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Duo 280c LED Backlight

aladds

Well-known member
I recently acquired a Duo 280c from a fellow forum member @pizzigri for a project which is (as yet) still temporarily paused, but until that resumes I was just playing with the Duo to make it "as good as possible" - this has involved replacing capacitors, 3D printing new feet (still haven't done some nice rubber bumpers for the lid, but I'll get to it) and lubing the keyboard keys (only to discover that the reason this one is bad is because the back metal plate is warped, so I'll be looking at that separately).

Anyway I was aware of these existing and thought that I'd like to give one of the kits a go. So I ordered one, trimmed the LED board to length, swapped out the CFL tube and powered the board externally. I got this:

IMG_8779.jpg


I thought that looked pretty awesome, so I poked around at the inverter board, found 24v to power it, 5v for the enable line and (temporarily) for the backlight brightness (I know there will be a variable voltage for that somewhere on there, I've just not taken the time to find it yet) and after stripping a few now extraneous components from the board I got the new LED driver to fit within the top case.

IMG_8833.jpg


I genuinely beleive that this is the best replacement option we have for failed CFLs in all PowerBooks going forwards. Even spinning a new PCB to replace the inverter board completely wouldn't even be particularly difficult.
 

aladds

Well-known member
Further investigation tells me that I must be missing something regarding how dimming the screen works on this thing. I belled out the connector on the Inverter board and found the following:
1Sleep LED
2Audio -ve
3Audio Gnd(?)
4Audio +ve
524v
6GND
724v
8Button Board pin 2
924v
10Button Board pin 4
11GND
12Button Board pin 1
13GND
14+5v (off in sleep)
15GND

Presumably it's possible that the "Audio GND" pin I've misunderstood (although it makes the most sense) or some other pin is serving a dual function, but I am pretty certain that the 25v and 5v lines don't change based on screen dimming (tried with both brightness controls and dimming via Powerbook control panel)

It's also possible that it changes the brightness through _current_ control so I'll check that too.
 

Paralel

Well-known member
How is the color temperature? A strip of WLED's and the CCFL that was in there are, I'm thinking, quite a bit different in color temperature. Do the colors seem to be true and accurate, or do they seem off in any way?
 

aladds

Well-known member
Brilliant! I’m looking forward to seeing how you make out on resolving the brightness control.
Looking up the LM1172 it should actually be quite straightforward, but of course knowing Apple they’ll have used a fancy current driver or something so they can have their fancy push buttons. It’s also possible that one of the grounds is different and used for backlight (see the bottom of page 14 in this PDF here)
How is the color temperature? A strip of WLED's and the CCFL that was in there are, I'm thinking, quite a bit different in color temperature. Do the colors seem to be true and accurate, or do they seem off in any way?
Honestly I think it’s better. This CCFL wasn’t actually too faded but some of my other PowerBooks are actually terrible in colour renditioning.
 

davidg5678

Well-known member
Good work! I did something very similar to my iBook clamshell backlight a few years ago, but never quite got the brightness control working perfectly. I'm curious to see what you figure out with the LM1172.

 

aladds

Well-known member
Well I was wrong in my table above, the "Audio GND?" pin (pin 3) is in fact exactly what I want...except it's inverted!
4.776v is backlight off, 3.344v is minimum brightness all the way down to 1.390v which is brightest.

But that's fine! The circuit the DF6113 chip (the one the new LEDs came with) supports inverse dimming, although the reference circuit in the datasheet and the board don't really line up. However....

With a somewhat straightforward Mod (10K resistor to pin 5 of the driver chip, direct from pin 3 of the inverter plug dimming works inversely, and perfectly. I could probably make it a little brighter/dimmer if I wanted by adjusting the value of this resistor. And I probably do, since at the lowest brightness level it does flicker. That said I did remove the input capacitor on the board so it fit better in the case. There is space for it, just not where it was, so it would be fine if I found a new home for it. (You can see this in the video)

So yeah, this was much easier than I thought (Caveat: I do have an EE degree)

If this isn't clear enough I can do a proper writeup, but in the meantime have some photos and a nice video of the dimming in action:

IMG_8841.jpg

IMG_8842.jpg

As an aside, I think the ENA/DIM pins might actually be the wrong way around on my boards.
 

