PowerBook G4 Backlight - NOT INVERTER OR CCFL

Hello all!

I've been trying to find some info out on a problem I have with my 867Mhz G4

I When switched on, the backlight flashes on for about 1/2 a second then remains dark.

I've already replaced the Inverter AND CCFL tube (very fiddly) and still have the same problem.

I have quite a bit of electronics experience in laptop repairs as I work at a PC shop. This problem seems to be deeper rooted than just the inverter etc...

I have seen mentions of the VDIM control to the inverter (dimming control) for other macbooks/powerbooks, but not for the one I have. The inverter in my G4 is attached to the underside of the DVDROM - part no. PWB-28053T


I have measured the voltages on the connector pins (cable is 100% fine btw) and I am seeing 16v, 5v and something around 2.8v
When I switch the mac on, one of the pins briefly outputs ~3.3v when the backlight flashes on, but then drops to 0.57v.

Just a few things to clarify...

1) NO WAY am I purchasing a new LCD, the one I have is immaculate (tested in another G4)

2) NO WAY am I purchasing a new Logic board (OVERPRICED)

3) NO WAY am I taking this to an Apple store (Out of warranty & I'd sooner eat the laptop piece by piece, probably be more useful)

Main reason? Apple cream profits for charging OBSCENE amounts to repair their own ****** products (Learnt from my Ipod) This is the first mac I have owned & used myself, and to be honest I am very disappointed in the quality & support (yes it's second hand but thats not my point)

Anyways... back to the problem.

Does anyone have experience of this problem with the same powerbook? Both inverter & CCFL have been replaced.

I'm thinking maybe a switching transistor on the logic board which should switch the VDIM when the display is activated. My hope is to hard-wire 3.3-3.5v to the VDIM input so that the backlight works (minus brightness control)

Are there any pinouts/service manuals/ANYTHING USEFUL for this?

Thanks for your time guys!

Just pulled out a universal inverter to test the screen - 12v & GND input.

Powered up my mac, backlight came on and I could use it as normal.

I think this pretty much confirms that the fault is on the logic board. Now I've bought a new inverter I don't need it. Fantastic waste of money! (apple in general)

I'll take another look at the cables running to the LCD even though the screen works fine in an identical powerbook (with same inverter board)

Any suggestions?

Ok! Don't all jump at once!

Anyways, done some more testing...

Stripped out the screen & magnetic switch so now I have just the Logic board & LCD connected. Accidently burnt out the old inverter whilst probing - just after reading about NOT doing that... Looks like I'll be needing the new inverter after all!

I have a universal inverter which just connects to a 12v supply - backlight works as normal. Still getting 1/2 a sec with the original inverter. I'm thinking its a transistor blown that's causing this problem.

Has anyone got a G4 Titanium 15" 867Mhz powerbook they can do some meter readings from? Would be a big help.

Readings from:

Q40 (Voltage & resistance to GND)
Q1 (Voltage & resistance to GND)
The 8 Test pads indicated (Voltages)




If someone can help out I will most definitley share my findings.

Thanks!

-EDIT
If I unplug the Inverter cable from the logic board then reconnect it, the backlight flashes on for about 1/2 second again... Strange...
I'll keep you posted...

Im curious

So that your post does not go totally to waste. What are your findings? i am having the same issues and i own a cell phone and computer repair shop and im stumped. Would love to discuss.


Patrick