TCL 55S421 powers on + LCD display + sound but no backlight

[howardc64]

Likely backlight strip but thought I check before tearing into this panel (read maybe easy to crack along the top edge due to glue so unclip the glued on bracket rather than separating glue... I'll definitely becareful!!! haha)

Flash light shows image on LCD display
Have sound (there is notification sound when powering off)

PSU P401 connector BL_ON 0v -> 3.3v when powering on

PSU P601 connector (4 wires to 2 LED strips, Each strip = 2 segments with 8+7 6V LEDs). When powering TV from off to on
Pin 2 0v -> 0v
Pin 4 85v -> 120v and gradually settle to 85v
Pin 7 0v -> 0v
Pin 9 85v -> 116v and gradually settles to 85v

Pin 4 and 9 mostly holds at 85v while TV powered off after being powered on. If AC plug is removed, these will slowly drain down.

Does this suggest LEDs are good or bad?

Don't have LED strip tester but thinking about buying one. I should be able to connect tester on P601 pin 2+4 and pin 7+9 and see either strip light up if good right?

[nomoresonys]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

do you get a backlight flash when you power up?

[nomoresonys]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

check tcon fuse might be f101, see if you have about 12 volts on each side, using chassis ground.

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

do you get a backlight flash when you power up?

No backlight flash ever.

check tcon fuse might be f101, see if you have about 12 volts on each side, using chassis ground.

Uploaded more clear tcon pic on original posting. Fuse on top left of ribbon connector from main board reads 12v on both sides as well as the probe point to the right (Board says F1 5A/32V, I've seen mislabel before on Vizio TV boards)

[R_J]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

You already proved your t-con is ok with the flashlight test, You likely have an open backlight string causing the backlight driver to shut down.

[nomoresonys]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

yep, he did, i read it and forgot it before post #3,

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

3 more data point before diving into panel tear down for backlight access.

From power off to on

A_DIM 0.7v -> 2.6v
P_ON 3.1v -> 3.1v (never changes)
P_DIM 1.2v -> 3.4v

Don't know whats normal.

And just to double check, should be able to drive the LED strings from the PSU connector to see if they are good before panel teardown right? Should see 90v? (15 x 6v LED / string)

[neilc6]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

Time to buy an LED tester if you want to repair TVs. Easy to confirm an open circuit in the string plus you can power the LEDs independent of the power supply to check the screen before ordering new strips. Power the strips with the connector unplugged.

[howardc64]

Re: TCL 55S423 powers on + LCD display + sound but no backlight

Time to buy an LED tester if you want to repair TVs. Easy to confirm an open circuit in the string plus you can power the LEDs independent of the power supply to check the screen before ordering new strips. Power the strips with the connector unplugged.

Yeah got a tester on order. Will confirm failed LED strings before ordering parts + disassembly. Report back after. Failed backlights seems to be common problem on these TCLs.

Also corrected the model # 55S423 but unfortunately can't change original post title.

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

UPDATE

LED tester came in and sure enough 1 string has an open while the other string lights up fine towards 80v+

Waiting for 6V LED to arrive and will try to do individual LED repair on this one. Will post follow up later.

[neilc6]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

Individual LED replacement is never recommended especially with new strip prices low with lots of sellers on Aliexpress and ebay. All the LEDs have wear and will eventually fail. Every time you open up the TV, you risk damaging the panel. Think that model only uses two rows of strips.

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

Individual LED replacement is never recommended especially with new strip prices low with lots of sellers on Aliexpress and ebay. All the LEDs have wear and will eventually fail. Every time you open up the TV, you risk damaging the panel. Think that model only uses two rows of strips.

Good point. Although this model cause me some pause. Some show the white reflector sheet is hard to take out without damage resorting to cutting rather than peeling off with the "glued"? back. Thus my reason to think single LED repair. Also all the aftermarket replacement strips I've used seems cheaper construction than original usually with thinner metal strip. I've often wondered their longevity. But I suppose OEM already been used awhile as well.

Regardless, turning down backlight is must after the repair

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

Tried single LED replacement. Definitely not for the first timer.

- Cutting off old LED left a copper plate that shorted anode/cathode. Not sure why. Perhaps was part of the old LED? Had to exacto knife to separate.
- Soldering iron is impossibly difficult. LED bar is a big heat sink taking away heat on the iron tip. Difficult to mount LED flat rather than tilted on one side.
- LED color temperature don't match
- Result is lightly tilted LED and likely higher than normal height under the lens
- Hot air requires low temp solder paste (had none on hand). Otherwise, too easy to toast the reflective white paper nearby.

Combined all these issues produced a circular ring bright spot when viewing flat bright colors in the replaced LED region.

Back to the entire LED bar replacement method. On this set, does require cutting the white reflective paper to get the bar out without ripping it.

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

One more note on reassembly for future DIYers.

The rear frame that LEDs/diffusers/LCD panel itself mounts flex a little. When placed on its back, top and bottom of the frame where diffuser sheets are slotted into the keys will not go back in properly due to this flex. Need to place a small support on the rear top to eliminate this flex. Height of the support probably depends on if boards are removed (I used a 1.5" tall support with boards removed)

[howardc64]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

UPDATE

LED bars came in from Aliexpress ($12). Came with 2sided tape on the back. Installed it and here are few notes for future efforts

- Seems impossible to remove the white reflector paper above the LED bars without damaging it. So I did some cuts to get the bars out. Cuts are all in the center of the paper bridge over the LED bars. Plastic diffuser stands goes over some of these bridges. So if careful, can get old bar out and put new bar in without any problem.

- Old LED bars are only glued on under each LED and taped to the tub in 2 sections. A thin blade tool cut the bar glue pretty easily from under the bar. Clean the tool after each cut as the black glue will make their way onto the white reflector sheet (pick off any that gets on with tweezers) Xacto knife cut the tape on the edge of the bar. Some tapes are under the reflector sheet bridge so need to lift up the cut bridge to cut the tape. See pics.

- LED on 1 of the old string was warmer color than the other so best to just replace all bars.

[dskall]

Re: TCL 55S421 powers on + LCD display + sound but no backlight

I've replaced individual leds before. Color never matches. New are always bluer. I think they yellow as they age.