Mellow Yellow
 

I have an older Sony VPL HS-10 LCD projector that has slowly shown some yellow on the screen. It covers about a third of the screen and is no longer subtle.
I have replaced the bulb...no effect. I removed the lens dust cover...no effect.
I assume it is something in the light pathway, but I can't seem to get to it.
Any ideas on how to trouble shoot this problem??

Thanks.