Thank you to the contributors of this thread, you saved me a lot of frustration after my first display appeared to have slowly "died" over the first 18 months of use of my home built Prusa i3. as soon as I installed a new display it was giving me the same contrast issue the 18 month old display had eroded to. After reading the posts in this thread, I tried 440 Ohms, and settled on 660 Ohms (becby tritodd - Controllers