Panasonic TX-40CX400B (Vestel) Backlight issue

[compcycle]

Hi I have a strange one here.
No backlights at all.
Tested backlights, all good, 28v drawn per strip, stable and no lights out.
Tested backlights whole circuit, 145V drawn all lights on.
Fitted good known power supply, no backlights but tv working.
Fitted original power supply in good known tv, works fine including backlights.
Power supply is 17IPS20
plugged good known backlights in to faulty TV - dont light up.
So, it appears im left with main board working, but not issuing signal to psu to turn on backlights? With my backlight tester plugged int panel lights are on and see menus on tv.
Is it possible for main board not to issue signal to switch lights on?

[Andrew F. Ali]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Yes. It is possible for the Main Bd. to not send the BL-ON signal to turn on the backlights. That could be a faulty CPU. If you don't/can't do a component repair, replace the Main Bd. Since you seem to have another working set as you were able to switch PSUs and backlights to determine a Main Bd. issue. Take the known good Main Bd. and try it on the TV you are fixing IF the Main Bds are the same.

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Update: plugged a main board from a different set into the psu, and backlights come on, so its def main board. I wonder if the backlights are / can be switched off in the service menu?

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Thanks Andrew, they are different sets that use the same power supply, unfortunately i havent got a main board for this tv, its a UHD 3D jobby. Strange that it works if I power the backlights externaly, menus come up fine

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

i wonder if i could supply the bl-on signal to the psu externally to see what happens, i expect its just a matter of pulling an input high?

[Andrew F. Ali]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

At 3.0Vdc Max it will work with an external power supply, but I am not too sure how the dimming control would work. You may have a reduction in contrast ratio that will affect the overall picture quality. You may also get backlight bleed in the dark areas of the picture.

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

According to the schematic im looking at i appear to already have 3.2v on BL-ON, however i have 0v on PWM_DIM could that be it?

[Andrew F. Ali]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

The PWM_DIM would vary the backlight brightness level. Trace the PWM_DIM from the connector on the Main Bd. and see if it goes through a resistor or transistor we could replace. However, when self-testing the PSU we don't supply a voltage to the PWM pin to turn the backlights ON, just the BL-ON pin.

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Ok I'll look at that in the morning when I get back to it cheers

[compcycle]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Just to post the result, noticed that Pwm_dim was actually being pulled to ground. Disconnected the wire to it from main board and backlights came on. Left it floating for now and tv seems ok, just obviously no backlights control. Perfectly useable, seeing as I can't find a main board for less than 80 quid!

[Andrew F. Ali]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Could you determine what on the Main Bd. is pilling down the pwm_dim voltage?. I don't think leaving it disconnected would cause a problem. Trace the copper trace on the Main Bd. and see if it is possible to determine the cause. Let me know what you find.

[budm]

Re: Panasonic TX-40CX400B (Vestel) Backlight issue

Right now the LEDs are probably driven at max.
What is the dcV on that PWM pin when you disconnected that PWM wire going back to the main board. You may be able to dim down the LEDs by using resistor between GND and the PWM pin.