Samsung UE55H6500SL : plus d’image après remplacement des LED rétroéclairage

vitimio3 wrote:

@ElektrodaBot
pour un televiseur Samsung UE55H6500SL , apres avoir remplacer les leds du rétroéclairage , je n'ai plus d'image , son ok, plus de contraste et lumière .
Les micros leds sur la Tcom s...

Initial assessment of available information
- Symptom: after replacing the backlight LED strips on a Samsung UE55H6500SL, you have sound but no picture, no contrast or light; status LEDs on the T‑Con board are lit.
- Implication: T‑Con has its supply (typically 12 V) and the set boots, but the panel appears dark. Because the failure started right after the backlight job, the most probable root cause is in the LED chain or LED driver section on the PSU, not in video processing.

Direct answer to the question
Most likely the LED driver is not starting (open circuit, short to chassis, wrong strip type/orientation) or it starts then shuts down in protection. Verify first whether an image is present without backlight (flashlight test). Then measure BL_ON/DIM control from mainboard and the LED driver output voltage on the PSU. If BL_ON is high but LED output is zero or pulses then collapses, re‑inspect the strips, connectors, and the LED driver fuse/shunt on the PSU.

Detailed problem analysis
Work from the simplest checks to targeted measurements.

1) Confirm whether the LCD produces an image (backlight aside)
- Flashlight test: in a dark room, turn the TV on and shine a strong flashlight very close to the screen at an angle. If you can barely see menus or motion, the panel/T‑Con/mainboard video path are OK; the fault is only in the backlight path. If you see absolutely nothing, keep the backlight checks but also plan basic T‑Con tests below.

2) Visual checks after a backlight replacement
- LED strip connectors and jumpers between bars: fully seated, no bent pins, no reversed plug.
- Strip orientation/polarity: verify the +/– markings on the strips match the PSU harness polarity. Some kits have A/B (or L/R) bars that must be placed in exact positions.
- Shorts to chassis: inspect for solder bridges, nicked solder mask, or a lens or pad that touches the metal rail.
- Compatibility: identical reference, same number of LEDs per string, same LED type (3 V vs 6 V “dual” packages). A mismatch will prevent proper current regulation.

3) PSU/LED driver controls (low‑voltage side)
- On the PSU <-> mainboard connector, identify:
- 5V_STBY: ≈5 V present in standby and ON.
- PS_ON: 0 V in standby; >2.5–3.3 V when the mainboard requests PSU ON.
- BL_ON (a.k.a. LED_ON): ≈3–5 V when the mainboard requests backlight ON.
- DIM/PWM: either a PWM signal or a DC level (typically 0–3.3 V) that sets brightness.
- Measurements with a multimeter:
- If PS_ON is high and BL_ON is high during power‑on, the LED driver should attempt to start.
- If BL_ON stays low (≈0 V), the mainboard is not enabling the backlight (rare in this scenario, but possible if it detected a fault).

4) LED driver output (high‑voltage side)
- Measure between LED+ and LED– on the PSU output to the strips (cold/isolated side). Expect a high DC voltage appropriate for a 55" chain (commonly ~120–220 VDC under load; can rise higher briefly at start).
- Case A: Voltage never rises (stays near 0 V) despite BL_ON high → LED driver not starting (driver IC fault, open fuse/shunt, or interlock).
- Case B: Voltage ramps up then collapses quickly, possibly repeating (“pumping”) → driver detects open string or overcurrent → protection. Look for:
- Open circuit in one bar or an unplugged jumper.
- Shorted LED/package or a bar shorting to the heatsink.
- Case C: Voltage holds in a reasonable range but no light → wrong strip type/polarity, or bars installed but optically blocked (unlikely).

5) Quick isolation without full disassembly
- If the strips are split into several series strings, temporarily disconnect one string at a time (power OFF first) and try to power again. A string that causes immediate shutdown indicates the fault is on that branch. Note: many models wire all bars in series; only use this step if your set has multiple outputs.

6) Integrity of the LED driver hardware on the PSU
- Inspect/measure:
- The small SMD fuse in the LED path (often right next to LED connector) → continuity must be present.
- The current‑sense (shunt) resistors (very low ohmic value, e.g., 0.1–0.47 Ω total) → not open.
- Driver IC area for cracks/burn or cold solder joints.
- A frequent accident is powering the TV with the LED harness disconnected; the driver can spike and blow the LED fuse/sense path.

7) Bench/forced test (only if comfortable and aware of safety)
- With the mainboard disconnected from the PSU:
- Use a resistor (≈1 kΩ) from 5V_STBY to PS_ON to start the PSU.
- Use another resistor (≈1 kΩ) from 5V_STBY to BL_ON to command the backlight ON.
- Observe the LED output voltage behavior. If it still won’t start or collapses, the issue is on the PSU/LED side (driver or strips). If it starts and the backlight illuminates fully, the strips and driver are OK; then suspect mainboard control, cabling, or a protection signal.

