HP 4550 Service Manual - Page 117

Once the ITB counter gear has rotated one complete turn, the tabbed lever drops into the gear notch. This causes the life detection lever to block light from the ITB unit life sensor (PS1). When light is blocked from PS1, the DC controller sends the formatter a warning that the end of the ITB unit life has been reached. Figure 5-17 ITB unit life detection Secondary transfer roller unit The secondary transfer roller is normally detached from the ITB. This operation presses media against the ITB with the secondary transfer roller at the time of secondary transfer. During "first page out," the secondary transfer roller engages with the belt prior to drum-to-belt image transfer to stabilize the belt from side-to-side movement. The roller stays engaged while the black image is being transferred to the ITB. Just before the black image reaches the secondary transfer roller, the transfer roller disengages from the ITB. Following image transfer from the photosensitive drum to the ITB and after the paper is positioned in front of the registration rollers, the DC controller turns on the secondary transfer roller clutch (CL4). Once CL4 is on, the secondary transfer roller contact/separation cam rotates and lifts the secondary transfer roller mount. This presses the secondary transfer roller against the intermediate transfer belt prior to the paper arriving from the registration rollers. Bias for the roller is turned on to enable image transfer from the belt to the paper. Once the print job is finished and the paper has passed through the unit, the DC controller then turns CL4 off and secondary transfer roller disengages from the ITB. C7085-90921 Chapter 5 Theory of operation 115