it is easy to think the lamps are working when they are not, as they are so close and bright. Just use a meter on them. Also remember the lower fuser...
I had this P14 on the same model a few years back, took out the print head and found the problem was a seized adjustment system on the K unit. Freed...
Why not get the "Konica Engineers " back to sort it out...for free! If it was printing all colors before they arrived with the gears, it should do so...