Diffrent picture between the original document and master picture

refer to youtube video title

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HOW TO: Clean a Riso GR Drum - YouTube

All you really have to do is put in the ink color you want and run a bunch of prints. Going from a light to dark color like black is fast--like about 3000 prints. Black to light color is going to be a lot more prints. Best is to print something with heavy ink. the more ink you put out the fast it will change color.

We suffer the same issue. Have you find the solution?

Open front door. If you look at the metal housing below the drum you will see a plate mounteg with two screws. Get it off and you will see a small lever. Turn it right to loosen the drum. Be extremely careful when pulling out the drum. If the drum is not in zero-position it might be a bit difficult (e.g. have a look at the master clamp). Please note that it may be difficult to insert the drum again.

I am looking for a replacement black drum for a riso 3750

1. check the upward sensor for adjustments and the sensor on the stripper pad unit for adjustments

My assumption is that it has something to do with the Purple ink. If you have a spare try swaping it out to see if the error goes away.

Yes you are right. The 'stripper finger' is not lifted enough so it scratches the master.

I know this problem from a GR. But the procedure should be nearly the same for your machine. Behind the back cover you will find a small rubber roller. Diameter 12mm (1/2") or so. This roller runs at the excentic curve of the drum. You might find that the rubber is worn of and so the finger cannot be lifted enough any more. You have to replace it.
(Hint: For a fast momentarily solution you may fix it by several windings of adhesive tape.)

Means a clamp error. Inspect the clamp on the drum to make sure it moves smoothly and that that clamping mechanism in the back of the machine works as well.

check the dust or any scar in scanner (glass).