C7000 spots face down when it prints first side

[jpablo_mza]

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Good evening, I need urgent help with a machine, it is a c7000p only when printing on long papers the 2nd transfer stains the underside of the paper, the stain is only black, it is verified and change the first transfer of the black, second transfer roller upper and lower, it was tested from all the trays of both the machine and the LU, the length of the paper was verified, only stain when printing the first face, never when the sheet comes from the duplex, the stain is only pure black, Not composed, any suggestion is very welcome

[jpablo_mza]

Alguna sugerencia..???

[Phil B.]

Alguna sugerencia..???

english?

have you checked fuser for pressure roller buildup?