JVC LT-55CF890 backlight on but no image

[notallbad]

This is a Fire TV Edition
The tv powers on, sound and can hear the remote response (clicks on the tv) but no images, no logo, menu or picture.

This set only has power and main boards.

Help appreciated!

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

photos

[diif]

Re: JVC LT-55CF890 backlight on but no image

Voltage on the fuse both sides ?

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

12.5v on both sides of the fuse on the (t-con) panel board fuse

[diif]

Re: JVC LT-55CF890 backlight on but no image

Check for any test voltages, check the diodes and caps for shorts.

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

the 3 diodes on the (tcon) panel board seem ok, caps dont appear to have shorts.
Not exactly sure which are the test points in all cases for the voltages labeled but there are 1.8, 3.3, 1.05 and 12v present around the board.

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

When initial mains power applied the red led standby light comes on solid, when powered up from standby the red led flashes 18 times and then when powered off goes solid, next time its powered from stby it goes out with no flashes. Until next time its completely powered down then its back to 18 flashes.
I guess this is normal being a smart tv?

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

power board voltages

12v stby (stby- 12.55v, on - 12.55v)
power pin 9 5v_stby (stby - 0v, on - 0v)
dimming (stby - 2.75v, on - 3.4v)
BL_on_off (stby - 0v, on - 5v)
stby_on/off_not (stby - 0v, on - 0v)
power pin 1 24v_vcc (stby - 24v, on - 24v)

After a short period of time (5 mins) the stby volts change
12v stby (stby- 8v, on)
power pin 9 5v_stby (stby - 0v)
dimming (stby - 0v)
BL_on_off (stby - 0v)
stby_on/off_not (stby - 1.4v)
power pin 1 24v_vcc (stby - 5v)

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

The power pin 9 5v stby is not connected to anything on the pcb side of the board so can be ignored.

So looking at the power board voltages I would say they look ok.

Anyone care to confirm please?

[diif]

Re: JVC LT-55CF890 backlight on but no image

I was suggesting you check the voltages on the slim boards that connect to the panel, the rest are working if you have everything but picture.

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

Yh sorry, I was partly supplying more detail as well as checking all basics were covered and correct.

I believe at the moment that the power board and backlights are ok. Suspect the firmware is ok.
But not yet convinced the main board cpu chip is 100%, which if it is ok it then only remains for the panel board(s) to be faulty. Bummer!

[comptek]

Re: JVC LT-55CF890 backlight on but no image

Check screen slim boards for anything getting hot caps or dc-dc converter etc

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

Shall confirm with photos later but although there is hvin_12v and hvin_24v at the connector theres also a hvin_12v on one of the slim panel boards that does not appear to be there and noticed a chip labelled 8100vb (3 pin - 2 top, 1 bottom) quite hot, think its a diode. Theres a small i.c, 6 pin i think in close proximity that could be a dc-dc converter.
I have a temp gun so shall check around the boards on those small smd caps.
But again will confirm later.
Thanks for the help!

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

It would seem the slim panel board is hotter in the upper portion as per zoomed in photo below, around 41 degrees whereas its around 24 to 30 degrees elsewhere.

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

Theres an smd G64 6 pin ic, mosfet.

12.5v on pin 4 Source
12.4v on pin 3 Gate
0v on pins 1, 2, 5 & 6.

Drain is connected to the hvin_12v test point that reads 0v.

How can I bypass or otherwise check its the mosfet at fault?
thanks

[diif]

Re: JVC LT-55CF890 backlight on but no image

Is it hot ? You can check mosfets in diode mode but it's not in the photo of the hotter area, have you checked the components there ?

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

Theres an ic SM4190 which I'm assuming is a DC-DC converter, but cant find any datasheet for it.

But seems its not generating any of the required voltages. i.e -14v, -7.4v, -3.3v, -4.15v, 9v or 32v.

I have panel slim boards from a 49" tv which are very similar but no longer connected to the panel but when powered up gives me voltages for comparison.

The other day I did get a perfect picture, I did not do anything to cause it that I know of, but had to power it off and now cant get it back since.

[dskall]

Re: JVC LT-55CF890 backlight on but no image

SM4190 appears to be a processing chip. What voltages are you getting at your ldvs input on the paired lines going into main processor? You should read about 1.2v if data is coming from mainboard.

[notallbad]

Re: JVC LT-55CF890 backlight on but no image

These are the voltage comparisons from the connected panel boards and the spare disconnected from panel board.

[dskall]

Re: JVC LT-55CF890 backlight on but no image

Still looks like a processing chip, not voltage chip.