Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

[Chungalin]

This Panasonic LED TV TX-42ASE650 tries to turn on with green LED blinking normally. Backlight flashes shortly 3 times and then it goes to standby with red LED blinking every 4 seconds.

Tried to unplug 12 pin connector that goes to LED backlight and it does the same.

PSU is TXN/P1ZGUB. Everything looks pretty. I haven’t found schematics for this model. These are the components I can identify:

Primary
FA5696N IC7201 Fuji Electric PFC regulator
FA5760N IC7301 Fuji Electric Half bridge switching controller
TK6P60W Q7301 Toshiba N Mosfet
TK6P60W Q7302 Tosbiha N Mosfet

Secondary
NR131S IC7503 SanKen Buck regulator
TPC8132 Q7402 Toshiba P Mosfet
TPC8132 Q7403 Toshiba P Mosfet
TPC8132 Q7404 Toshiba P Mosfet
2SB1181 Q7803 Rohm PNP BJT LED strip switcher
2SB1181 Q7804 Rohm PNP BJT LED strip switcher
2SB1181 Q7805 Rohm PNP BJT LED strip switcher
2SB1181 Q7806 Rohm PNP BJT LED strip switcher
2SB1181 Q7809 Rohm PNP BJT LED strip switcher

Cannot identify LED driver IC7800. SMD component markings are often eroded in the wave soldering process. Also, I haven’t identified TO-220F mosfets that are under heatsinks facing the board.

There’re several test points:
SRV-5.3V is actually 5.2V all the time
SRV-16V is about 16V while the TV tries to turn on
SRV-24V is about 24V while the TV tries to turn on
SRV-VLED peaks at 50V while backlight flashes and then drops slowly

Looks like there’re 5 independently driven LED strips. 2SB1181 transistors are related to each strip. I’ve tried to spot differences between each LED strip voltage measuring pins at PSU connector while the short lapse that backlight is on: all five pins have about 25V. When backlight lights, three of them drop to about 14V, one drops to about 6V, and one drops to almost 0V. My DMM is cheap and slow but this behaviour is deterministic between checks.

So, time to open panel to check LEDs, right?

BTW, I cannot identify this panel. Cannot see any sticker. From T-CON I see it’s a LG panel, but nothing else. No sticker under PSU. Perhaps its sticker is under MAIN board, but I don’t plan to remove it just to see that.

"Badcaps" is a good name for a good reason. However, lately is looks as though "Badleds" might be appropriate :-)

I was about to say this, but someone else said it before. I couldn’t agree more.

[dskall]

Re: Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

Need some good pictures. Probably bad led's. Need min/max for good peak voltages.

[Chungalin]

Re: Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

It’s a bad LED in middle strip.

Opening this panel is not difficult, but the real pain in the ass is all those pins holding the white background sheet. Some of them are under PSU or MAIN boards. I have removed all but one, that’s right under MAIN board. I think it would be better to push the pins from behind before opening panel. The long ones are in central positions and short ones in peripheric positions.

I’ve replaced bad LED via stacking method, but I’ve had to place original lens because it’s very different from the ones that are used in my generic strips. Even this way I’m afraid that some brightness difference will be noticeable.

[dskall]

Re: Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

I have same problem with vizio. Led color a little off and uneven brightness on panel. Short of replacing all led's on panel may have to live with imperfection.

[Chungalin]

Re: Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

Panel model is LG Display LC420DUF-VGF1. LED strips are 6916L-1807A ("L1"), 6916L-1808A ("R1"), 6916L-1809A ("L2"), 6916L-1810A ("R2"). They're splitted in half (L1+R1, L2+R2).

I'll get original replacements as I cannot accept the brightness difference with other LEDs.

[KHz01]

Re: Panasonic TX-42ASE650 repair, red LED blink every 4 seconds, backlight problem?

I have the same problem...same model and symptoms...
a) Did you manage to repair the TV?
b) How did you manage to identify the bad LED strip?
To me it look like the complete backlight flashes ( 3 times)