Ever ended up in a herd of scared cows after driving down a steep bumpy trail by following tomtom directions? I did.
Measuring the voltages across the single backlight LEDs, it looks like the first one and the last one in the chain are high-resistance-ish. With a power supply in constant current mode, I was able to check the single LEDs for operation.
Indeed, the two LEDs were faulty (?!?!).
The next logical step: replace/bridge them:
I opted for 0805 resistors in the 10...20 Ohms range (12R7) :
aaaand voila - Press and keep pressed the power button to let the unit boot to a summary screen. Releasing the power button continues the boot process.
I still have to replace the burnt 5V USB input protection diode.
Fun fact: It now tells me the TFT type *facepalm*: Sharp lq043t1dg01
Problem: The display does not work.
Hint 1: If the device is turned on, it plays a jingle.
Conclusion 1: SoC, flash ok, ram ok, most critical supply voltages ok. This narrows the problem down to the display itself, the SoC, the interconnect (flat flex...) or power supply.
Hint 2: In complete darkness, it is possible to see a dark but visible image.
Conclusion 2: This narrows the problem down to display backlight circuitcy, contrast voltages, backlight supply, backlight brightness control, ...
Hint 3: The reverse polarity protection / overvoltage protection diode on the USB connector died a hot death (burn marks around)
Conclusion 3: Smells like a power supply issue.
The display creates the contrast voltages itself with a charge pump IC on the flat flex cable. A 4wire resistive touchscreen controller IC (again on the flat-flex) drives the touchscreen. Backlight driver is located on the main board.
From this flat-flex, a flat-flex connector goes to the display, and another one goes to the touchscreen. All these parts are covered in sticky tape, glued to the display chassis etc. It requires patience to remove this and dig down into the display...
Hint 4: With or without backlight LEDs attached to the driver IC, it delivers approx 40V DC (with some ripple, of course). Most of those LED driver chips are constant-current output with max. voltage limit.
Conclusion 4:No difference between load connected or disconnected means it tries to drive the set current, but fails due to too high resistance. Are the LEDs ok?
Coming soon to a theater near you: poking around inside a small display and burning flat-flex with a soldering iron... (lets see if it turns out that way :D)