Hisense f42k20e turns on, screen goes dark

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

So is that possibly the issue? 0v meaning it's dimmed (off)? When I first turn on the tv and the backlights come on it looks as though they are at max brightness and then quickly fade to black. That makes sense though, the fim function is fading/dimming them, but not at a specific level, just absolute minimum.

I figured that BRI had something to do with BRIghtness given the abbreviation.

The power supply makes little popping and buzzing sounds randomly while the tv is turned on.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

Can you measure the resistance of the BRI connection to chassis at the power supply and the mainboard. Set switched off and mainboard disconnected.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

I can, but I need a little more explanation of what I need to do.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

It's just to check that there's no short circuit on the BRI line either at the power supply end or the mainboard side. Initially you can leave the mainboard connected to the power supply pin and just as you measured the BRI voltage, just set your meter to ohms or Kohm and of course the set switched off and give the reading shown on the meter.

[budm]

Re: Hisense f42k20e turns on, screen goes dark

Remove the 'BRI' wire and see if the backlights will stay on.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

It's just to check that there's no short circuit on the BRI line either at the power supply end or the mainboard side. Initially you can leave the mainboard connected to the power supply pin and just as you measured the BRI voltage, just set your meter to ohms or Kohm and of course the set switched off and give the reading shown on the meter.

I got 26.9 in 200kohms on both the main board and power supply sides of BRI wire. That is with them connected.

[dick_barton]

Re: Hisense f42k20e turns on, screen goes dark

I believe that the BRI pin may also need to be connected to 5V to bring on the backlights one the wire is removed.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

I believe that the BRI pin may also need to be connected to 5V to bring on the backlights one the wire is removed.

I think you are correct.. I disconnected the BRI wire and the backlights did not come on at all.

[budm]

Re: Hisense f42k20e turns on, screen goes dark

So it does not have pull-up resistor for the BRI pin, but check the DCV on BRI PIN anyway just in case; otherwise use 1K resistor between BRI and STBY 5V.

[nomoresonys]

Re: Hisense f42k20e turns on, screen goes dark

You mentioned you still don't have a resistor, resistor is recommended, if you are real careful not to touch anything else, you can use a jumper wire, resistor is used so you don't short or blow anything by accidental contact or wrong jumping. Can try jumper if you do it correctly and carefully but resistor is the best policy.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

You mentioned you still don't have a resistor, resistor is recommended, if you are real careful not to touch anything else, you can use a jumper wire, resistor is used so you don't short or blow anything by accidental contact or wrong jumping. Can try jumper if you do it correctly and carefully but resistor is the best policy.

Using a jumper wire wouldn't hurt the BRI circuit would it? Should I place the wire back into the plug, or leave it out when jumping it?

[nomoresonys]

Re: Hisense f42k20e turns on, screen goes dark

No, leave it out during test.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Jumpered it and the backlights don't come on at all.

Edit:

I went to check voltage on the BRI pin with the jumper in place as before. I accidently slipped the probe on the connector but made a new discovery. The probe connected BRI to SW (BL_ON) which is less than 4v, and the backlights came on. I thought Uh OH at first, but then referred back to my previous readings and found that SW is almost the same voltage as STB. I placed the probe between the two pins again and held it for a few seconds. As long as those two pins (BRI and SW) are jumped the backlights stay on. More interesting however is that the screen was blue with the No Signal message (aka working properly).

I did however recheck BRI pin voltage and it was -2v. Checked SW and it's 3.84v. I don't understand why I'm getting a negative reading on BRI.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Jumped it with a wire and it works great. What's the possible risk in making this permanent and using it as is? Side from this potential possiblity, what would be the proper fix/what needs to be replaced?

[nomoresonys]

Re: Hisense f42k20e turns on, screen goes dark

Not exactly sure but it seems like maybe the mainboard isn't sending proper signal to turn backlighrts on.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

I guess we'll see what budm has to say. I already know that the 2+ string isn't getting full voltage under normal circumstances. I'm just curious if this is anyway a potential fix/bypass.

I'm going to leave it as is with BRI and SW jumpered and check the voltage of the LEDs again.

Edit:
1+ = 279v
2+ = 67v

This is with the backlights on, blue screen appears as normal with No Signal message, and no darkspots. I can change the Brightness in settings even with the BRI wire completely disconnected, but Backlight level does not change. 0 or 100 backlight is the same. I've been watching OTA TV on it for about a half hour so far with it jumped.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

Watching TV for over an hour so far with no issues (other than not being able to adjust Backlight). I'm thinking about securing the jumper and buttening it up. If I were to reconnect the BRI wire WITH the jumper connected, would that enable me to control Backlight?

[budm]

Re: Hisense f42k20e turns on, screen goes dark

You need to lower the BRI feeding Voltage to may be around 2.5V that may put the backlights level to about 50%, you can us variable power supply (or test with 1.5V and 3V batteries to test the effect on the backlights level) to test if the backlights level will change.

[x_orange90_x]

Re: Hisense f42k20e turns on, screen goes dark

You need to lower the BRI feeding Voltage to may be around 2.5V that may put the backlights level to about 50%, you can us variable power supply (or test with 1.5V and 3V batteries to test the effect on the backlights level) to test if the backlights level will change.

So as is the backlight would be 100%. Would reconnecting the BRI wire have any impact/allow me to change backlight level via the menu?

What would be the "permanent" fix in order to reduce backlight to 75 or 50?

I just answered one of my questions... Reconnecting the BRI wire while having BRI and SW jumpered.. Backlights turn off after a few seconds. It seems to me that whatever signal going through the BRI wire is not correct and causes the main board to turn off the LEDs. However, as long as the wire is disconnected and I have the jumper in place, it seems to be happy.

[budm]

Re: Hisense f42k20e turns on, screen goes dark

So as is the backlight would be 100%. Would reconnecting the BRI wire have any impact/allow me to change backlight level via the menu?

What would be the "permanent" fix in order to reduce backlight to 75 or 50?

I just answered one of my questions... Reconnecting the BRI wire while having BRI and SW jumpered.. Backlights turn off after a few seconds. It seems to me that whatever signal going through the BRI wire is not correct and causes the main board to turn off the LEDs. However, as long as the wire is disconnected and I have the jumper in place, it seems to be happy.

Well, you already know that you have BRI 0V which you reported, so now you force the BRI by jumping the BRI to Voltage source then you have backlights which is why you can no longer control the backlights level via user menu. Typical BRI analog control is that at 5V = max backlights, 0V = FULLY DIM.
I already suggested you to try lowering the BRI feeding Voltage to see it will vary the backlights level or not.
https://www.badcaps.net/forum/attach...4&d=1579461631
BTW, it looks like you have strand of wire touching GND pin, you need to clean that up that is why I recommend resistor.

Reconnecting the BRI wire while having BRI and SW jumpered.. Backlights turn off after a few seconds.
The main control BRI but the main board is having problem and keeping that line grounded instead of providing the Voltage level for the BRI.
You know, you are lucky so far from blowing parts on the main board not using resistor for protection.