Paul__B:
I think you are either using an unusual display, or feeding it 6V instead of 5 because this differs from the vast majority of previous discussions here. What is the actual contrast voltage measured between Vo and Vcc? And perhaps a picture of your module (and its retail source)?
No it's a pretty bog standard 1602 display. As for the source, I haven't a clue (it was part of an xmas pressie from my daughter). The only markings on it are "1602A" and then further down, "QC1602A r2.0"
Just hooked it up to an arduino mega2560. Vss to Vdd measures 4.96 volts. V0 is connected directly to Vss via three 1K resistors in series.
Reading Vss to V0 I get 1.08v
From V0 to Vcc I get 3.88v
Using the only camera I have at hand here's a couple of images. Sorry I can't get this thing to focus at short distance so hence the fuzziness. I can assure you though, the display is clear and free from any abberant white bits.
On the back the longer bit of heatshrink houses those three 1k resistors, and the shorter bit holds a 56 ohm resistor for the backlight (yes I know probably redundant).
