Sanyo DP42849 No Backlight

[lookimback]

I'm back finally (living up to my name I guess). New project is a Sony DP42849.
Problem is no backlight. Get one flicker when turned on.

Tried testing with CCFL, didn't light.

Gets Sound, can see pictures with flashlight.

Found these 2 threads. Tried everything in them.
https://www.badcaps.net/forum/showthread.php?t=26686

https://www.badcaps.net/forum/showth...DP42849&page=1

Took the transformer right out of the board. Certain its good. Resistance is 117 Ohms across 5 and 10 while in board. Pins 5 - 6 and 9 - 10 are about 26 Ohms each.

All the resistors mentioned in the two threads were good.

Tested all the diodes. All were good. Most I took out of the board to test.

I have 5v, 12v, 14.5v but no 24v. Not sure if I'm supposed to have 24v.

Got one hell of a shock last night off the secondary side of the transformer. Wasn't even plugged in at the time. So I know its getting power out the transformer, just not enough to drive the lamps.

Think I might have it narrowed down to an over voltage problem on ICs 701, 702 and 703. (see pic, ICs.jpg).

IC702 Is marked 258A and I was able to find datasheets for it. Datasheets LM258 and LM258W attached. Don't know how to tell which is which.

Here's what I have on IC702
12v across pins 4(vcc-) and 8(vcc+)
4.3v across pins 2(ln1-) and 3(ln1+)
4.3v across pins 5(ln2-) and 6(ln2+).
Pins 1(Out1) and 7(Out2) are 0v.

Also ICs 701 and 703 (right and left of IC702) each have 1 output that is 0v. Datasheet LM224A Attached.

IC701 pin 1 (Output 1) 0v. Voltage across pins 2 (Inverting Input 1) and 3 (Non-Inverting Input 1) 4.3v. Voltage across pins 4(vcc+) and 11(vcc-) is 12v.
I get about 9v output on pins 7, 8 and 14. Voltages across pins 5 - 6 bridged, 9 - 10 = 2.1v, and 12 - 13 = 3.7v

On IC 703 pin 8 (Output 3) 0v. Voltage across pins 9 (Inverting Input 3) and 10 (Non-Inverting Input 3) 4.3v. Voltage across pins 4(vcc+) and 11(vcc-) is 12v.
I get about 5.8v output on pins 1, 7 and 14. Voltages across pins 2 - 3 = 3.1v, 5 - 6 = 2.8v, and 12 - 13 = 3.1v

So what I see is common is that where I have a 4.3v signal, the corresponding output is 0. So, I'm thinking over voltage.

[lookimback]

Re: Sanyo DP42849 No Backlight

Ok, heres an update. I missed this post in one of those other threads. I'm actually not getting power to pin 16 of the PWM IC, IC301. I traced it through the transistor Q303. I traced the circuit which leads to the base of Q303 all the way back to IC303 which is an optocoupler (pc123 datasheet attached.) I can see I have voltage on the emitter but not the collector. I'm going to pull it out and test it but I don't think its the problem. I also pulled IC701 and IC703 off the board. I switched them to see if it would make a difference. Only difference is that I have 0v on pin 7 of IC701. The two signal input pins for that output pin are bridged so I'm thinking I might never have had voltage on that pin or else it was coming from somewhere else. In any event, I don't think they are the problem. Since they were only 58 cents each, and I was putting in an order for a different TV, I decided to order them anyway. Now I'm on IC702. There are dead outputs on it as well. I ordered one since it was only 50 cents. Also decided to recap the entire board, though I doubt that will solve the problem. I'm kind of leaning toward the problem being one of the optocouplers, but not IC303. I'm going to pull each one out and test them. I was going to just order a couple but the part isn't available from Digi-key or Mouser. Now I just found a cross-reference sheet for them and an exact substitute is only 60 cents. Used board is over $100 so I'm not going to give up on this.

[lookimback]

Re: Sanyo DP42849 No Backlight

Solved. Its the transformers that go bad in these. I found a complete TV on eBay for $50 but it was local pickup only in Indiana. I suspected a bad Tcon and was correct. I talked to the seller and he agreed to disassemble it and ship me all the boards for $65. Couldn't beat that. While waiting for them to arrive, I found another power board on eBay for $65. Since these sell for $115 and up, I bought it for resale. Lucky for me that I did. The board from Indiana worked but whined. The third time I powered it on, the transformer burned up. I actually saw the arc. So I took the transformer from the original board and tried it. Still didn't work. Finally, I took the transformer from board 3 which I knew worked. Bingo, the TV powered right on.

The primary side of the transformer is what was bad on my original board. It wasn't shorted but it had low resistance. It was around .3 ohms. That's probably just the resistance on probes. The good transformer gives me a reading of .4 between pins 1 and 2, and .6 between pins 3 and 4. Neg probe to 2 and 3. I'm thinking both sides should be .6 so maybe this one is on its way out as well. As for the secondary side, you should get around 25 to 28 ohms between pins 5-6 and 9-10.
I hope this will help someone.


I'm planning on rewrapping the bad transformer. Any advice on selecting the proper wire will be much appreciated. Thanks.