LG 37LC7D-AB again - Backlight problems

[mangozac]

After my latest round of repairs to this set she worked well for a couple of weeks and then started exhibiting a new issue of no backlight. I still had sound and shining a torch at the screen showed the panel itself to be working. At first the backlight was intermittent - it wouldn't come on when the TV was switched on, but cycling mains power solved it. That worked a couple of times but then it wouldn't come on at all.

The backlight inverter is part of the PSU board:


The INV_ON signal seemed to be functioning, although at a measured ~1.2V is lower than I would expect, since the signal is coming from a 3.3V microcontroller.

I pulled the board and set it up on my bench. ON/OFF and 3.4V_ON were both tied to STBY_5V to power everything up. I connected INV_ON to 3.4V (since this signal usually comes from a 3.3V microcontroller). I hooked up a 15W CFL as described in BudM's Instructions and was surprised to find that the bulb lit up - although only for about a second before switching back off. It does this each time power is applied.

So the question is: is this turning the bulb back off likely to be due to the CFL bulb load being too small? Would this indicate that the inverter circuit is working? I had figured that the intermittent failure method of the backlight meant that the backlight itself was probably ok.

[rievax_60]

Re: LG 37LC7D-AB again - Backlight problems

The lamp voltage is usually monitored by the control IC like in this example by C12 and C13 dividing the lamp voltage and applying it to the Vsen pin.
http://circuits.datasheetdir.com/59/...0-circuits.jpg
As CCFL lamps wear, the voltage increases. At some point the inverter might start shutting down.

[mangozac]

Re: LG 37LC7D-AB again - Backlight problems

Interesting. Although the transformer on this board has an extra winding that seems to provide some feedback to the controller IC, it does also feature these voltage divider caps. The controller IC is an MSC1691A but I can't find any datasheet.

I did notice the error output on the schematic you linked to, and it reminded me of the error pin on one of the connectors on my board here. I measured this pin: it started at 5V and when the bulb was switched off it went to 0V to indicate the error. This would almost certainly indicate that the test bulb I have connected is incorrect.

I'm pretty sure the backlight on the TV never even came on for 1 second (you can see the glow on the rear sides of the panel even if the LCD pixels are all off), so I suspect that the driver board may be fine and I actually have a connection issue...

[mangozac]

Re: LG 37LC7D-AB again - Backlight problems

Tentative update: it seems to be working again. I hate intermittent issues!

Since the CFL test was giving positive results I decided to rule out any problems with the MICOM signals and backlight itself and hooked the PSU board up to the backlight on the TV with all of the power supply enable link wires left in place. The backlight came on and stayed on.

OK, maybe the fault is with the MICOM->PSU connections? I pulled off my link wires and connected the board correctly and the TV was all working fine.

So I think the issue was/is a bad connection somewhere. I'll just have to keep an eye on it. I'll leave the back of the TV secured with just two screws for a while