JVC LT-47X579 NoBacklight

[Ltank]

Set had fallen a short distance according to customer. Removed back and every board to inspect under magnifier. No cracks in boards or solder joints visable. Reseated connection
Set has sound. PS board not labeled to jumper PS. No service manual either. PS is LCA90817. How do I jump PS to force backlight on?

[Ltank]

Re: JVC LT-47X579 NoBacklight

Pi s

[tom66]

Re: JVC LT-47X579 NoBacklight

Do you have 24V and do you have BL ON/OFF signal on the inverter board?

[Ltank]

Re: JVC LT-47X579 NoBacklight

I have 24vdc to inverter but the boards have no labels telling you what pin is what.
no labels on main board or PS. The inverter is labeled.got 3.3vdc to BL-On

[tom66]

Re: JVC LT-47X579 NoBacklight

The "standard" which is common on newer TVs is:

24V
24V
24V
24V
24V
GND
GND
GND
GND
GND
Error (often unused) L: No error / H: Error
BL ON/OFF L: Off H: On
Dim1 Analog (0-3V)
Dim2 PWM (Square wave 10~90% @ 100~400Hz)

Not certain about analog/digital pin out it can vary between models
But all that is really important is the BL ON/OFF.

[Ltank]

Re: JVC LT-47X579 NoBacklight

When set is first turned on the backlights flash on breifly then turn off. Sound stays on.
the BL-On stays at 3.3 vdc I even used meter to monitor all the pins one at time to check voltages when set is turned on to look for a command that turns on then off. Any
voltages that come up stay on. Not what I was expecting to see. Does anyone have any info on Power supply or other boards for this set?

[Ltank]

Re: JVC LT-47X579 NoBacklight

Bl-on stays constant 3.3vdc
I_PWM constant 3vdc
No error or Inv-on on

[tom66]

Re: JVC LT-47X579 NoBacklight

Have you checked all the fuses on the inverter board?

[budm]

Re: JVC LT-47X579 NoBacklight

Since these are CMO inverter boards, check all the fuses including small black fuse with '15' or '20' on top. It looks something like this.

[Ltank]

Re: JVC LT-47X579 NoBacklight

All fuses are good. Backlight breifly comes on when turning on.

[tom66]

Re: JVC LT-47X579 NoBacklight

Ah, you have two (main) possibilites then:
- Bad transformer
- Bad bulb

Since it was mentioned that the TV fell a broken bulb is a lot more likely.
However, you should check the transformer secondary resistances too.

[budm]

Re: JVC LT-47X579 NoBacklight

OK, we need to check resistance of the high voltage windings of the inverter transoformers next. Can you read the P/N as printed on the body so we can look for the pinout of the secondaries side of the transformers.

[tom66]

Re: JVC LT-47X579 NoBacklight

Also check the transformers for cracks/fractures in the ferrite of the transformers.

[Ltank]

Re: JVC LT-47X579 NoBacklight

Ok thanks will do it tomorrow. What is the best way to check each bulb? One of those small testers?
If the wires to the bulbs have 2 wires on each end but the testers have two leads?

[tom66]

Re: JVC LT-47X579 NoBacklight

Do you have a spare CCFL? From a 32" or bigger TV? Or a spare strip fluroscent light? You can place it near the inverter outputs across all of them at once. Whichever part of the bulb which is over a connector that does not light up is a bad transformer or shorted bulb connection.
I saw someone else on here use that trick and it looked pretty neat.

To verify you should then check the secondary resistance of the transformers. Only if you can rule out the inverter suspect the bulbs.

[budm]

Re: JVC LT-47X579 NoBacklight

I use this to locate bad inverter transformer.
https://www.badcaps.net/forum/showth...x+dx-40l261a12

[Ltank]

Re: JVC LT-47X579 NoBacklight

I disconnected all the bulbs from the two inverters then plugged in one bulb at a time to both inverters and turned set on to see if that bulb lights up. I did this for all ten two wire connectors to each board on both sides ot the set. Every bulb did the same thing. When set is turned on the bulb would flash on temperary then goes out. I placed a neon bulb next to each transformer on the inverter and it glowed as long as the bulbs were lite up. You could see a picture on the screen each time . Can I rule out the T-Con and screen?

[Ltank]

Re: JVC LT-47X579 NoBacklight

When the backlight does turn on, at that time I neasure a 4vdc on CN23#12 Error on the flat ribbon cable that goes to CN25 on the othe inverter. The error code probably generated on the master inverter. If all the bulbs light up does not give an indication of curent draw on each bulb. Not having any service info. I hate to buy new inverters and it not be the problem.