TCL 32A12H TV no backlight

[dicky96]

Hi guys

My latest repair is this TCL 32A12LH TV, which looks like something that has been thrown together with various bits of off the shelf..... errr...stuff

I have no back light - It's got 24V from the PSU board to the inverter therefore looks like the SMPS is running.

There are two other wires from PSU to inverter one has 4.8V on it, the other 2.4V. Had a look on my scope. no logic signals or pwm on them but there does seem to be a considerable (excessive?) amount of ripple - see pic (this is the 2.4V on 1V/cm DC).

Could anyone tell me what signal/voltage should I see on those 4.8v/2.4v lines to activate the inverter?

Cheers
Rich

[budm]

Re: TCL 32A12H TV no backlight

Did you remove the metal off the then inverter board to check the fuses and the caps yet.
One signal is BL-ON, another one is for Dimming.
BTW, is that SAMSUNG LCD panel?

[dicky96]

Re: TCL 32A12H TV no backlight

Got the metal off - the 7A fuse on 24V rail near the power connector is OK

Can't spot any other fuses

The two electrolytic caps are not happy bunnies!

I first thought one of these had been replaced before at some time as they are very similar values but different makes, but the glue holding them to the PCB looks the same on both so i guess they are both original?

I don't suppose anything special needed here? - they are just connected from the 24V rail to 0V in parallel

No shorts around the four STU432S Fets, but then if there was I would have expected a blown fuse.

A quick check shows the STU432S are connected in pairs in some sort of parallel arrangement - The source of one pair going to 0V and the drain of the other pair going to 24V

The driver chip is an OZ9966SN - I am struggling to find a proper datasheet for this, just this image http://www.datasheet4u.com/pdf/OZ9966SN-pdf/1093178 which at least gives some info.

The screen is a samsung

[dick_barton]

Re: TCL 32A12H TV no backlight

Those two electrolytics are in desperate need of changing and I'd do that straight away.

[dicky96]

Re: TCL 32A12H TV no backlight

Unfortunately the only suitable replacements I had here were pulls off an old PSU or something. They looked OK on cap and esr meters. They were a bit too big to get the metal screen back on the inverter pcb but I guessed they would be OK for test purposes so I tacked them in place.

Once I did that the 24V would come on for a few seconds then go off again. Sometimes I would hear a noise like a relay click just after power up.

I was trying to get my scope on the two signal wires to see if it is trying to strike the inverter but the 24V was not staying present long enough. then after several attempts the 24V did come on for some seconds, and while I was looking at the signal lines one of the replacement caps popped quite spectacularly!

I guess it wasn't as OK as I thought. Have got to go back to UK next week anyway so will order some new replacements as the bar this TV belongs to are not urgently needing it.

There's nothing special about the original caps is there other than being small enough diameter to fit under the pcb sheild?

NXH 35V 470uF SAM YOUNG (M)/105C
Rubicon 35V 390uF ZLH T0918 105C PET

Had a quick look at the electrolytics on the output side of the main SMPS and they all look OK visibly

[budm]

Re: TCL 32A12H TV no backlight

I would at least recap the caps for the 24V on the power supply board too at the same time.

[dick_barton]

Re: TCL 32A12H TV no backlight

+1 That would be a wise move to do so at the same time.

[dicky96]

Re: TCL 32A12H TV no backlight

I think that is excellent advice. Will do. Cheers.

[ReeceyBurger123]

Re: TCL 32A12H TV no backlight

Also bad cap on the mb I noticed

[budm]

Re: TCL 32A12H TV no backlight

Also bad cap on the mb I noticed

I just notice that too, it has black stuff on top and the top is domed.

[dicky96]

Re: TCL 32A12H TV no backlight

I replaced the 470uF 35V cap on the inverter board again and also the domed 1000uF 16V cap that was spotted on the main board Interestingly that one read OK on my ESR meter but only 100uF on my cap meter.

Anyway with the three caps replaced the TV now powers up.

Cheers guys