Setting the contrast only affects the LCD bias voltage. It has no effect on the lifespan of the LCD, but I've noticed that it can drift away from its center position over (many) years.
I'm a bit confused by your description, though:
if i change the contrast-setting to the far left, everything is good to read.
That does indeed sound like the center point has drifted away.
One step to the left, its "okay-ish" ... one more step to the left and reading values becomes difficult.
Wait, how can you go further left than "far left"? I assume you meant one step to the right?
Anyway, the good news is the LCD voltage center point can be reset quite easily on the 867B:
- Rotate the dial to SET UP
- Change the contrast so that the display provides the best clarity
- Press the "Save Set Up" soft key
- Rotate the dial directly to the mV/HiZ position
- Push the calibration button (hole on the right side, just below the DC barrel jack, that may be covered by a calibration sticker)
- Press the "Set Lcd Voltage" soft key
- Press the "Abort Cal." soft key
That's it. The new LCD voltage center position is stored!
If you change back to SET UP, the current position should be in the middle. Since you were previously at the far left, you may want to test whether you can now improve contrast even further. If the contrast gets even better, you may want to repeat the steps above to update the center position once more.