68kMLA Classic Interface
This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.
| Click here to select a new forum. | | Backlit Portable: No boot, lines on display | Posted by: SuperSVGA on 2022-07-05 17:37:08
I ran into a backlit portable that had this same regulation problem. Never did figure that one out when all the traces were good. You still have it? On mine it looks like the 1.2V reference varies based on battery voltage, which definitely should not happen. I replaced it which gave it a small boost, but it's still not stable.
Right now I'm looking at op-amp output A which I haven't traced out but I'm guessing is a negative feedback setup. My guess there's a voltage or current issue somewhere there that's causing the zener reference to drop when the battery voltage is lower. | Posted by: techknight on 2022-07-05 17:53:20
You still have it? On mine it looks like the 1.2V reference varies based on battery voltage, which definitely should not happen. I replaced it which gave it a small boost, but it's still not stable.
Right now I'm looking at op-amp output A which I haven't traced out but I'm guessing is a negative feedback setup. My guess there's a voltage or current issue somewhere there that's causing the zener reference to drop when the battery voltage is lower. I do, and i replaced all the semiconductors in that section of the board with no change either. So i shelved it.
But it absolutely would not go into regulation, and i spent hours on that thing chasing voltages. the 47uf cap is also important as its a float charge smoothing cap.
I even moved the resistor packs from a known working unit over, and nadda. | Posted by: SuperSVGA on 2022-07-05 18:09:29
I put a 470k resistor between battery voltage and the VREF, and now I get a stable 1.23V and ~5.2V no matter what I put the battery voltage at. Getting closer... | | < 2 |
|