Had a problem with a chinese display, similar to the one in this post, just i2c instead of SPI.
The display was very dark, voltage @D1 cathode was only 4.5V insted of 15V. I connected an external 12V source to D1 cathode and the display got bright.
As this is not the ideal solution, I tried to find the source of the problem and reverse enginered the schematic. I figured out that they used a 4.7k (R9) pullup connected to the enable pin of the converter chip. In the datashet of the chip, en is directly connected to Vin without pullup:
Datasheet tps61041
So I just connected 5V from R9 to the En pin of the converter (tps61041). This solved the problem.
