LG RZ-27LZ55 momentary backlight

[zx81]

Hi Guys please could you help me out with this LG RZ-27LZ55

When I power the set up I get the backlight to come on it will stay on for 5-6 seconds sometime it only comes on for a second or so then shut down the set again.

I have tested the voltages I am getting 24v showing around 18v then the voltage drops down to zero while I'm reading. Same with the 33v. Standby and 12v are present.

Sound is also present

Pics attached

Thanks

zx81

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Any pointers as to where to start would be very much appreciated guy.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Can anyone suggest where to start with this could it be an inverter issue that's pulling the voltage down and shutting down the power supply or should I investigate the power suppy ?
Any pointers would be very much appreciated.

Cheers

zx81

[CapLeaker]

Re: LG RZ-27LZ55 momentary backlight

Disconnect the main board from the PSU, then force the PSU on with the back lights connected. Use a 1k ohm resistor from STBY voltage to PSON, and another 1k resistor from 5v to BLon. See what you get.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Thanks for the reply CapLeaker will try that out and post my results.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Disconnect the main board from the PSU, then force the PSU on with the back lights connected. Use a 1k ohm resistor from STBY voltage to PSON, and another 1k resistor from 5v to BLon. See what you get.

CapLeaker can I forgo the resistors and just short out the connections suggested?

Sorry for the newb question. Pretty much starting out and have loads to learn.
Does the power supply need to see a load and that's why we need to use the resistors?
Will the resistors be a dummy load?

Thanks

zx81

[budm]

Re: LG RZ-27LZ55 momentary backlight

The forced on resistor is for current limiting incase there is shorted circuit, the current be limited to 5V/1000 Ohms = 5mA not enough to damage anything. If you slip on that 5V wire and accidentally connect to the wrong pin, or shorted pin, or chassis, you can damage the power supply.
It is not dummy load resistor.
And when you do the force on, you MUST disconnect those cables (the PS-ON, BL-ON) going back to the main board FIRST.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

The forced on resistor is for current limiting incase there is shorted circuit, the current be limited to 5V/1000 Ohms = 5mA not enough to damage anything. If you slip on that 5V wire and accidentally connect to the wrong pin, or shorted pin, or chassis, you can damage the power supply.
It is not dummy load resistor.

Thanks for the explanation budm.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Hi Guys,

I've just uploaded some close up pictures of the connectors on the power supply and a connector going to the main board. The connectors have different labelling compared to Capseek's instructions. No STBY or PSON labels. Would someone be kind enough to point out which points I should connect the 1k resistors in my particular case? I will be removing the main board connector and use that side of the connecters to push the resistors into. Is it ok to do so?

Does the wattage of the resistors matter ? I don't have any !k resistors and will have to pick some up locally.

Why does the power supply have 33v ? is it really 33v or is it 3.3v.

[budm]

Re: LG RZ-27LZ55 momentary backlight

PWR-CTL (Power Control) = PS-ON (Power supply ON)
INV-CTL (Inverter Control) = BL-ON (Backlights ON)
INV-BRI (Backlights Brightness) = Backlights Dimming Controll, Note: there is no indication to tell us if it is PWM-Dimmimg (Pulse Width Modulation) or A-Dimming (Analog)

[zx81]

Re: LG RZ-27LZ55 momentary backlight

PWR-CTL (Power Control) = PS-ON (Power supply ON)
INV-CTL (Inverter Control) = BL-ON (Backlights ON)
INV-BRI (Backlights Brightness) = Backlights Dimming Controll, Note: there is no indication to tell us if it is PWM-Dimmimg (Pulse Width Modulation) or A-Dimming (Analog)

Thanks Bud.

Any idea what the 33v is used for or am I reading it incorrectly is it 3.3v?

I am picking up some 1k resistors tomorrow is wattage anything I should look out for will 0.5-0.6watt be okay?

[budm]

Re: LG RZ-27LZ55 momentary backlight

The 33V may be for the electronic tuner, if it is 3.3V it would have been labeled with 3V3 not 3.3 since sometime the DOT can be hard to see.
1/4W is more than big enough, just be sure to remove the wire/s going back to the main board before forcing on since we do not want to back feed the force on signal back to the main board.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

The 33V may be for the electronic tuner, if it is 3.3V it would have been labeled with 3V3 not 3.3 since sometime the DOT can be hard to see.
1/4W is more than big enough, just be sure to remove the wire/s going back to the main board before forcing on since we do not want to back feed the force on signal back to the main board.

Got it.

Will pick up some resistors tomorrow and report back with my findings.

Cheers.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

I have got a couple of resistors ready to go now. But I am confused where my standby is.
The connector does not show Stby? Or 5v stby.

There is another connector on the main board that shows ST 5V if I follow that connector back to the power supply it goes to an area near the primary side of the power supply where there is an IR jack and rs232 port. There is a printed white divider line that may be indicating that it is on the cold side of the power supply.

I'm a little confused and thought it best to seek advice before I blow anything up.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

Hi guys can anyone offer any advice on the correct connection points to put the resistors?

[budm]

Re: LG RZ-27LZ55 momentary backlight

OK, before doing the force on, we need the pictures of the whole back side of the TV.
I see that it has read connector and white connector with 24V and inv control lines, also other two connectors with PWR-CTL and PWR2, so I need to see which connector goes to the main board and which connector is not used.

[budm]

Re: LG RZ-27LZ55 momentary backlight

https://www.badcaps.net/forum/attach...5&d=1426004736
Is this the connector on the power supply board or on the main board?
If is on the main board then what DCV do get on all those pin when TV is OFF and when power switch is activated?

[zx81]

Re: LG RZ-27LZ55 momentary backlight

I will upload better pics tomorrow.

I'll do some photoshop work to make a very high quality flat/evenly lit pic to upload that will show everything clearly. Until then I'll hold off trying the force on.

[zx81]

Re: LG RZ-27LZ55 momentary backlight

https://www.badcaps.net/forum/attach...5&d=1426004736
Is this the connector on the power supply board or on the main board?
If is on the main board then what DCV do get on all those pin when TV is OFF and when power switch is activated?

Hi Bud I'm away from the set right now. I'll update this thread tomorrow with the info you've requested and a real high resolution pic of the whole main board and power supply. I may put some overlays in photoshop to help show things more clearly.

[budm]

Re: LG RZ-27LZ55 momentary backlight

OK, just make sure to give me the Voltage readings on those pins, thanks.