troubleshooting PowerBook LCD backlight issues

[mdanh2002]

Hi, I have a Powerbook 190cs and a Powerbook 165 that are having weird backlight issues. On both laptops, upon power on, the backlight will lit up for a while (5-10 seconds), then turns off. With this, I can still use the 165 since the screen is B&W, whereas the 190cs with color screen is totally unusable without the backlight. However, strangely on the 190cs, if I do not power it on for around 2-3 days, the LCD backlight will work fine for around 2-3 hours once it is turned on before the problem will happen again. Once the problem happens on the 190cs, I have to let it rest for a while before retrying. On the 165, the backlight symptoms are persistent and will not change regardless of how long I let it 'rest' before powering on.

On the 165, the issue happens regardless of whereas the machine boots from floppy or HDD. On the 190cs, the issue does not happen with floppy boot but only when booting from the hard disk drive. The 190cs has the PRAM battery removed, if that matters. (I have another 5300cs which also has the PRAM battery removed, but does not exhibit such symptoms)

The 190cs is on System 7.6.1 and the 165 is on System 7.5.5.

I guess the backlight hardware is most likely ok, since it works fine for a while when power is applied. It is also not a loose cable issue since I did not touch the cables when the problems happen. Any thoughts?

 

[Macdrone]

how about caps?  there are capacitors on the screen boards on the sides that leak, and the inverter board.  At this age it would be a great idea to clean off the goo and replace.

 

[mdanh2002]

It could be caps on the 165. However I am reluctant to open it, since the cable between the LCD and the motherboard is 'almost' broken and I do not want to risk ruining it.

For the 190cs, I doubt so. I checked the inverter board and it looks very clean, almost like the day it left the factory. I measured most caps on the LCD boards using an ESR meter and they seem to be fine. I also swapped the inverter board with another Powerbook 5300cs that I have whose LCD is cracked, and the same symptoms persisted. The symptoms did not happen when the swapped inverter board was on the other Powerbook, the backlight there could remain on for the whole day with no issues.

 

[Rajel]

Just because it looks clean doesn't mean the capacitors are good. They can leak, or they can dry up.
Many systems I see have caps that "look" perfect, but are in desperate need of replacement.

Backlight, it's likely the inverter caps. I'd go ahead and do the rest of the caps on the screen as well. I still need to do my 190cs caps, even though the system works mostly fine, and the caps look ok.
Electrolytic capacitors are almost always the most likely cause of issues like these. Do not trust them, and replace with new if there is even the slightest possibility.

 

[Rajel]

I see you did swap the inverter as a test, in that case it could be other caps behind the board. The power section near the battery has several large caps that likely need replacement.
Basically just swap out every electrolytic cap in the system you can find, and go from there. My 190cs has issues with certain hardware configurations (SSD vs. CF card for hard drive) and it's almost certainly due to the caps next to the battery having mostly failed.

 

[mdanh2002]

The 190cs only has this problem when it boots from the hard drive. It does not exhibit the issues when booting from the floppy disk. And the problem always happens shortly after the "Welcome to Macintosh" text is displayed (backlight is turned off, making it very hard to see the screen). Something I observed today is that, if the problem happens, a PRAM reset (Command-Option-P-R) will restore the backlight to working condition again on the 190cs. I will take it apart this weekend and check again for leaky caps and see what I can find.

For the 165, its backlight will always turn off around 5 seconds after power is applied, regardless of whether there is a boot disk. The PRAM reset trick does not solve the problem on this machine. 

On a side note, I have a 5300cs that works fine but does not accept CF cards. Drive Setup Lite always says "Disk Initialization Failed" when running on CF card (the card is detected with the correct capacity). I have tried various CF cards with the same results. When running on a traditional IDE hard drive, everything works fine. My 190cs does not have such issues. This is strange.

 

[Rajel]

Dry caps you won't necessarily see. Like I said, it can *look* perfectly fine, even test ok with a meter but that won't show things like ringing and ripple from the caps, you'd need a scope for that.
Also, temperature changes on bad caps can cause significant failure, this is what I suspect the issue is for me since it works great with the CF card that runs nice and cool, but with the SSD (it runs quite warm) it fails after a few minutes unless the system is running while open.