[Solved] Troubleshooting Vestel 17IPS71 Power Supply: No Backlight, Sound Only Issue

I have a TV which uses a Vestel 17IPS71 power supply. Recently it has sometimes failed to start up. The backlight flashes on only briefly, then the sound comes on but there is no picture. I found this thread https://www.elektroda.com/rtvforum/topic3337446.html although that describes a slightly different symptom.

The first time this happened I established that there was no voltage output to the backlight, so I resoldered a few joints on the back of the PCB in the backlight boost converter area that looked dubious. This fixed the problem for a couple of weeks, but it has now come back.

The output from the PSU to the backlight is just 2 wires so I assume there is a single string of LEDs. A few seconds after turning on the TV with the remote my multimeter registers about 38V across those wires for a moment, then it drops slowly down to zero. From the values of R105, R106 and R107 (read from the resistors and checked using a multimeter) I calculate the over-voltage detection threshold to be just under 100V, so I don't think it's tripping because of over-voltage.

I am tempted to try replacing the MP3394S regulator IC and the FDD120AN mosfet next. Before I do that, does anyone know of any common failure points on this PSU?
AI: Could you provide the specific model number of your TV?
Linsar 32LED450S
AI: Are there any error codes or indicator lights on the TV when this issue occurs?
The red Standby light behaves normally, i.e. it is on when the TV is on standby, then when I turn the TV on with the remote it flashes and then goes out.

Hello my dear.
It is time to replace the backlight. There are 1 or more LEDs that tend to malfunction.
Good repair

>>21328426 You must change the backlight! Buy new LED BARS and don't change the LEDs because you will have the same problem after a few days. You must open the panel and see the backlight code number first. Be careful with volt and panel code.

Thanks. Both of you. I further dismantled the TV and applied a bench PSU (actually 2 in series because it needed 67V) to the backlight connector. Two LEDs in each of the two strips didn't illuminate at low current, and at the full current (130mA) two still remained unlit while the other two lit more dimly than the rest. I have ordered a new pair of backlight strips.

So I replaced the backlight strips but unfortunately in reassembling the TV a FFC from the screen became detached from the driver board. There's no FFC socket, it looks to me that the FFC was bonded directly to the driver board in some way. Can these connections be restored or should I scrap the TV?

This is the disconnected FFC. 



and this is one of the others that is still connected.

 

https://www.youtube.com/watch?v=8A1TPmJ4Eko

It is doable, just needs to be kept clean and preferably with a microscope.

Thanks @ivtv1 and @Telo. I wasn't sure that the disconnected FFC was the only problem, because the symptom I was getting after replacing the backlight wasn't the dark vertical band on the screen that I expected from that broken connection, it was more than that - it looked like a loss of sync. Anyway, I followed that video, using acetone to remove the residue of ACF, then no-clean flux and solder to tin the fingers on the FFC and PCB, removed all the solder bridges between the fingers, then more no-clean flux and the soldering iron (without solder) to solder them together. I reckon I did a reasonable job.

However, after reassembling the TV the screen still shows loss of sync. I guess I must have damaged something else. I tried reseating the FFC that runs between the main PCB and the driver PCB at both ends (which have proper FFC connectors) to no avail. So I shall scrap the TV. It's quite old and the smart apps were so excruciatingly slow as to be unusable anyway, so it's not a big loss.

Thanks to everyone who posted suggestions. Replacing the backlight strips did fix the original problem of the backlight turning off immediately.

The original issue (PSU turning the backlight power off almost immediately) was solved by replacing the backlight strips.