Attachments

  • IMG_8845.mov
    13.8 MB
  • IMG_8846.mov
    17 MB

aladds

Well-known member
Good work! I did something very similar to my iBook clamshell backlight a few years ago, but never quite got the brightness control working perfectly. I'm curious to see what you figure out with the LM1172.

Interesting! Did you change the feedback resistors when doing your mod? The datasheet has a current calculation formula and I could imagine that affects the linearity of it.
It also looks like we reached the same conclusion about the inverted controls but solved it in very different ways! I decided to make an "engineering judgement" (informed guess!) that I could probably just use the resistor on pin 5, and happened to be right. That said I didn't look closely at the circuit your board had, so they could have been quite different.

Other than the lowest settings being flickery (which as I say might be due to the admittedly still missing capacitor) I'm happy with how my setup has turned out.

Regarding your linearity issue: how much of the original backlight circuit did you leave in place? I still have the LM1172 on the board, which I am considering removing, but I could imagine it's acting as a load and keeping my output a little more stable...

(Also I love how much space there is inside the Clamshell's screen!!)
 

davidg5678

Well-known member
Interesting! Did you change the feedback resistors when doing your mod? The datasheet has a current calculation formula and I could imagine that affects the linearity of it.
It also looks like we reached the same conclusion about the inverted controls but solved it in very different ways! I decided to make an "engineering judgement" (informed guess!) that I could probably just use the resistor on pin 5, and happened to be right. That said I didn't look closely at the circuit your board had, so they could have been quite different.

Other than the lowest settings being flickery (which as I say might be due to the admittedly still missing capacitor) I'm happy with how my setup has turned out.

Regarding your linearity issue: how much of the original backlight circuit did you leave in place? I still have the LM1172 on the board, which I am considering removing, but I could imagine it's acting as a load and keeping my output a little more stable...

(Also I love how much space there is inside the Clamshell's screen!!)
I didn't use any resistors when I did my mod. I also removed the CCFL driver board entirely, but I don't know if this was the right decision.

It looks like your LED driver board differs slightly from mine, but they are based on the DF6113 IC.

You mentioned that lower brightness settings made your LEDs flicker, and this sounds similar to issues I had with my modification: Something I found odd about my iBook is that when I would turn the brightness down, the backlight would actually get brighter before getting dimmer (flickering throughout). It would then brighten again before finally turning the backlight off entirely. If you have any success adding a capacitor to your LED driver, I'd be interested to hear about it.

I like your idea of using a resistor on pin 5, and I wonder if I can add something similar to my iBook. When the screen gets stuck at the maximum brightness setting, it is painful to look at (it also overheats the LED driver), so I bet adding a resistor would help cut the brightness down a lot. I think I have a spare LED driver PCB lying around somewhere, so this is something I'll definitely tinker with at some point.
 

aladds

Well-known member
You mentioned that lower brightness settings made your LEDs flicker, and this sounds similar to issues I had with my modification: Something I found odd about my iBook is that when I would turn the brightness down, the backlight would actually get brighter before getting dimmer (flickering throughout). It would then brighten again before finally turning the backlight off entirely. If you have any success adding a capacitor to your LED driver, I'd be interested to hear about it.
If you look at the second video in my post above, I appreciate that I switch between brightnesses quickly, but it's the same effect as I've seen on other PowerBooks with normal CCFL backlights on a very low setting - the steps up and down are "in order" but the bottom one (or two, depending on the resistor I used) flicker "within" their range.

Yeah I'll look at capacitors!
 
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