Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

[fuxxy]

Sanyo LCD TV
Model: FVM5082

Power Board:
Model: B109-J01

Digital Board:
Model: Z6WF#316A-1599

Analog Board:
Model: Z6WE#411B-0629
PCB30007

I found this TV in the trash by the road. It will power on, and the power LED turns green. The backlights will not turn on. I get display on the LCD, just simply no backlight.

This seems like an odd configuration to me. The backlight inverters are located behind the LCD panel, on a daughterboard that the CCFL's connect directly to. A 14 pin cable connects the daughterboard to the power supply board.

I'm in the process of following the flowchart in the service manual one more time. It's been a couple months since I've messed with this TV and I want to be sure of the information I'm posting. Last time, I remember the flowchart diagnosis of "Replace Analog board".

2 Questions so far:
It appears that this design will allow the other backlights to run, even if one is bad. Do you guys agree?
Is there a way to force the backlights to turn on? (To diagnose which CCFL's are bad, if any.)

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

Service Manual

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

Flowchart Process:

Power LED turns ON, but LCD backlight is not ON or on ly turns ON and then OFF.
For analog signal board: Is the "BL_SW" voltage (3.3V) Supplied from "K8B Pin13"?
- I get 3.1V from pin 13 of Analog Board connector K8B.

For Power board: Is the 0V supplied from "CN3 Pin10" during LED on?
- I get 0.5V from Pin 10 of Power Board connector CN3.

Panel is broken. Replace "Panel".

The LCD display itself is not bad. I'm assuming the "Panel" is a preassembled unit containing the Backlights, inverter, and LCD.

[budm]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

You have correct signal, the way to look at it is like this:
If the inverter circuit is good but you have a couple of bad lamps, then the inverter circuit should kick on and try to fire up the lamps for it to know if it encounter bad lamps or not, so the at least some of the lamp should at least come on before the protection circuit kicks in.
Since you did not even see the backlights flash on for seconds then we need to verify that the Inverter transformers driver circuit (which is on the power supply/Inverter transformer driver combo board) is OK or not.
So first of all, the red round fuse FL1 for the Inverter transformer is OK?

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

I replaced FL1 when I last opened this set a few months ago. I don't remember testing it after putting it back together and it still not working.

FL1 is blown.

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

I think I know your next question, so I'll go ahead and answer it.
Fuse FL1 feeds MOSFETS Q|1 and Q|2. Both mosfets source-to-drain resistance is zero(shorted).

[budm]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

I think I know your next question, so I'll go ahead and answer it.
Fuse FL1 feeds MOSFETS Q|1 and Q|2. Both mosfets source-to-drain resistance is zero(shorted).

Well, you found the problem.

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

I replaced MOSFETs Q|1 and Q|2 with what appears to be a replacement, even if the case size is different. Originals are Farnell 12N50F, and I replaced them with 2SK3522.

The replacement fuse popped as soon as I plugged in the mains power.

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

What are the specs I should be concerned with when sourcing a replacement part?
To me, the replacements I found seem compatible, but now I'm not so sure..

Original: MagnaChip MDP12N50F

[budm]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

The input capacitance of the 2SK (2280pf typ.) is much higher than the original 12n50 is 1010pf, this may cause cross conduction.
I would use the original MOSFETs and also check all the resistors/diodes connected to the GATE of those MOSFETs for damages.

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

Tested the resistors and diodes attached to the gate seen in this pic, all appear within tolerance.

I'll keep searching through my parts bin for compatible MOSFETs, and I'll order if I can't find them. It appears an NTE2928 is almost identical as well.

D|3 0.547v
R|20 - 10K Ohms
R|16 - 47 Ohms
R|17 - 22 Ohms

D|6 - 0.548v
R|35 - 10K Ohms
R|28 - 47 Ohms
R|33 - 22 Ohms

C|22 - I don't have a capacitance meter to test
R|32 - 51 Ohms

RJ18 - Shorted (Jumper?)

[budm]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

Those two snubber caps C134, C135 are good with no leakage resistance?

[fuxxy]

Re: Sanyo LCDTV FVM5082 - Powers on, Video displayed, but no backlights.

Those two snubber caps C134, C135 are good with no leakage resistance?

Both capacitors, when removed from the board, read "OL" resistance (infinite)