[HD2 General] motoi_bogdan:
if the leds are in series, there are 2 scenarios:

1. replacement leds have lower current rating. Let's say an original hd2 led needs 10mA for max brightness. And also, let's say the replacement led has only 6mA requirement...and for that level.. they, of course, produce less light. If you place one original led in series to one replacement, the original will try to take 10mA but the replacement, also being in the same circuit, can handle only 6mA. the rest 4mA transforms to heat (are lost). If try to draw more current, more miliamps would get transformed into heat and in a worst case scenario you burn out the replacement leds.

2. replacements are actually brighter but they require more current to do their job. The driver IC can't handle it so it heats up. This also stresses the other leds (originals). Think of those lamps with dimmers, most of them use a 40W-60W bulb. What would happen with the dimmer circuit if you simply replace the bulb with a 100W one :D

There's also a bonus 3'rd option. Replacements=originals but they all work at less then nominal current because driver ic got busted for whatever reason.

Try to power the backlight and check to see how the led's brightness compares to one - another.

Could be that the service guy thought the backlight is broken (when in fact the driver ic was broken) and replaced some leds only to see that it has no effect.

Try to start the phone without the display connected. Does that chip still heats up? I'm guessing (due to it's size, no. of pins and placement) it drives the backlight + tft matrix. If it doesn't heat now, we can assume that either the backlight draws too much current or the chip itself is faulty and can't handle full (but still normal..) load of the backlight current requirements.

Possible solution... well hack the backlight power supply. Here's an example.

Case 1 - if the - pin of the backlight power supply is the same as the main ground of the motherboard and the the same as the - pin on the battery.

Case 2 - if the - pin of the backlight power supply is different then that of the battery and main ground of the motherboard.

Ok... now what do we have here. Well the idea is pretty simple, probably too simple. We use the voltage from the driver ic (normally for the backlight power) to drive a normal npn transistor via a limiting resistor. The strain of driving the backlight is thus transferred to this transistor. We then use the backlight led's and another limiting resistor as the collector load for the transistor being used. We also use the battery power pins for voltage supply for the backlight, via this transistor. If the ground points are common, case 1 is to be applied, if not, there is case 2, where another resistor will limit the current drop between the 2 circuits.

Well.. i said, the idea is probably too simple. Here are some problems with it. If PWM is used to control the backlight, well.. a bipolar transistor won't do good in that circuit. Bipolars are drived mostly by current, not voltage. Probably a mosfet will do better here, because it can be better driven by apparent voltage variations as "seen" by it. Also.. the idea to use the battery pins to directly supply the backlight isn't that elegant. When the battery will get empty, it's voltage will decrease and so will the backlight level. We got a pretty bit voltage swing around here. A more elegant solution will be to tap on directly to the main voltage converter used in hd2's motherboad but again... it won't be happy with an added current hungry consumer. The most "politically correct" solution would be a switching mode constant voltage supply to be made, but there's no room inside hd2 casing for this sort of things.

If the problem chip simply heats on whatever you do and even with or without the display connected to it... there isn't much to do. You have to either replace .. it, or the motherboard.