I've seen a lot of 500s with the video chip issue (maybe I hit the jackpot and got a bad batch of them or something), but that problem is characterized by very erratic behavior, so if target mode works well and is stable, that might be an indicator that it's not the video issue. Actually, connecting to an external display is probably the best way to test this -- if an external display is stable and works well, that means the video chip is good, at which point I'd say the screen itself may be dead, my logic being that you've replaced the inverter cable with a (hopefully) known good one with no luck, ruling out that possibility, so if the display on an external monitor is stable, and the inverter cable is not the problem, that leaves the screen itself.
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Another way to rule out the video chip issue: While powering up clamp the space just left of the trackpad with your left hand, applying some pressure. When the video chip issue is present, pressure in this area can force the chip to function, and sometimes the display will come back temporarily. So basically if squeezing this area has an effect on the display, you know you've got the video chip issue. And like I said testing an external display is the other way to determine this.
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Another way to rule out the video chip issue: While powering up clamp the space just left of the trackpad with your left hand, applying some pressure. When the video chip issue is present, pressure in this area can force the chip to function, and sometimes the display will come back temporarily. So basically if squeezing this area has an effect on the display, you know you've got the video chip issue. And like I said testing an external display is the other way to determine this, i.e. if you get no external image, you've likely got the problem.
I've seen a lot of 500s with the video chip issue (maybe I hit the jackpot and got a bad batch of them or something), but that problem is characterized by very erratic behavior, so if target mode works well and is stable, that might be an indicator that it's not the video issue. Actually, connecting to an external display is probably the best way to test this -- if an external display is stable and works well, that means the video chip is good, at which point I'd say the screen itself may be dead, my logic being that you've replaced the inverter cable with a (hopefully) known good one with no luck, ruling out that possibility, so if the display on an external monitor is stable, and the inverter cable is not the problem, that leaves the screen itself.
Another way to rule out the video chip issue: While powering up clamp the space just left of the trackpad with your left hand, applying some pressure. When the video chip issue is present, pressure in this area can force the chip to function, and sometimes the display will come back temporarily. So basically if squeezing this area has an effect on the display, you know you've got the video chip issue. And like I said testing an external display is the other way to determine this.