Apparently there is a problem with the contrast control of the LCD (which on this kind of displays actually adjusts the viewing angle, not really the contrast). The service manual shows an adjustment trimpot for it (1R120 on the power supply board) but this kind of stuff doesn't usually just move by itself... could be a bad contact somewhere or a faulty trimpot.
Anyway the circuit is just a controlled DC voltage source that is fed directly to the LCD itself and it is nothing critical. If you feel comfortable opening the speaker and locating this trimpot in question, I guess you might just as well try to adjust it. If the contrast jumps by just tapping on the trimpot, it has a bad contact. Correct adjustment value is 1.9V on pin 5 of connector P301 to the display module.