Hi,

I am working with a Howtek d4000, silverfast, on os 10 with a firewire adapter. I have been working with this scanner for a few years now. Sometimes I get clean, great scans, but lately I have been getting horizontal lines (parallel to the spin of the drum) across the image. They are not consistent- each scan give slightly different lines. There seems to be a correlation between lower resolution scans and more lines, i.e., the pre-scan or test scans at 100 dpi are filled with lines. As I go up in resolution, the lines get less. I am attaching images so you can see.
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Also, on the pre-scan this morning there seems to be a bright spot in the upper right hand 1/4 of the scan area, like too much light shining on it- I have no idea what this is or if it is affecting the scans. Click image for larger version. 

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I normally scan 8x10 negatives at 2000 dpi (the computer cannot save images larger than that) and they all seem to have pronounced lines, but last week I scanned a set of B&W 4x5s at 4000 dpi and got perfect scans- no lines at all. I have lubed the lead screw and changed the rubber belt recently- with no real change in lines.

I wonder whether the drum is slipping somehow? Or, if there is a calibration issue?

Any help or ideas would be awesome- I cannot figure out what, exactly, is wrong.

Thanks,

Noah