You're right ic-racer! I had to remove the top half of the head and then remove the structure that holds the lamp assembly. All the filters are then easily accessible.

No the filter is not broken. I'm actually trying to implement the solution to split grade printing with this head that you suggested to me a few months ago. I have some time now. I'd like to replace the supplementary filter with either a yellow or magenta filter of equal strength to the other filters in the head. I got the supplementary filter out. It measures 83 mm x 87 mm x 2 mm. Now, where do I get a dichroic filter to replace this one with? I tried Jensen Optical but he asked about $1600. I'm not sure that he understood, or that I explained well, what my need was...

Also, as the filters are accessible now, is it worth cleaning them individually, or better to not take the risk of breaking them...?