Samsung LE40A756 no backlight

[PowerAmpFreak]

Samsung LE40A756 tries to start the backlight (checked the INV ON/OFF signal) but doesn't succeed and then starts over and over...
The powersupply is BN44-00197A with integrated HV supply.
Caps replaced, the obvious supply voltages seems to be OK, but no HV output from inverter.
Inverter power circuit measures OK.
Any good advice on this?

[tom66]

Re: Samsung LE40A756 no backlight

Schematic.
http://www.tgohome.com/ServiceManual...&Search=Submit

May be useful...

[selldoor]

Re: Samsung LE40A756 no backlight

Picture of the inverter board?
Any fuses on there

[tom66]

Re: Samsung LE40A756 no backlight

This uses integrated high voltage mains driven inverter.

[selldoor]

Re: Samsung LE40A756 no backlight

Ok Tom - so does it have a distribution board - is it one where if a lamp is out you can trick it by shorting the lamp connector and it will run with a dark stripe where the bulb is out?



EDIT EDIT yes it does have a board 21 transformers on it!!

[budm]

Re: Samsung LE40A756 no backlight

What happen if you disconnect the cable between the main board and the power supply board? It does have Balancer board with current transformers for each lamp. There should be FB pin on the inverter board, and it should have 0VDC if the lamps do not draw over current.

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

It appears to be something wrong with mainboard after all. The INVERTER ON signal just goes +39mV when set "should" start inverter. This is true even if the cable between mainboard and psu is disconnected. I haven't forced INVERTER ON signal to +5V yet, but as far as I can see, all powersupply voltages are correct from powersupply.
So it boils down to some mainboard problem..

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

With the risk of sounding a bit confusing (I take the risk ) close to the 6-pin ribboncable going from mainboard to psu, containing the INVERTER ON signal, there is proper +5V as soon as inverter should start, but that proper +5V is pulled down by a transistor on mainboard. Have to dig deep into this I assume...

[tom66]

Re: Samsung LE40A756 no backlight

Could be a bad eeprom or t-con fault (unplug t-con -- a shorted t-con can crash main board.)

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

Could be a bad eeprom or t-con fault (unplug t-con -- a shorted t-con can crash main board.)

Yeah, could be a corrupt eeprom. Wonder if it can be cured by shorting SDA and SCL during powerup (as can be done on other LE40XXXX...)
Didn't know, though, a bad t-con could crash mainboard. Will check! Thanks, Tomie

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

Unplugging T-con won't do any difference. The set reacts to poweron, then enables the PSU. The cooling fan starts to spin. Two seconds later, the fan stops and set tries to restart. Just like it doesn't get the proper voltages from psu, but they're all there.
STBY 5.3V measures actually 5.1VDC, and 13V measures 12,3VDC, but I guess those are close enough. So, conclusion is corrupt mainboard?

[selldoor]

Re: Samsung LE40A756 no backlight

Did you try jumping the bl on yet?

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

Did you try jumping the bl on yet?

Never did, it has been shown that problem is not in the backlight drive area. The mainboard does not want to power up the backlight, probably because it's corrupt.

[tom66]

Re: Samsung LE40A756 no backlight

Chek DET5V or 5VDET

[PowerAmpFreak]

Re: Samsung LE40A756 no backlight

Chek DET5V or 5VDET

That is OK. It's directly connected to 5V so it comes alive as soon as PSU ON gets on. It appears the owner of this set wants to terminate repair attempts. So this may be the end of all this. I will get back as soon as I know for sure.