# Dead LCD Backlight on Mac Powerbook G4 Aluminum 17"



## johnmc22 (May 13, 2010)

OK, so I'm pretty handy with my Powerbook. In the past, I've replaced 2 different hard drives, a DVD-R drive and a few other things. In short, I've torn my Mac apart and put it back together successfully on numerous occasions. By this latest problem has really stumped me. I've read up on everything I can find on the Internet and changed out many parts, but still my LCD backlight is dead. Here is what I've done so far...

1) First assumed it was the Inverter board, so I ordered a replacement, opened up the screen (man, that was a bear) and installed it. Still, no backlight.
2) Next, assumed it was the inverter cable. Repeated above process with new cable. No light.
3) Next, assumed it might be the backlight itself, so I ordered one (complete with compatible 2-pin connector so no soldering needed). I didn't actually install it (which is an even bigger bear than lid removal), but I did plug it into the inverter output to test it out and... you guessed it... no love.
4) I figured that somewhere along the way, I burned out the replacement inverter board that I bought (OK, OK, I put a meter across the outputs... oops), so I bought ANOTHER replacement inverter. Plugged it in, powered it up... still no backlight (tried both original bulb and new replacement bulb).

At this point, I am completely stumped. I checked the input voltages to the inverter board and the seem OK, although I can't find anything on the Internet that states exactly what they should be. But they aren't zero (at least not all of them). Here's what they are...

Silver wire (pin 1, perhaps): 0V
Blue wire (2): +5V
Gray wire (3): 0V
Black wire (4): +12V
Green wire (5): hardwired to ground

So, I'm in for about 90 bucks so far in parts and the thing is still dead. Oh yeah and, by the way, I know it's the backlight because I can see the faint images on the screen AND the laptop works fine when connected to an external monitor. I already have a newer MacBook Pro that I use for everyday stuff, but it would really be nice to get the old G4 working again. Anyone have any suggestions about where to go from here?

John


----------



## richdave (Nov 17, 2004)

So it seems you have input voltages. Can you measure the o/p voltage? Is it possible that one of the wires to the CCFL are shorted to ground causing the inverter to fail immediately? Is there a micro/nano fuse on the inverter board that may have blown if the o/p has in fact been shorted?


----------



## johnmc22 (May 13, 2010)

richdave said:


> So it seems you have input voltages. Can you measure the o/p voltage? Is it possible that one of the wires to the CCFL are shorted to ground causing the inverter to fail immediately? Is there a micro/nano fuse on the inverter board that may have blown if the o/p has in fact been shorted?


Actually, I tried to measure the output voltage a few weeks ago by putting a meter across it and it read zero (on AC Volts setting). Then, I saw an article on the internet stating that I shouldn't put a meter across the outputs because it could blow up the inverter board. That article also showed a pretty cool technique for determining if the Inverter Board was working, so I got the $20 meter that was suggested and tried measuring an output frequency. I got nothing. Zero. 

Here's the technique I used for testing an Inverter... http://www.fonerbooks.com/test.htm

It was this article that led me to think that, perhaps, I blew up the Inverter Board by putting a meter across it, so I purchased yet another Inverter Board. Still no backlight and still zero frequency reading on the meter.

Regarding a fuse, I cannot see any fuse but, honestly, I'm not exactly sure what a microfuse might look like, But I don't see anything that looks "fuse-like".

Also, you mentioned that maybe the CCFL was shorted causing the Inverter to shutdown or short out, I considered that as well, so when I got my latest, pristine Inverter Board, I first connected it to the replacement CCFL (also pristine) rather than the internal one that might be shorted. Still, no light.

Man, this one is tough. There's got to be something I'm missing.

John


----------



## richdave (Nov 17, 2004)

Assuming your meter can safely accommodate the 500 - 1000VAC out of the inverter and is spec'd to read within the 20 - 50kHz range, I cant imagine why it would "blow up" the inverter... If on the other hand it is a low end meter designed for mains voltages only, then there is some risk I guess.

Is it possible there is some sort of "enable" signal from the logic board that is missing? can you try to reverse engineer/trace the inverter circuit? Or at least ID the main components on it?

try THIS link for some examples of small fuses. They are usually called pico fuses BTW. My mistake!

These are the more common forms in my experience


Example 1

Example 2


----------



## richdave (Nov 17, 2004)

Looking at some reference designs for CCFL inverters, a common thread is that there will be an enable signal so perhaps this is in fact missing. Getting the detail on the IC controller on the board would be a good place to start though. Also there will be a "bright" signal to vary the intensity and possibly a sync signal too depending on the cct employed.

Just a thought as I type, Is the lid button operating? Could the system think the lid is closed thus it turns off the back light?


----------



## johnmc22 (May 13, 2010)

Thanks so much for your attention to this. Your comments are helping me to think of new approaches to things. I will analyze the traces on the logic board to see if I can follow it to the IC Controller chip. If there is an "enable", I could possibly find the appropriate voltage somewhere on the board to hardwire it.

