My Insperon 1420 laptop display goes dim when the laptop is plugged in I press the Fn up arrow to brighten it and it does nothing. I unplug the laptop and the display brightens. Any ideas if its the motherboard, LCD screen or ac jack. I have tried a different power cord. And couldnt imagaine it being the LCD screen because it brightens when unplugged. Any Ideas??
It's either a bad inverter (backlight power supply) or a bad mainboard - the former is under $50 to replace (you'll need to remove it to find a replacement), while the latter is more $350-450+, so try the inverter first.
why does it brighten up when its unplugged
See above. It's either a faulty inverter that's doing the opposite of what it should be doing, or it's a faulty mainboard that's not sensing the adapter correctly.
I wanted to share. While this may not address your issue, I think it may be of value to people with hardware that is out of warranty.
Anyway, I had a similar problem with a Dell Inspiron 8000 (almost 10 years old now). About a year or two ago, it started doing what you describe. Turn it on while it is plugged into the AC adapter, and after some variable amount of time, the screen goes dim. Unplugged the AC adapter, screen returns to full brightness. Plug in the AC adapter, 10 seconds later, the screen is dim (still barely readable, but really annoying). I was at the point I was thinking about a new laptop.
So, finally, I took the display apart, checked all the connections (even used a little very fine sandpaper to be sure there the connections where not corrupted), put it back together, and the same thing happened again. Then, I took it appart again, and left the display out of it's case. Now, when on AC power, the display stayed bright (for hours). This was odd. I noticed that the inverter (not suprisingly) was warm/hot. I also noticed that the inside of the plastic display case is lined with metal.
I then recalled that I had taken the monitor/laptop apart several years ago to replace the CPU fan, and fix a loose connection in the display cable. So, I wondered: could the metal in the plastic case be there to disipate heat away from the inverter? and, if so, did my prior display fidling interfere with this, resulting in an inefficent transfer of heat --> overheating the inverter when on AC power (does it get more power on AC then when it is on battery?) --> causing the inverter to shut down.
Well, since this laptop is way out of warranty, I took some aluminum foil and scotch tape; made a little aluminum foil pad; taped it to the steel in the plastic case in position that the inverter would sit on it when the display was in the case; re-assembled the display/laptop; and tured it on.
I kept in on for a week, plugged into AC power, and the display stayed bright.
So, if it's a warranty problem, I would not recommend this. But if you (or anyone else goggling around the internet with this problem) has an out-of-warrenty display with the same problem, this may be a 10cent solution for you.
BTW, be sure not to short anything with the foil.
This FYI is strictly use at your own risk.