Re: Samsung LN40A530P1F backlights go off

I believe I have found the answer to the back light level problem.
First: Put the TV into "Service Mode". For the Samsung LN40A530P1F, that is done with the following remote control key sequence: Pwr off > Mute > 1 > 8 > 2 > Pwr on.
The TV will power up with the service menu. Select "Option Byte".
Under the "Option Byte" Menu, Select "Dimming Selection". There are two choices: "INT" and "PWM_EXT". Use the sideways arrows to select "PWM_EXT".
Cycle the power and the Samsung will power on and Back light level will now work correctly. You should be able to set Back light level anywhere from 0 through 10. The brightness will change incrementally and the back lights should not shut off.

I found this by experiment. Trial and error. I do not know what "INT" and "PWM_EXT" mean. The main board is BN41-00975C. It is used is several different Samsung models. I assume the "INT" selection was for some other model.

This is the voltage on the "PWR_DIM" line after switching "Dimming Selection" to "PWM_INT". "Level" is the back light setting.
Level - Voltage
10 - 1.48
9 - 1.36
8 - 1.22
7 - 1.10
6 - 1.00
5 - 0.87
4 - 0.76
3 - 0.64
2 - 0.55
1 - 0.55
0 - 0.55

In the event that "back light level" gets set below 5 and the back lights get turned off, you will not be able to see the screen to put the back light level back above 5.
This is how to do a blind back light level reset: Power on > Menu > Enter > Down arrow > Enter > Right arrow (at least 5 times) > Menu > cycle power off-on. The TV should start up with the back lights on.

[budm]

Re: Samsung LN40A530P1F backlights go off

Level - Voltage
10 - 4.74
9 - 4.55
8 - 4.42
7 - 4.11
6 - 3.73
5 - 3.21
4 - 0.80
3 - 0.48
2 - 0.26
1 - 0.12
0 - 0.09

How about putting the pin names on each pin so we will know what they are?
What is the P/N of the power supply/inverter board combo? BN44-?????
http://www.shopjimmy.com/catalogsear...ng+LN40A530P1F
LCD panel P/N?

"I do not know what "INT" and "PWM_EXT" mean." Dimming control methods: PWM (Variable pulse width, 200~300 Hz Freq.) for dimming control, or DC level control for dimming, it depends on the inverter board setup since one of the control pin has to be GND. Your Power supply/inverter board looks to be using PWM_EXT.
"n the event that "back light level" gets set below 5 and the back lights get turned off" If it is working properly, that should not cause the BACKLIGHTS to go off.

[CDGraves]

Re: Samsung LN40A530P1F backlights go off

"How about putting the pin names on each pin so we will know what they are?"
In my first message, I posted a photo of the measurement point. Connector CNI08, Pin3.

"What is the P/N of the power supply/inverter board combo? BN44-?????"
BN44-00197. Why is that important in this case?

"I do not know what "INT" and "PWM_EXT" mean."
The point is: There is a choice for "Dimming Selection" value. Someone prior to me had set it wrong. That is what caused the problem. Changing it to the other is what fixed the problem. It is a pretty reasonable guess that INT is internal, EXT is external and PWM is Pulse Wave Modulation.

"Your Power supply/inverter board looks to be using PWM_EXT."
No kidding.

"If it is working properly, that should not cause the BACKLIGHTS to go off."
If it was working properly, I would not have posted this.
If the back light setting causes the back lights to go out, you are trapped. You can not see what is on the screen. These are simple instructions to get the back lights back when an incorrect setting keeps them off.

The reason for posting this fix, is to help anybody else who runs into the same problem. I spend a significant amount of time on it. Hopefully someone will find it useful.

[budm]

Re: Samsung LN40A530P1F backlights go off

"The point is: There is a choice for "Dimming Selection" value."
It is not the value, it is the method of dimming the backlights.
The int: the main board supplies 0~5V (Typ., or some may use 0.5~2.5) Variable DC to control (the pulse width of Inverter PWM IC) the backlights level.
The PWM as it name imply, the main board sends the variable pulse width between 10~90% (Typ. or some may use 0~100%) for controlling the backlights level. At 50% Pulse width your DCV meter will show the AVG DCV of 2.5VDC (50% of the 5V Pulse).

"In my first message, I posted a photo of the measurement point. Connector CNI08, Pin3." so I guess I need to figure out what the rest of the pin names are.

"BN44-00197. Why is that important in this case?'" So I can look at the pictures of connectors and see what the pin names are.
BTW, if you remove PWM_DIM wire, the backlights will be fully on due to built-in pull-up resistor in the inverter circuit so if the setup is messed up you can still get the backlghts.