ISIS ISI-42-913-LED3D Backlight strobing

[tn245]

This is some cheaply made Vestel rubbish I think. When it came to me several components on the PSU had blown. I fixed up the PSU as best I could and now it works without blowing the fuse.

The TV is functioning and showing a picture, menus, etc.. but the backlight strobes on and off a lot on startup, and then intermittently after that. Turning up the BL brightness makes it flash constantly, turning it down makes it flash less and if it's dim it stays on. The +24 to the inverter board is steady until the BL lights up, but then drops out by a couple of Vs and fluctuates once the BL is flashing. My DMM doesn't really keep up though.

I've disconnected the main board and turned on the backlights with a resistor between BL_on and +5V and the flashing is there, so it's either the PSU or inverter board. I think I managed to get it to stay on with BL set to fairly dim by blasting the boards with a hairdryer, but then it stayed the same when it cooled and I couldn't repeat it.

My guess is capacitors, but there's a lot on there and I'd like to be sure before shelling out on a whole set. They all look fine and I'm still waiting for my ESR meter to arrive from China. It'd be nice to narrow it down a bit.

PSU is 17pw25-4 and schematic can be had from here:


Any suggestions? I'd be very grateful for any pointers.

[tom66]

Re: ISIS ISI-42-913-LED3D Backlight strobing

The output capacitors on the 24V are used as part of the PFC circuit. If these fail, they can cause issues like this. I would suggest replacing all the 24V capacitors. In general these power supplies are among the worst for reliability due to the multiple parallel diodes and crappy psuedo-PFC design.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Just to be clear, we're talking about these capacitors? And replace all of them?

Thanks tom66.

[tom66]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Yes, those. Use Panasonic FC, FM or FR.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

The caps on my board are actually different values than the ones in the diagram. I guess I should go with what's on the board right?

[tom66]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Yeah. Vestel appear to pick up whatever capacitors fell on the floor that particular day to make their TVs.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Actually on this board there's only 3x3300uF and 1x1000uF caps in place of all those caps in the diagram. So that's 10900uF total instead of 22800uF on the schematic. Is this some kind of cost-cutting measure or do I really not need all those caps (it is an LED backlight)?? What values would you replace with?

[tom66]

Re: ISIS ISI-42-913-LED3D Backlight strobing

A 26" of this I did (CCFL backlit) had more capacitance, so I would go for more. A 26" inverter draws about 65 to 80W. LED driver will draw around 100W for a 42" screen. Maybe 3x4700uF and 1x1000uF after inductor.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Ok that sounds good. I'll get those ordered. I think you're right about those caps being the culprit. I tried the hairdryer again directed at those caps and it stopped the strobing when set on a low backlight.

Thanks again!

[Ltank]

Re: ISIS ISI-42-913-LED3D Backlight strobing

On the inverter board check the big capacitors. I have had about 4 sets that the capacitors had a high ESR or changed values. I replaced them and the backlights worked fine no strobing effect. They have not caused a return due to failure. Its an easy fix, cheaper than a board.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Thanks Ltank. I will replace those too then.

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

So... replaced inverter caps, tested - still strobing.

Replaced 24V PSU caps - still strobing.

What's next?

[tom66]

Re: ISIS ISI-42-913-LED3D Backlight strobing

What happens if you disconnect 24V to inverter in operation and reconnect it? Could be power supply instability from starting with high load. In other words bad design.

Also, the connector on the main board is not properly seated:
https://www.badcaps.net/forum/attach...7&d=1384615446

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Ah, that connector has been seated for all the testing. It just wasn't for that photo because I had just had the boards out and put them there to pose for the pic.

I tried your suggestion and on reconnecting 24v to inverter it carries on strobing.

Could this problem be caused by something in this section of the circuit (see attachments)?

The repair I did involved several of these smd components. It looks really dodgy because the previous owner had obviously attempted the repair himself and torn up several of the pcb pads and tracks. I had to solder two wires to connect up D942 to IC900 and the glue you see there was to try to prevent any movement breaking the solder joints or ripping any more track. I meticulously tested to check if everything was connected up right but still wasn't 100% happy with the repair. Also does C909 look a bit off-colour to you? Does that mean anything?

Components replaced in the repair:
F900,Q935,R974,R977,D907,IC900,D942,R923,D941,Q909,R980,R981,D922-938

[tn245]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Just wanted to update that this TV was fixed with a brand new power supply, and works great!

[jorg]

Re: ISIS ISI-42-913-LED3D Backlight strobing

My son has this model too and it died while he was using his Play Station.

I checked the PSU (17PW25-4) and the plug supplying the main board (17MB65-2) didn't seem to be seated properly. After rectifying this and powering back up, the Standby LED illuminates for about 2-3 seconds before the set powers off. Everything is dead apart from a beep when any of the fascia buttons are pressed.

The capacitors on the PSU appear to be OK with no signs of bulging etc. I tested all the diodes, the FET (Q935) R974 and R977. The incoming mains and Radial Fuse are fine and there is no signs of damage to any components or dry joints.

I have looked at the main board and checked the fuse and again there is no sign of damage or dry joints.

The Inverter is LG, the same as the screen and I eliminated this by disconnecting, and also disconnecting the video and speaker outputs from the main board.

Is the behaviour of this TV familiar to anyone?

Thanks.

[stj]

Re: ISIS ISI-42-913-LED3D Backlight strobing

can you post a foto of the main board?

[jorg]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Please find attached.

[blandau]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Check the PWM feedback circuits and PWM IC frequency.
If PS/Inverter board is good, suspect panel

[Shotgun25]

Re: ISIS ISI-42-913-LED3D Backlight strobing

Just wanted to say that the service manual posted at https://www.badcaps.net/forum/showthread.php?t=39481 for the vestel board 17MB65-2 in a Celcus TV fixed a no audio from internal speakers, but sound from headphones issue.
The posts above in this thread helped me narrow down what was working in the TV so thanks.

No audio fix instructions below.

Turn TV on
Press menu
Type 4725
Type 4725 again
Service Menu appears
Open Diagnostic
Select factory reset
Press OK
Wait approximately 2mins
Accept 'first run' options
Tune any channels or switch to your desired source
Test sound works

I suspect this is only a temporary fix, there may be some hardware that is aging, but the fix can be applied as many times as it works

Hope that helps someone.