Toshiba 32AV500U picture but no backlight

[DakotaKid]

I have a Toshiba 32AV500U LCD that powers on but stays black. Flashlight test shows that the OSD menu does come up. Sound works when hooked to source. Outputs on power supply appear to be good at each connector with 24V (23.4V) going to right and left inverter boards. Each inverter board is fused with a 5A pico fuse on the power input. The left inverter fuse reads 0.01 ohms, the right inverter fuse reads .9 ohms.

I need to re-test the BKON to make sure it has power. Caps on power supply appear to be good, many are rubycon. No sign of burn marks or failed components.

Pictures (bad I know, I will replace them when I can get it in better light with a better camera) are attached. This is the power supply and the left and right inverters.

[retiredcaps]

Originally Posted by DakotaKid

I have a Toshiba 32AV500U LCD that powers on but stays black.

Check the four ICs on each inverter for shorts.

[DakotaKid]

Indeed retired!! I found two shorted IC's on the right (looking at back) inverter board. The the two leftmost IC's are shorted. These are FDD8447L MOSFETs and you can see in the close up pic I've attached that the second one looks like it has gotten hot at some point because of the discolored metal on the tab.

Other than ordering a couple of these, what next? What should I check to determine the root cause of the failure? I believe the right part is Digikey FDD8447LCT-ND

[retiredcaps]

Originally Posted by DakotaKid

I believe the right part is Digikey FDD8447LCT-ND

Personally, I would order 10 of these for $9.04 and replace all 8.

Remove the ones that are shorted and verify that they are shorted out of circuit.

[retiredcaps]

PS. I would resolder the inverter transformer pins on both boards regardless if they look good or not.

[DakotaKid]

Originally Posted by retiredcaps

...replace all 8 ICs...verify that they are shorted out of circuit...resolder the inverter transformer pins on both boards.

Originally Posted by budm

...replace caps on each inverter board...do some resistance checking on the ICs.

Thank you so much! I have ordered the parts to do all of the above as recommended. I will report back next week after making these initial repairs.
Again, thanks for your time.
DK

[DakotaKid]

Update: I replaced the 8447's, the IC controller chip, the two large caps and I still have a problem but I've made slight progress. I now have what I think is called 2 seconds to black. The backlight comes on then immediately shuts off. While this is happening I hear a high pitched whine from the inverter transformer while the backlight is starting. After the inverter shuts down the TV indicator light stays green (power on) versus red (power off).

I have a schematic of the inverter boards but it doesn't list cap values other than the two large caps so I don't know the value of any of the SMD caps. It also doesn't list expected voltages at various test points. The board does have 23.6V coming in. All voltages at the power supply outputs seem correct.

Any help on what to look at next is appreciated.

DK

[budm]

I would replace the two elctrolytics caps (2 on each inverter board at the same time), also look real close at the IC that drives the MOSFETs, you may want to download the data sheet of the IC so you can do some resistance checking on the IC otherwise you may end up damage the new MOSFETs.

[DakotaKid]

It has master and slave inverters, the notoriously failure prone HIU-813-S along with HIU-813-M. Disconnecting the slave results in no backlight, not even the 2 seconds to black. The two inverters have a connecting cable between the IC controllers on each board in addition to the 24V power cables going to each. I'm not sure if I'd term this 2 seconds to black or brief flash but the high pitch whine I mentioned is definitely coming from the slave transformer.

I am wondering if the IC on the slave inverter board is going into shutdown protection mode...and if so...why.

[DakotaKid]

I am going to hook up a 24V load per budm writeup to make sure it is not the PS causing the issue. I don't think it is.

[budm]

I do not think you are having the 24Vdc problem either, I think it is still something on the inverter boards.
By the way, you replace the IC and caps in the master board?
And you look at the screen from the front, if the master inverter is on the left side of the screen (as you face the screen), when you trun on the TV, do the see the left side lights up half way?
Or if the master inverter in on the right side of the screen (as you face the screen), do you see the right side of the screen lights up halfway?
Try looking at at in dark room.

[tom66]

Bud - I'm not sure how the TV's CCFLs are laid out. For what it's worth, on the Toshiba 32WLT66 (with an LG panel), unplugging either did not appear to change which side was lit up - I suspect LG interleave the bulbs in the panel, making it harder to diagnose. These master/slave configurations are very difficult to debug and repair.

Since the slave is buzzing, I would look there - but I've only replaced inverter transformers or changed fuses, never done any real diagnosis of them. I may have access to 37" inverters at some point (LG 37LC2D is still in office) to check some things, I believe they are the same as 32".

OP: Does the buzzing create a smell like ozone? Ozone is produced by electrical arcing, among other things.

[budm]

I work on the 42" Vizio (LG/PHILIPS with famous MASTER/SLAVE that need mod) that had blown IC and MOSFET on the master, the slave has no blown part, the symptom I see on the screen is that only half the screen lit up for seconds. That was how I look real close at the master board.
Still trying to understand how the 2-inverter function works, I think they work like push-pull setup but not sure.
By the way, I just check the LG LC37LCD2, it draws 3VA in standby.

[tom66]

[duplicate post...! ugh]

[tom66]

3W! Crazy... Must have been before the 1W legislation in EU or maybe they can get away with it because the TV has a timer function and can be set up to output Freeview/DVB-T to a recorder at a scheduled time. It was made in 2006 so it's quite old.

I will have a look at it soon - I will unplug either side and see what it does.

[budm]

Its 3VA, it shows 0Watt on my Kill-A-Watt meter.

[tom66]

Originally Posted by budm

Its 3VA, it shows 0Watt on my Kill-A-Watt meter.

Well, that sounds better, unless your Kill-A-Watt has difficulty with low wattage readings. Mine must be at least 2W because it can draw up to 2W from 3.3V...

[DakotaKid]

It only buzzes a few seconds but I haven't smelled ozone or any acrid odor. Looking at front, the M is on the right, S on the left. I will try turning it on in a bit with the room dark and see if there is a lighting pattern. I haven't studied real closely but the 9897 IC controllers are interconnected between the two inverter boards, I presume so they both shut down if either isn't working properly in a sort of latching arrangement. I didn't replace the IC or the FETs on the Master since the damage on the Slave was so obvious. Also, according to the service manual on my model they did a revision and went to a single inverter with dual outputs, probably due to so many issues with the M/S setup. I don't know if they changed the wiring configuration of the ccfls in the revised panel or just extended the connector over to the other side.

In thinking thru this further, is there any reason why I shouldn't able to replace my dual inverter setup with a single board with both inverter circuits on it as long as the PS 24V lines, grounds, dim, etc. all match up? Also, is there any easy way to test the ccfl tubes for damage in case something happened when the slave failed?

Thanks for the help and input guys.

DK

[DakotaKid]

The backlight doesn't seem to show any distinct pattern when it comes one, maybe a bit dim though if anything. After pressing the power button the backlight flashes quickly once then off a couple of seconds then back on for about a second or two then goes off. The green power light stays on throughout and after all of this occurs.

[retiredcaps]

I can't quite tell from the photos, but see if you can identify the secondary pins on the inverter transformer and see if they are within 3% of each other.

You are looking for something like this

http://www.lcdparts.net/TransformerD...ProductID=3494