Ricoh Aficio 550 printing very light

Question

The copies are all coming out very light, changing the darkness has no effect.  The toner tube seems to have plenty of toner in it.  I read that the toner tubes have to be able to hold a "charge" and sometime they can't even if they have plenty of toner.  Is it possible a new toner tube would fix the problem?  I'd like to explore the simple posibilities before calling out a service person.  Are there other simple solutions to try to fix a copier that is printing out very light?