Hisense f42k20e turns on, screen goes dark

[x_orange90_x]

I have a hisense f42k20e that powers on and the power light flashes between red and blue about 6-7x, then a couple more times and remains blue. The panel lights up bright white then fades to black. I can see images on the screen after it fades. When it lights up there is a quick buzzing noise that sounds like it's coming from the power supply, and when the screen goes black the buzzing stops.

I took the following voltages on the LED strips:

1+ -- spike to 280v, drops to 133v
2+ -- spike to 130v, drops to 0v

I'm not sure what these voltages are really telling me. They don't match what I have seen in the past with other tvs.

[budm]

Re: Hisense f42k20e turns on, screen goes dark

Do you get the same reading if the connector to the LEDs is not connected?
It sounds like you have open circuit in the LED string.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

Any pictures of your circuit boards would be useful.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Do you get the same reading if the connector to the LEDs is not connected?
It sounds like you have open circuit in the LED string.

Yes, I get the same readings without them even plugged in.

In the picture attached, the coil with the black bands on it makes a low pitch buzzing sounds when the tv is in standby, and stops when it's switched on. The large yellow block with the sticker on it makes a strange sound when the leds light up, then stops when they go off (even with them disconnected). The sounds it makes is a little reminiscent of a hard laptop drive if that makes any sense.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

I just realized the first picture of the back of the set is incorrect. Please ignore that one.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

If you place your meter probes between the two legs of C810, what DC voltage do you have in standby and out of standby?

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

If you place your meter probes between the two legs of C810, what DC voltage do you have in standby and out of standby?

C810:
Standby - 172v
ON - 164v

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

The lower voltage is generally when in standby. When the PFC circuit is off then the voltage tends to be around 160V and when on approx 380V. It looks like the PFC circuit is not working.
Can you find a schematic for the power supply.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

The lower voltage is generally when in standby. When the PFC circuit is off then the voltage tends to be around 160V and when on approx 380V. It looks like the PFC circuit is not working.
Can you find a schematic for the power supply.

]

I found a couple but I have to verify which one is correct.

Edit:
When removing the power supply to inspect the bottom, I found this little guy on the floor (picture attached). I don't know if it came from this tv, or my Vizio I posted about in another thread. After I found it I plugged the power supply back in and the tv still acted the same.

What is this and where might it go?

[budm]

Re: Hisense f42k20e turns on, screen goes dark

That is the a fuse. You should leave the meter probes on the two legs of the main cap again and see if the Voltage jumps up higher than 160V when first turn on the TV, since the boosted Voltage for the LED has gone up and then drops down it may have been due to the protection circuit kick in so the PFC also gets shutdown too.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

That is the a fuse. You should leave the meter probes on the two legs of the main cap again and see if the Voltage jumps up higher than 160V when first turn on the TV, since the boosted Voltage for the LED has gone up and then drops down it may have been due to the protection circuit kick in so the PFC also gets shutdown too.

I had them on there for a good 10 seconds or so after powering on. Maybe that wasn't long enough?

That fuse may have come from another non-tv circuit board I had out.

I can try checking the cap voltage again and see if I get anything different.

Edit:
Just checked again. I left them on there for 30 seconds after powering on. 173v standby, down to 166v after powering on and it stayed there.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Here is a couple different schematics. I cannot figure out which one is correct. I've tried comparing them to my board but I don't really know what I'm looking at. Hopefully one of you masters can

This one you scroll down below the ad and it says Get Manual. Click that to download it:



The other schematic is attached to this post. The power supply board is on page 36 and 37.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

When the screen fades to black can you still see a picture when shining a bright light at an angle at the screen and do you have sound.

What dc voltage do you have across C819.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

I can see the image with a flashlight. I haven't checked for sound yet. I will check that and the voltage of C819 tonight.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

Also what voltages do you have on connector XP807

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Sorry for the delay.. Here are those voltages:

C819:
Standby - 17.12v
On - 18.64v drops to 17.92v

XP807:
Standby---
14V - 16.26v
14V - 16.26v
GND
STB - 0.04v
12V - 12.31v
12V - 12.31v
SW - 0.01v
BRI - 0.00v
GND
GND

On---
14V - 15.82v
14V - 15.82v
GND
STB - 5.18v
12V - 12.30v
12V - 12.30v
SW - ?
BRI - ?
GND
GND

I somehow missed one of the last two pins of the connector. I have 3.78v written down, but I'm not sure which pin it belongs to. I want to say it is SW though. I can recheck those two tonight if they're vitally important.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

Your backlight voltage (SW) is missing and should be >2V or more. Can you double check it since you have given SW - ?, but there must be some dc voltage there even if sat at 0v.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Your backlight voltage (SW) is missing and should be >2V or more. Can you double check it since you have given SW - ?, but there must be some dc voltage there even if sat at 0v.

I'm thinking that's the 3.78v I had written down. I will recheck those this evening when I get home. What is BRI for?

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Ok, it is confirmed. I have:

SW - 3.78v
BRI - 0.00v

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

So it looks as though you have all the correct voltages except BRI. Unfortunately I don't know what that is, possibly the equivalent of the DIM signal.

If anyone else knows then I would be interested in finding out it's function.


Edit
Found an earlier thread where Budm gives the answer

'SW' is the BL_ON (INVERTER ON)
'BRI' is the Backlight Dimming control.