Regarding the lid button... good thing to check. I hadn't thought of that one. There is no obvious lid button, so it must be somewhere in the latch or hinge mechanism. Or maybe it's a magnetic switch. But, after some thought, I do believe the lid button is operating correctly since the laptop does correctly "go to sleep" consistently about 30 seconds after shutting the lid. 

By the way, I did find a fuse on the Inverter board. Interestingly, on my original Inverter board, the fuse was blown. But on the two replacement boards I have in my possession, the fuse was OK. I did do a test and short across the fuse on the original inverter board and then plugged it back into the system for startup test. No go... no light. I'm thinking the enable signal missing might be the most promising direction. I am pursuing that one now.

John


----------



## richdave (Nov 17, 2004)

Just curious, is the fuse on the input or the output side of the cct? Are you able to post a picture of the board showing the fuse location and type?

If you can ID the controller and post it I'll try to get some detail on it. 

The blown fuse on the original suggests an overload/short but if that was true, then I'd expect the others to blow too, however IF the fault also damaged the input cct's then you may now have 2 faults now... whatever caused the fuse to blow in the first place and another subsequent failure somewhere on the logic board perhaps? have you checked continuity of the cable from the logic board to the inverter input?


----------



## johnmc22 (May 13, 2010)

This problem is solved... sort of. The bottom line was that the logic board was NOT outputting any voltage for the "dim control" input to the Inverter board. According to some other info found on the Internet, the "dim control" pin should have a voltage ranging from 0 to +4 volts depending on the dimming desired (where 0V = no light, 4V = max light). Since the logic board wasn't outputting any voltage on that pin, the backlight remained off. So, since I had no intention of buying a new logic board for such an old computer, I decided to just manually put +4V on the pin. Here's how I did it... 

First off, the CORRECT pin names and voltages to the input connector of the Inverter board are here (again, this is for Powerbook G4 17" Aluminum). This is slightly different than what I wrote in my original message...
CHASSIS GROUND - Green wire (connector pin 1): Hardwired to chassis ground
+12V SUPPLY - Black wire (connector pin 2): +12V
CABLE GROUND - Gray wire (connector pin 3): 0V
+5V SUPPLY - Blue wire (connector pin 4): +5V
DIM CONTROL/ENABLE - Silver wire (connector pin 5): 0 to +4V depending on dimming

So, it was the DIM CONTROL that was dead from the logic board. I had both 12v and 5v available to me at the input connector to the Inverter board so, my first thought was to just connect the 5v to DIM CONTROL. At first it worked but, after about 1 minute, the backlight would shut off, presumably because the Inverter was being "overdriven" and it has some kind of overcurrent shutdown circuitry. So, I went to plan B and, though experimentation, found that a 3K ohm resistor placed between pin 4 and pin 5 brought the voltage on the DIM CONTROL to +3.8V which kept the backlight bright without it shutting down. See pictures below.

Now, for the bad news. Not only will the screen not dim (which I can live with), but it also won't shutoff when going into sleep mode (the display goes away, but the backlight remains lit). For the record, it DOES shutoff when the computer is ShutDown. But, this poses yet another challenge. Since I generally only use my computer from home while it is plugged in, I may not worry about this one for now. But, for many people, this might be a showstopper. None the less, I did get the backlight working.

John


----------



## TiHKaL42 (Nov 11, 2010)

Hi I have a very similar problem with a PB G4 17".

The back light does not work but the display still works.

My problem is that I am missing the +5V feed from pin 4 to the inverter board. I've tested the voltage at the logic board connection socket. This is what I get.

Pin 1 : Ground
Pin 2 : +12v
Pin3 : 0v
Pin4 : 0.09v Should be +5v
Pin5 : 1v

Would it be possible for me to pull a +5v feed from elsewhere on the logic board to power the inverter and if so where would be a safe place to get it from.

My first thought was one of the fans but I don't know if this will be safe or not.

Thanks in advance for any advice


----------



## johnmc22 (May 13, 2010)

Question: If you fire it up and measure the voltage on pin 5, can you make that voltage ramp up to it's maximum by holding down the "Screen Brighten" button? If so, then something to try would be to max out the screen brightness (and leave it on max all the time) and short together pins 4 & 5. That should put 4v on pin 4 which I would think would be sufficient to "enable" the screen output. You could still find 5v somewhere on the mother board as a backup, but this might be a quick fix (plus, it also verifies the dimming circuitry is working). -John


----------



## TiHKaL42 (Nov 11, 2010)

Thanks for the advice. I had to return this to it's owner and the problem wasn't resolved.

Adding a +5v feed to both these pins had no effect. It'll become a reasonably functioning desk top with a screen plugged into it. I didn't get a chance to test this do you know if the powerbook screen could be removed completely? Then just have an external screen attached without affecting operation.


----------



## johnmc22 (May 13, 2010)

If you applied 5v to both pins 4 & 5 and applied 12v on pin 2 (with the other two pins grounded), then you really should see the backlight light up regardless of the motherboard state. Based on that info, it sounds like either the inverter board or the backlight bulb itself is the culprit. Regardless, as far as I know, you should be able to remove the screen entirely without otherwise affecting operation.


----------

