Philips 40PFL3706/F7 No Backlight Logo flashes on

[neilc6]

When turned on, the backlight comes on briefly and you can see the Philips logo on screen but then the backlight promptly shuts off.

Service manual available here
https://www.electronica-pt.com/esque...-pl11.1-25763/

Samsung panel: LTA400HM09 7HBL0A256H

On-state voltages
Connector to LCD
Panel+24V: 24.7V
Backlight-SW: 2.7V
Backlight-Adj (pin14): 2.6V (reads 3.2V when backlight flashes on)
Backlight-Adj (pin13): 2.2V (reads 2.8V when backlight flashes on, 0V on schematic but not connected to anything on PS board so reading from inverter board)

Connector to main board
Backlight-SW: 3.2V
Backlight-ADJ(DC): 2.6V
P-ON-H2: 3.1V
AL+4.3V: 4.4V
DETECT: 3.3V
P-ON+12V: 11.9V

The schematics show expected voltages and my measured voltages all agree. When the backlight is off and tv is on, all voltages are steady except Backlight-Adj.

No bulged caps on power supply and inverter boards. All fuses ok. Doesn't appear to have a PFC circuit or protect signals for shut down.

[dskall]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

Do you see any dark area when backlights come on?

[Andrew F. Ali]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

CCFL/EEFL Backlight issue. Though HV Transformers could be issue, too. I would measure and compare secondary resistance readings. If they are equal or very close readings. I would focus attention to backlights.

[neilc6]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

Hard to tell when only the logo is on screen if there are dark areas.

Older rev inverter board is around $15 on ebay part LJ97-03241A. Pics show a PCB SSI400_08A01 Rev0.2 while mine is newer Rev0.3 with Rubycon electrolytic caps part LJ97-03241B. Shopjimmy sells this refurbished. Wonder if they are replacing the transformers. http://www.ebay.com/itm/Samsung-LJ97...MAAOSwPYZVADvD

Panel looks like it is used in the following models so gonna done some more searching.
Dynex DX-40L261A12
Emerson LC401EM2F LC401EM3F
Hisense F40V87C
Magnavox 40MF401B/F7

This post is similar to my problem
https://www.badcaps.net/forum/showthread.php?t=39166

[dskall]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

I would be measuring transformer resistances before throwing parts at it

[neilc6]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

From looking at the transformer the middle winding is primary.

Transformer 1 - 593 ohm, 556 ohm

Transformer 2 - 589 ohm, 589 ohm

Small transformer - 12 ohm

[Andrew F. Ali]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

From looking at the transformer the middle winding is primary.

Transformer 1 - 593 ohm, 556 ohm

Transformer 2 - 589 ohm, 589 ohm

Small transformer - 12 ohm

Transformer 1 is suspect

[dick_barton]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

+1
I believe that you look for < 3% difference in the winding resistance for a good transformer

[neilc6]

Re: Philips 40PFL3706/F7 No Backlight Logo flashes on

Got the replacement board today but the seller sent me three boards instead of just one. I contacted them and they said they just wanted to get rid of them.

Measured the secondary coil impedance of the replacement board and it is around 562 ohms for both outputs and both transformers. Quickly tested and the tv stayed on so put the shield back and buttoned everything backup. Testing again, no logo at all. Oops, forgot to plug the board back in.

One minor issue is I saw a black vertical line 1 pixel width that went away. Possibly an intermittent tab bond or panel issue. Will do some extended testing to see how intermittent it is.

If you need an inverter board for this TV, let me know.