Sound and picture but no backlight

[JCdes]

Re: Sound and picture but no backlight

Top side

[budm]

Re: Sound and picture but no backlight

There should be printing on the body of Q201.

[JCdes]

Re: Sound and picture but no backlight

A couple of posts back I said theres no voltage at gnd

[JCdes]

Re: Sound and picture but no backlight

D669 I think...

[Topcat]

Re: Sound and picture but no backlight

The argument has been removed, do not resume it.

[JCdes]

Re: Sound and picture but no backlight

There should be printing on the body of Q201.

D669A is the part#

[dick_barton]

Re: Sound and picture but no backlight

They look like transistors. Data sheet.
http://www.datasheetq.com/datasheet-.../Iscsemi/D669A

[JCdes]

Re: Sound and picture but no backlight

I noticed there's 0v at d204

[dick_barton]

Re: Sound and picture but no backlight

Have you check D204 with your meter set to diode mode?

JP8 whicjh is a 0 ohm resistor is on the other side of the board close to this diode. I would check that it hasn't gone open circiut.
I would check all the 0 ohm resistor in that area

[JCdes]

Re: Sound and picture but no backlight

Will do right away

[JCdes]

Re: Sound and picture but no backlight

Jp8, jp5, jp16 are all ok...

[JCdes]

Re: Sound and picture but no backlight

Got continuity both ways on d204 but then all resistors underneath do connect them. I would have to pull it out tomorrow to test it.

[budm]

Re: Sound and picture but no backlight

D204 has a bunch of low Ohms (2R20) resistors connected in parallel with it so it will show conduction in both directions.

[budm]

Re: Sound and picture but no backlight

BTW, are you getting the 380 ~ 400 VDC PFC Boosted Voltage on main filter cap EC101 ( measured between the two legs) when TV is turned on?
What are the Voltage rating of the caps EC11/12/201/202?

[budm]

Re: Sound and picture but no backlight

They look like transistors. Data sheet.
http://www.datasheetq.com/datasheet-.../Iscsemi/D669A

They are too cheap to use the typical MOSFET for the PWM control of the constant control (LOW side control).

[JCdes]

Re: Sound and picture but no backlight

BTW, are you getting the 380 ~ 400 VDC PFC Boosted Voltage on main filter cap EC101 ( measured between the two legs) when TV is turned on?
What are the Voltage rating of the caps EC11/12/201/202?

For ec101 I'm getting 376v, ec11 and 12 are 100uf 100v... 201 and 202 are 47uf 160v.

[budm]

Re: Sound and picture but no backlight

For ec101 I'm getting 376v, ec11 and 12 are 100uf 100v... 201 and 202 are 47uf 160v.

OK, the PFC Boost Voltage is working.

Next, meter set to 20Vdc, all connectors in place, black probe on the chassis, red probe on 1 of 3 pins of Q201 at a time then turn the TV on while watching the meter to see how high it goes up to then turn off the TV then repeat the same test on the other 2 pin of Q201.
BTW, look at the spec sheet, post #67, of the Transistor so you can see which pin is Base and Emitter, middle pin is Collector (Connected to the Cathode of the LED string). Emitter is connected to resistor R244 (5R10), the other end of the resistor is then connected to circuit ground via D204//with resistors (R246/A, 247/A, 248/A) to complete the current path for the LED string, as you can see, the Cathode of the LED strings are not connected directly to circuit ground so if the transistors are not turned on then the LED strings will not light up.
R212 (1K) is connected to the FB pin 5 of the IC to monitor the Vdrops on D204//with resistors (R246/A, 247/A, 248/A) for maintaining the constant current through the LED strings.
BTW, what is wattage of the 100 Ohms resistor you have? We can use that to bypass the Cathode return of one of the LED string to circuit ground via 100 Ohms resistor just to find out if the LED string will try to come on or not.

[JCdes]

Re: Sound and picture but no backlight

I get 0 on pin 1, 0 on pin 2 and .64 on pin 3. Nothing changes when I turn it on. I can check the wattage on the resistors I got tomorrow.

[budm]

Re: Sound and picture but no backlight

I get 0 on pin 1, 0 on pin 2 and .64 on pin 3. Nothing changes when I turn it on.

0.64V on pin 3 (Base) is on border line of Bias Voltage to run it on since the Emitter is connected to the Diode to ground.
Can you put the 100 Ohms resistor to the middle (Collector) pin of Q201 or on the LED connector pin that goes to the middle pin of Q201, the other end of the resistor will be connected to the circuit ground (chassis), you should see the one of the LED string try to come on, just watch out for the resistor may be over heated but it is just for testing only. Do not leave the TV on for too long.

[JCdes]

Re: Sound and picture but no backlight

I can do that tomorrow. I got all the tools and goodies in my hobby shack outside the house. I'll do it and report back tomorrow. I really appreciate all the help. Resistor to center leg and to chassis right?