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  1. #21
    Trusted Tech 50+ Posts
    Join Date
    Sep 2009
    Location
    Waterbury, CT
    Posts
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    Copy Mark on Black Halftones

    Little hard to tell from your image but it looks like a jitter or speed matching problem caused by the transfer belt or fuser interacting with the paper - worse on heavy stock.

    Transfer belt unit is OK and rollers replaced? Second transfer too?

    KM says this is a characteristic of the machine and cannot be totally fixed. Hmmmmm....

    This is a cut from article 40 in the KM Troubleshooting Guide for the C6501:

    The next things can be done to reduce it:
    • Activate all transfer pinches when making black prints by service setting SW11, Bit 05.
    Admin settings: Set Thick paper black mode to ON.
    This under [utility] 03 Adm. settings. 06 Common Settings.
    Then 1 times pres NEXT. Set it ON.
    Normally when you make a black monocolor copy only the black first transfer roller is active. What this does is activate all the rollers. Effect is to put a brake on the transfer belt and hopefully stabilize its speed and reduce the jitter.

    • Minimise speed variations by means of service belt line speed adjustment. OK so play with it between -5 and -35 or more if you dare. Check the FD Mag adjustment when you are done so image size is correct.

    • Reduce 2nd transfer step pressure.
    After enabling by service (SW1, bit0) the key operator can reduce the pressure by changing the paper tray setting.
    DIP SW setting shows Process Adjust button in the paper settings. In Process Adjust section there is a setting for 2nd transfer pressure - again the idea is to reduce the possibility of jitter so try normal and light.

    • Use different raster Dot 2 / type 2 instead of Dot 1.
    This screen is to adjust in:
    Utility [button]
    03 Machine adm. settings.
    01 Sytem setting.
    05 Expert adjustment.
    07 Quality adjustment.
    05 Custom screen.
    Then set for Line 2 and Dot 2 the type to TYPE 2.
    Changing the print screen can mask jitter marks by altering the pattern of dots laid down by the lasers under some circumstances - definitely worth a try. Change the screens used by the copier in machine admin mode and then select Dot 2 instead of Dot 1 in the print driver.

    BEST SOLUTION SO FAR
    1. Do all the stuff above.
    2. Lie to the machine about the paper weight in the tray - select 81-105gsm. This makes the motors run at high speed - more torque means less jitter.
    3. You have to go to the Process Adjust screen mentioned above and raise the fuser temp and pressure to compensate for higher speed and make sure fusing is correct.

    This is not a complete fix but pretty darn close.

    Best Regards from the snows of Connecticut!

  2. #22
    Field Supervisor 1,000+ Posts RRodgers's Avatar
    Join Date
    Jun 2009
    Location
    San Bernardino
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    56
    Do the Fuser line speed adjustment and that should fix it. Play with it (line speed) until it goes away.

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