8) Verifying the strips themselves (out of circuit)
- Use a LED strip tester to drive each bar individually. The bar should light uniformly and draw similar current to its siblings. A bar that only flickers on a tester or requires much higher voltage indicates a bad LED or poor inter‑LED connection.
- Compare the count of “LED packages” per bar (e.g., 7/8/9) to the originals; mixing 3 V and 6 V packages in a chain is a common silent killer.

9) If backlight proves OK but still no visible image
- Reseat and clean the FFCs (flat cables) between mainboard↔T‑Con and T‑Con↔panel source boards. Ensure the locks are fully engaged and contacts are clean (IPA).
- One‑side isolation: power the set with only one T‑Con‑to‑panel ribbon connected at a time.
- If half a picture appears with one ribbon, the other side/panel driver is loading the T‑Con.
- If no half‑image in either case, measure on T‑Con test points:
- AVDD ≈ 15–17 V, VGH (a.k.a. VON) ≈ +25…+30 V, VGL (a.k.a. VOFF) ≈ −5…−10 V.
- If these rails are missing while T‑Con LEDs are lit, the T‑Con may be in protection due to a panel short. However, given the timing with your backlight repair, this is statistically less likely than a backlight chain/driver issue.

Current information and trends
- Many replacement kits on the market mix 3 V and 6 V LED packages or alter the number of series LEDs per bar. Even if “compatible” is advertised, a different forward‑voltage stack often triggers the Samsung LED driver protection. Always match the original bar references and topology.
- Quality variability: a single weak new LED may pass a quick tester check but fail under rated current, causing shutdown.

Supporting explanations and details
- The LED driver is a constant‑current boost (or flyback) stage. It raises voltage until the programmed current flows through the LED string(s). If it reaches its maximum voltage without current (open circuit), or sees too much current (short), it shuts down to protect itself. This is why you often observe a brief flash or a rising voltage that collapses.

Practical guidelines
- Safety: the LED output is high voltage. Use an isolated multimeter and keep one hand free when probing. Never hot‑plug the LED harness.
- Document the pinout from PSU silkscreen before testing. Take clear photos for reference.
- If you powered the set with LED strips disconnected, proactively check the LED fuse/shunt resistors.

Possible disclaimers or additional notes
- Exact connector labels and voltages can vary slightly by PSU version; rely on the board silkscreen where available.
- Do not assume the backlight will auto‑turn‑on with the mainboard unplugged on every Samsung PSU; some require forcing PS_ON and BL_ON as described.

Suggestions for further research
- Service manual or PSU board number (BN44‑…): schematics help identify BL_ON/PS_ON/DIM pins and the LED driver topology.
- Reliable sources for exact replacement bar sets matching the original reference.

Brief summary
- First, confirm if an image exists with the flashlight test. If yes, stay focused on the backlight path.
- Measure BL_ON and the LED output. If BL_ON is high but LED output is zero or collapses, you likely have an open/shorted bar, wrong bar type/orientation, or a blown LED fuse/shunt on the PSU.
- If the backlight lights under a forced BL_ON/PS_ON test, shift attention to the mainboard control path; otherwise, re‑inspect/measure the LED strips and LED driver section.

Questions for you (to narrow it down quickly)
1. Do you see a faint image with a flashlight?
2. What are the measured voltages:
- BL_ON (in ON state)?
- LED+ to LED– during power‑on (does it rise then drop)?
3. Exact reference of the PSU board (BN44‑xxxxx) and the LED bar set you installed (photo of markings).
4. At power‑on, do you observe any brief flash from the backlight or is it completely dark?
5. Did the TV ever power up with the LED harness disconnected during the repair?

Post a couple of clear photos of the PSU (around the LED connector and driver area) and we can point to the next component‑level checks.

vitimio3 wrote:

@ElektrodaBot
suite à votre réponse sur TV Samsung UE55H6500SL voici ce que j'ai pu vérifier; toujours pas d'images ,le rétroéclairage est ok ,référence carte BN44-00711A.

Initial assessment of the new facts
- Model: Samsung UE55H6500SL.
- PSU: BN44-00711A.
- After your checks: backlight is confirmed OK, sound OK, still no image; T‑Con has its status LEDs lit.

This moves the fault out of the LED/PSU backlight loop and into the video path: T‑Con → panel, or (less likely) mainboard → T‑Con LVDS.

Note: UE55H6500 does not use a One‑Connect cable/box; ignore advice referring to that. Also, this LED chassis does not feed the T‑Con with 24 V; the T‑Con runs from ~12–13 V via the mainboard/T‑Con fuse.

Direct answer
Focus now on the T‑Con power rails and panel load. Measure the T‑Con input fuse for 12 V, then check AVDD, VGH and VGL on the T‑Con. If any of these rails are missing or collapse with the panel ribbons connected, isolate the short by testing with one panel ribbon at a time. If the rails are stable and present but there is still no picture, check that the LVDS from the mainboard is actually toggling; otherwise suspect the T‑Con itself.

Detailed problem analysis
1) Confirm the symptom precisely (quick flashlight sanity check)
- In a dark room, TV ON, shine a strong flashlight at the glass at a shallow angle.
- Faint menus/motion visible → LCD/T‑Con/video path alive (not your case if you already verified “no image at all”).
- No faint image but screen glows uniformly → backlight only; video path is down.

