Taranis LCD, no or very low contrast

[Ozzy]

I have a spare Taranis that developed a LCD with no or very low contrast, the back LED lights are fine. The rest of the Tx works perfectly. By lowering the brightness and increasing contrast to max, I can just read the lcd at the right angle. I bought a replacement LCD from Aloft and same. Not knowing, I should of bought the main board.

I also tried reseating the LCD ribbon cable, no love. It must be something on the main board. I am pretty good with electronics and excellent soldering, I just don't have the schematic to start troubleshooting. Any suggestions where to start looking on the main pcb? Even a p/n to the LCD would help with the LCD pinout or pin numbers driving the LCD. Another thread to ask in? TIA!!!

[Kilrah]

Taranis or Taranis Plus? They use different (and incompatible) LCDs. Make sure you ordered the right one.

[Ozzy]

Regular Taranis, I already swapped it, and it works the same, barely readable.

[Ozzy]

More info, not solved yet. I scoped and pinned out the LCD connector. I was hoping to find an analog signal to adjust the LCD contrast (I have seen this before on a digital goto telescope (LX200GPS-14"). The LCD pin number #3 is a strange voltage, I adjusted the contrast while watching this pin, and it does not change voltage levels, even on a good Taranis. Four other pins are switching 0-3v. Any ideas or suggestions welcome.

LCD Pin#
1 3v
2 0v
3 Switching 1.4v to 1.6v ??? Unknown where this connects.
4 0v
5 Switching 0-3v Connects to STM32 pin#5
6 Switching 0-3v Connects to STM32 pin#6
7 0v
8 0v
9 3v
10 Switching 0-3v Connects to STM32 pin#58
11 3v
12 3v
13 3v
14 3v
15 3v
16 3v
17 3v
18 Switching 0-3v Connects to STM32 pin#57
19 0v
20 3v

[Kilrah]

All your measurements match what I can see. Pin 3 is unconnected.
The only thing that looks strange is your supply voltage, should be 3.3V, not 3V, would be looking at that first.

[Ozzy]

Good point, I looked closer at the voltage levels.

Looking at the Ti TPS5401 (U3) buck regulator,
R76=29.4K
R77=10.0K
Vfb=0.8V
Vout calc = 3.152V

I measured on my good Taranis:
R76=29.33K
R77=9.97K
Vfb=0.8
Vout calc = 3.146V.

I measured my good and bad Taranis LCD voltage = 3.124V and 3.129V. ~23mV should not be out of spec.

Any other ideas to look at? Thanks.