How to clean a dark blurry shadow in the image?

Hello,

This is my first post here, after having read many of the beautiful guides and repair advice! I hope to find a Repair Café in my area soon, and contribute to the right of repair as much as I can. I am already keeping a old Mac under Linux, and the next laptop would be Linux, and with a high repair index from iFixIt !! Frame.work, Why Computing, I am looking at you!!

Back to my projector issue. I have just replace the lamp with module, from original part, and it works well. But then, after about an hour, I dark shadow, or blurry circle (I don't know how to describe it well) is in the middle of the image. Here is a photo of the image from the projector.

I already cleaned up the lens, smoothly externally, and remove most of the dust inside the projector, but no change. Changing the zoom or focus handle do not change the aspect of the circle.

Please would you any suggestion to where to look/clean?

I don't have a repair guide, and I have open the projector, and unscrewed the main structure part. But not any board or electronic circuit, yet.

Kind regards,
Hervé