2) T‑Con input supply and fuse
- On the T‑Con, locate the small SMD fuse (often “F1”, “F100”, or “F101”) near the power connector.
- With TV ON:
- Measure 12–13 V on the fuse input and output. If 12 V is present on only one side → fuse open; replace and re‑test (find the cause before finalizing).
- If no 12 V at the fuse → trace back to the mainboard output to T‑Con.

3) T‑Con DC‑DC rails (the critical step)
- With the T‑Con powered (12 V present), measure at the labeled test pads:
- AVDD (a.k.a. VDA): typically 14–17 V.
- VGH (VON): typically +25…+32 V.
- VGL (VOFF): typically −5…−10 V.
- VCOM: typically ~6–8 V (about half of AVDD; exact value depends on panel).
- Interpret:
- All rails present and stable → T‑Con is powered correctly; go to LVDS/data checks.
- Rails missing or pulsing → T‑Con is in protection, most often due to a panel‑side short/load.
- VGH≈0 V or VGL≈0 V specifically → gate driver supply is being clamped by the panel.

4) One‑side panel isolation (find a shorted half)
- Power OFF. Disconnect the left panel ribbon (T‑Con ↔ panel). Power ON and observe/measures:
- If AVDD/VGH/VGL now come up and you see half‑image/stripes on the connected half → the disconnected side is shorted (panel COF/edge board fault).
- Repeat with the right ribbon disconnected.
- If rails remain down in both cases, disconnect both ribbons:
- If rails now come up with both ribbons OFF → panel is loading the T‑Con; identify which side as above.
- If rails still down with both ribbons OFF → T‑Con DC‑DC section is defective.

5) Static resistance checks (power OFF)
- Measure resistance to GND at the panel‑side pads (on the T‑Con) for:
- AVDD to GND: typically >5–20 kΩ. A few hundred ohms or less suggests a shorted MLCC on a panel edge board.
- VGH to GND: typically very high (>100 kΩ). A low reading (kΩ or less) indicates a gate line short.
- VGL to GND: may read through diodes; much lower than VGH is normal, but not near‑short.
- If low resistance is found, inspect the corresponding panel edge (long thin PCB on glass). Look for cracked MLCCs or a flex/COF touching the metal frame.

6) LVDS from mainboard to T‑Con
- If all T‑Con rails are correct yet the screen stays blank:
- Re‑seat/clean the LVDS cable at both ends (IPA, no residue; ensure latches lock).
- If available, use an oscilloscope on an LVDS pair at the T‑Con input; you should see high‑speed differential activity when the set is ON. No activity → mainboard video path/SoC not sending LVDS (less likely given your timeline, but possible).
- Some T‑Cons have a pattern‑test pad/jumper. If you can enable internal patterns and they display, the panel and T‑Con are fine; suspect mainboard/LVDS.

7) What often goes wrong after a backlight job
- Panel stress/COF micro‑cracks or a nicked edge board → pulls down VGH/VGL and kills the image.
- A tiny metal flake or mis‑seated bezel contacting a gate/source line → shorted MLCC on the edge board.
- ESD to panel/T‑Con during reassembly.

8) Replacement strategy (avoid “parts cannon”)
- Do not fit a replacement T‑Con until you have proven that AVDD/VGH/VGL remain normal with both panel ribbons connected. A shorted panel half will instantly stress the new T‑Con.
- If one side is confirmed shorted, sometimes removing a single shorted MLCC on the corresponding edge board restores operation (with potential minor artifacts). Otherwise, the panel is considered not economically repairable.

Current information and notes specific to BN44‑00711A
- BN44‑00711A provides 5 V standby and ~12–13 V main to the mainboard; the LED driver is on this PSU. The T‑Con is not fed with 24 V on this LED chassis.
- Since your backlight is stable, you can focus entirely on the 12 V path to T‑Con and its on‑board DC‑DC rails.

Practical guidelines
- Safety: Work unplugged; discharge caps; the LED driver output is high voltage, and the T‑Con has exposed fine‑pitch pads—ESD strap recommended.
- When measuring tiny test pads, use fine probes or clip leads to avoid slips that can short adjacent pads.
- Take clear photos before pulling ribbons; never hot‑plug the panel ribbons.

Brief summary
- Measure on T‑Con: 12 V at fuse, then AVDD, VGH, VGL, VCOM.
- Do the one‑ribbon test to see if a panel half is shorting the rails.
- If rails are good but still no image, check LVDS activity and re‑seat the cable; then suspect the T‑Con itself.

What I need from you to proceed
1. Voltage readings on the T‑Con (TV ON):
- 12 V at the input fuse (both sides).
- AVDD, VGH, VGL, VCOM values.
2. Results of the one‑ribbon isolation test (left only / right only / both disconnected).
3. With power OFF: resistance to GND of AVDD and VGH at the T‑Con panel‑side pads.
4. Whether the screen ever showed a brief flash or half‑image after your LED work, or it was black from the first power‑up.

Post the numbers and a photo of your T‑Con (both sides if possible). From that, we can pinpoint whether you have a T‑Con DC‑DC failure or a panel‑side short.