Keeping back lite on without signal

[tubetech]

I have a 17" Acer AL1717A LCD screen with a cracked LCD! I have removed the LCD and it makes a terrific Lightbox for viewing slides - however as there is no signal the lights go off after a short time. Can any one help me with a workaround to cheat it into thinking it has a signal.

Thanks
Tubetech

[PlainBill]

Re: Keeping back lite on without signal

I have a 17" Acer AL1717A LCD screen with a cracked LCD! I have removed the LCD and it makes a terrific Lightbox for viewing slides - however as there is no signal the lights go off after a short time. Can any one help me with a workaround to cheat it into thinking it has a signal.

Thanks
Tubetech

The cable that connects the power supply / inverter to the logic card contains two lines which control the inverter. These are often labeled BL_ON and BRI or something equivalent. Measure the voltage on these two lines, then rig up voltage dividers to provide those voltages. You can now eliminate the logic card.

PlainBill

[tubetech]

Re: Keeping back lite on without signal

Thanks for the reply PlainBill
I have an 8 way cable between "logic board" and PSU/Inv it has 2 Vcc 5volt, gnd, on/off, bright, gnd, volume, mute. so between the on/off and bright perhaps?

Tubetech

[PlainBill]

Re: Keeping back lite on without signal

Thanks for the reply PlainBill
I have an 8 way cable between "logic board" and PSU/Inv it has 2 Vcc 5volt, gnd, on/off, bright, gnd, volume, mute. so between the on/off and bright perhaps?

Tubetech

I'm not sure we are communicating correctly. Let me spell it out in detail:

Step 1. Measure the voltage from Gnd to the Bright pin while the backlight is on. Note this reading as Vbright.

Step 2. Measure the voltage from Gnd to the On/Off pin while the backlight is on. Note this reading as Von.

Step 3. Make up a pair of voltage dividers (two resistors in series). These dividers will be wired between 5V and Gnd. Choose the resistors for one divider so the junction will give Vbright. Choose the resistors for the other so the junction will give Von.

As an example, if Vbright = 2.9 volts, a 3.0 K in series with a 2.2 K will give about 2.9 volts at the junction (3.0 K to Gnd, 2.2K to 5V).

Step 4. Remove the logic card, give it to a friend who repairs monitors. Connect the two voltage dividers so Vbright and Von get their appropriate voltages.

As a matter of fact, if you are clever, you can do it with using only three resistors in series. This is left as an exercise to the student. Also, the Von voltage is not particularly critical. Changing the Vbright voltage will change the brightness.

PlainBill

[tubetech]

Re: Keeping back lite on without signal

That was a big help.
did as you suggested, ~ 3volt on the Bright and 5.1 volt on the on/off - arranged that - 1 quick flash then off.
thought to self, perhaps there should be time delay on the on off, so organised that with 1uf tag tantalum and diode with cap to +5 and diode to gnd on/off to junction, no diff.
Just tried waiting for count of ten and connecting on/off to +5 with a 100 ohm resister, comes on for count of 5 then off. Reconnected logic board and computer stays on.
Knew a girl like that once - but only once.....

thanks for your help

tubetech

[PlainBill]

Re: Keeping back lite on without signal

That was a big help.
did as you suggested, ~ 3volt on the Bright and 5.1 volt on the on/off - arranged that - 1 quick flash then off.
thought to self, perhaps there should be time delay on the on off, so organised that with 1uf tag tantalum and diode with cap to +5 and diode to gnd on/off to junction, no diff.
Just tried waiting for count of ten and connecting on/off to +5 with a 100 ohm resister, comes on for count of 5 then off. Reconnected logic board and computer stays on.
Knew a girl like that once - but only once.....

thanks for your help

tubetech

Quick guess - there wasn't enough of a load on the power supply. Try adding a 10 ohm, 5 watt resistor from 5V to Gnd. Or of course you could just leave the logic card hooked up.

PlainBill

[tubetech]

Re: Keeping back lite on without signal

thanks PlainBill - trier with logic card connected just the on/off cable cut didnt make any difference.
tubetech

[tubetech]

Re: Keeping back lite on without signal

Mind you I wouldn't mind leaving in the logic board - but without a signal it still goes off after a count of 10.
makes you wonder....

Tubetech