HP 4550 Service Manual - Page 116

When the printer receives a print command, the ITB is lifted into position against the drum. This is accomplished when the ITB lift cam is rotated and the cams press against the lift levers located on each side of the ITB. Drive force for the ITB lift cam is provided by M1 and applied to the cam when CL2 is activated. Once CL2 is activated to lift the ITB, both CL4 and CL3 are activated to position the transfer roller and cleaning roller against the transfer belt. The transfer roller is used to stabilize the belt's side-toside movement during its initial contact with the drum while the cleaning roller prepares the belt for the image. Just before the black image reaches the transfer roller, both CL4 and CL3 are deactivated again to remove the transfer roller and cleaning roller from the transfer belt. This procedure only occurs when the ITB is lifted into position against the drum. The print process continues as described in the image formation section. Once the full image is on the ITB, both CL4 and CL3 are activated, and the transfer roller and cleaning roller are moved into position against the ITB. This time the image is transferred from the ITB to the media. Any residual toner left on the ITB is charged positively for removal. Once the print job is complete, CL4 and CL3 are deactivated and the transfer and cleaning rollers are moved away from the belt. CL2 is then deactivated and the ITB is lowered to move it away from the drum. Home position detection The home position of the ITB is detected to precisely transfer the four toners to the intermediate transfer belt and align the leading edge of print paper with the leading edge of the image. In the area outside of the toner transfer area of the ITB, there are home position detection holes. With the ITB rotating, the home position detection sensor (PS5) detects a hole and sends the information to the DC controller. When the ITB home position detection signal is detected by the DC controller, it sends a top-ofpage signal to the formatter. The formatter sends the image data and the DC controller begins to create the image. For full-color printing, this process is repeated four times to form the image. Figure 5-16 Home position detection ITB unit life detection The ITB counter unit is located on the side of the ITB unit. The counter unit consists of a number gears, including the counter gear, and an ITB unit life detection lever. The gears indicate to the ITB counter gear the number of rotations (turns) made by the intermediate transfer belt. A "page count" is also stored on the formatter, which is used for informational purposes only. When a configuration page is printed, the percentage of ITB life remaining is indicated based upon this page count. If the ITB unit is replaced prematurely or the percentage of life is inaccurate, the page count in the formatter must be manually corrected. 114 Theory of operation C7085-90921