Powerbook 145 global contrast issues

[Zarwox]

I have this Powerbook 145 I've been working on for some time now. It's in quite nice shape, both the screen hinges and the original Conner CP2025 drive are still alright. The LCD of course needed some work. It had really bad and uneven contrast from the start but after an uneventfull recap using tantalums the display is now really crisp! When I manage to set the contrast that is.

Problem is that the global contrast when the machine is first powered on or wakes up from extended sleep is messd up and the screen is all white (picture only visible from extreme angles). Efter about an hour the contrast is working fine and is stable and I can adjust the slider all the way from screen almost fullly black to all white. I've had the machine powered on now for a few days straight and there is no issue whatsoever as long as I don't power it off or make it go to sleep.

Started to suspect the contrast/inverter board so I replaced the two electrolytics on there as well, even though they did not appear to leak and was still testing good, but it did not change the behaviour.

Took the LCD apart again and did some more cleaning and looked for bad traces. It made no change. Removed all caps again. Cleaned the board thoroughly and put new caps back on without using any extra added flux. Cleaned again. Still no change.

Did the same for the contrast/inverter board.

Now I'm running out of ideas. How does the contrast actually work on these machines? Are there any schematics?

 

[Shaddam IV]

have you tried a replacement inverter board from a different 145 or 160/165 machine?

 

[Zarwox]

Unfortunately I don't have any other compatible machine to do part swaps. However, as far as I can see the inverter board only contains the contrast pot. All the rest is for the backlight. I have traced the contrast pot on the board all the way to the connector and it seems good.

I'm beginning to suspect some kind of power supply issue maybe. There is a very slight rolling shimmer on the screen as the contrast stabilizes.

 

[Zarwox]

The DC/DC converter appears to be very sensitive to temperature. If I cool it softly with compressed air the screen goes blank and that rolling shimmer comes back. Took it off and put it back on again but it did no real change. Maybe the module is flakey somehow. It also makes me wonder about that ”SW1” bridge. Are there multiple grounds on this board maybe