Powerbook 3400c Logic Board Help

[tastyrerun]

I have a pair of 3400c machines each with a different issue.  The first machine shows artifacts around the mouse pointer and usually corrupts the display of text and video game graphics and the second works perfectly for a while but then the backlight shuts off on the display and stays off.  The machine is still running fine and I can shut it down and then if I wait a bit and boot it up again sometimes the backlight is back to working.

I had thought the issue with the backlight was probably an inverter issue so yesterday I swapped the logic boards between the two machines and, what do you know?  The backlight issue followed.  It's something on the logic board.  

Anybody got any advice to find the cause of either issue?  If I can fix at least one of the logic boards I'll be happy.  Are there caps I could replace?  FWIW, both boards look fantastic.  No obvious corrosion or leakage anywhere.  I've reset the PMU, put in a fresh PRAM battery and the original battery is still holding a charge (astonishingly).  The machine with the video problem is a 180 and the machine with the backlight problem is a 240.  Thanks in advance for any pointers you can provide. 

 

[bibilit]

IIRC very few capacitors on those boards.

 

[galgot]

I have the same backlite problem with a kanga (witch really annoys me cause... its a Kanga).

Also swapped screen, same result. must be a component on the board, but which one ?...

Like Bibilit said very few capacitors on these machines, 2 or 3 on the PMU board.

 

[AlpineRaven]

Around the area where the PRAM battery is located - would be the issue. It would be caused by slight battery leak which is very common issue for those.
Cheers

AP

 

[techknight]

Display artifacts is almost always bad RAM/VRAM. 

 

[galgot]

Hi Techknight, 
I tried changing the ram on that Kanga, still same problem, very dim screen.
Like a failing backlit, but its not that as I swapped screen with known good one already…
Also this Kanga doesn’t want to zap pram, or boot in OF…
(yes the PRAM batteries are removed).

 

[techknight]

well theres a difference between artifacts on the screen (graphics glitches) versus actual backlight brightness. 

Check the voltage going to the inverter board, it may be low. Also check the control signal to the inverter with an oscilloscope and make sure you have a clean PWM signal and not grounded (That will make it dim too)