AOC L32WA91 - No Backlights

[dick_barton]

Set is an AOC L32WA91. The service manual for the L32W961 power supply appears to be identical.

The set appears to be working, volume up / down but the backlights are not working.

There is not even a quick flicker from the lamps when the set it taken out of standby.

Removing the cables from PSU to the main board and switching on and the backlights still fail to come on or flicker.

Using a CCFL tester the tubes have been tested and they all light up.

The inverter transformers have been checked and readings given below.
For the transformers on the inverter board the minimum secondary resistance is 1445 ohms and the maximum 1496 which may be slightly above the recommended tolerance.

The driver chip is OZ9976 and shorting the timer input Pin 11 did not result in any change. The two mosfet drivers checked out OK out of circuit.

Power supply: 715G3332-1
Inverter: 715G3333-1

I'm suspecting the OZ9976 IC but any hints gratefully received

[ReeceyBurger123]

Re: AOC L32WA91 - No Backlights

Are you getting boosted Pfc voltage across the cap matey ? Also whats the input voltage to the driver fets ?

[dick_barton]

Re: AOC L32WA91 - No Backlights

Hello Reeceyburger123
I'm getting 394V across the cap (C920) out of standby.
At R801 (fusable resistor) feeding the mosfets the voltage is 390V DC with respect to the negative leg of C920.

[ReeceyBurger123]

Re: AOC L32WA91 - No Backlights

Ok buddy, sounds like your on the right track about that driver chip. As the fets are getting the feed voltage sounds to me like they are not being switched.

[dick_barton]

Re: AOC L32WA91 - No Backlights

Thanks for your input ReeceyBurger123. Time to get the scope out and check the drive but that will be tomorrow now. In the meantime I'll check out D802, D804 and C802.
Thanks

[budm]

Re: AOC L32WA91 - No Backlights

There are many version of this 715g3332-1.
Which one of these has the matching barcode #?


http://www.shopjimmy.com/catalogsear.../?q=715G3332-1
Hopefully you can get to the site, otherwise please provide what is printed on the barcode.

[dick_barton]

Re: AOC L32WA91 - No Backlights

Hello BudM
It looks very similar if not identical to this one.
AOC ADTV8LCCMACF Power Supply for L32W961

The partial barcode on my board is (T)8LCCMAEF
Thanks

[budm]

Re: AOC L32WA91 - No Backlights

Please look at the bottom side of the board, there are many pins that has 'RESOLDER' right next to the pins.
http://assets.shopjimmy.com/media/ca...f-bottom_1.jpg
This board is the power supply-inverter combo, the output ACV is about a couple hundres Volt and is fed to the step up transformer on the sub-inverter board to step the Voltage to thousands of Volts to drive the lamps.

[budm]

Re: AOC L32WA91 - No Backlights

The service manual for the L32W961 power supply appears to be identical.
So you have the service manual with the diagram for the power supply/inverter combo board?

[dick_barton]

Re: AOC L32WA91 - No Backlights

Hello Budm,
The bottom of the board looks identical. One of the first thing I did was to resolder the pads around the HV transformer and mosfet driver end since they all looked rather grey but that appeared to be due to the lead free solder.

Edit#

Yes I have that pdf in front of me on the laptop.

[budm]

Re: AOC L32WA91 - No Backlights

Looking at the board, the two power MOSFETs are driven by the Gate drive transformer so It will be good to see how that IC drive the primary side of the Gate drive transformer.

[dick_barton]

Re: AOC L32WA91 - No Backlights

They are driven by the OZ9976 which has 12V supply and is enabled. I'll have the scope out tomorrow to check if there is any drive.

[ReeceyBurger123]

Re: AOC L32WA91 - No Backlights

They are driven by the OZ9976 which has 12V supply and is enabled. I'll have the scope out tomorrow to check if there is any drive.

Thats what I would do.

[dick_barton]

Re: AOC L32WA91 - No Backlights

Here are the waveforms at the primary pins of T801
The scope signal is recorded 1/20th of its value so approximately 240V amplitude at 20ms

Edit
Thinking about it the results don't make sense. The OZ9976 is only driven by 12V so I cannot see how it could drive the primary at those voltages. Need to repaet test and repost results.

[dick_barton]

Re: AOC L32WA91 - No Backlights

I above measurement were incorrect and explains why they did not make sense.


Measuring the signal on T801 (secondary of transformer driving the mosfets), at switch on the drive signal remains low and after approximately 4-5 seconds the signal appears 13.99v peak to peak at 44.73Khz for 1.5 seconds and then falls back to 0V.
There is no flicker on the lamps during this time.

[dick_barton]

Re: AOC L32WA91 - No Backlights (Fixed)

So the OZ9976 was attempting to bring on the backlights so decided to replaced the inverter board. Set now up and running as normal.

[dick_barton]

Re: AOC L32WA91 - No Backlights

Have had a chance to look at the redundant inverter board. Most of the solder joints are very dull grey and could all do with resoldering.

Watch out for the jumper links that they do not fall out when doing this.

The secondary windings of all 12 transformers are linked in series and one of the links had a very poor solder joint which was probably the fault.

I'll keep the board to one side. It may come in useful at a later date.