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

 

[alexGS]

Sorry, none of this is any help but just to share some thoughts. I have a similar effect with my Powerbook 100 - the wrong contrast (too dark in my case) and the ‘rolling shimmer’. Seems to go away once it warms up. I thought it was because I used MLCC caps instead of tantalums, but I see you’ve used tantalums (nice job on the size choice and the soldering!)

On the 100 the LCD drive voltage (about 28-30V I think) comes from the logic board. With the 1xx, I presume it’s from that DC-DC converter you’ve removed and cleaned around? Good move, because I’ve found residual capacitor juice can cause erratic behaviour like this, and tends to be temperature-sensitive. I wonder if those adjacent surface-mount electrolytic caps are good-quality low-ESR types (I don’t recognise the markings sorry) as there’d be a chance the DC-DC converter performance would be affected.

 

[Zarwox]

Very interesting thoughts! This all sounds very similar to what you experience with your PB 100. The DC/DC converter appears to provide around 28V here as well but it's difficult to do proper measurements with the screen assembled. I will replace the capacitor on the DC/DC converter output side to something known good and maybe try adding some smaller 100nF MLCC in parallel as well. The 100uF on the 5V input is a decent Panasonic FK but the other one I actually don't know. Just picked the first one I found with the appropriate voltage rating thinking it would not be that critical.