HP CLJ 8500/8550 Repeating Color Image Problem

This article originally appeared in the February 2002 issue of The Image Source (vol. 5, issue 2)

One of our account representatives stopped by my office a little while ago with an image defect problem on an HP Color LaserJet 8500 that was boggling one of his customers. He was getting a repeating image across the page and had replaced the image drum and transfer kit to no avail. Seeking more information, the customer faxed us a copy of the problem. The repeating image was a little over two inches to the right of the color print on the page (Fig. 1). The strangest part about the image defects was that the customer said the repetitive print was only in color--there was no repetition in black. Because of this, the customer was thinking it might be a laser scanner problem or DC controller failure. However, the solution was a lot simpler and less expensive than what the customer first suspected.

Figure 1

To begin trouble-shooting this problem, we had to remember the image the customer faxed us was a letter size page, meaning the paper physically went through the printer long edge, or sideways. Thus, seeing a repeating image trailing about two inches to the right of the print actually meant it was repeating about two inches down the normal image path or paper path. Turning the page one-quarter turn clockwise produced the image as it was made in the printer. The next step in trouble-shooting this was using the repetitive defect ruler--a good tool found in the service manual. The distance between the repeating images on the fax was the same as on the customer's print, and measured 56mm (2 and 3/16 inches). The measurement pointed to the cleaning roller as the cause of the repetitive images. The next closest measurement to the cleaning rollers was the black developing roller, 52mm (2 and 1/16 inches). Since black was not repeating at all, this roller was ruled out.

Since we suspected a cleaning roller problem, we started by looking at its specific purpose. After four layers of toner are placed on the transfer drum, paper moves through the transfer section, pulling toner off the drum and down onto the paper. As this is happening, not all the toner leaves the transfer drum. The cleaning roller is engaged with the transfer drum at this time and changes the charge of the residue toner so it will be attracted back to the image drum. Once the residue toner is on the image drum, a scraper removes it from the image drum, sending it to a waste toner collection area (Fig. 2). At the completion of the cleaning process, the cleaning roller is pulled away from the transfer drum and allows the new image to be created for the next print.

Figure 2

Replacing the cleaning roller in this case did not fix the customer's problem when they installed a transfer kit, but the customer was in the right area. As noted above, the cleaning roller engages with the transfer drum when cleaning and disengages from it when it is finished. This is controlled by a solenoid, SL 1. If by chance the cleaning roller stays engaged with the transfer drum when it is not supposed to be, the cleaning roller will pick up whatever toner is on the transfer drum, and, rotating around, paste it back on to the transfer drum. This would create a repeating image across a letter size page with a distance of 2 and 3/16 inches, but if this is the case, why isn't the black toner repeating too?

To understand this, let's focus in on the transfer process where the image is being transferred to the paper. We will assume the cleaning roller has been in contact with the transfer drum through the whole image formation process. As each layer of toner is placed on the drum one at a time (yellow, magenta, cyan & black), the cleaning roller is creating repetitive prints of it on the transfer drum. As the third color, cyan, is placed on the transfer drum, the image rotates around, and the cleaning roller creates a repetitive print of the three colors. Then, the black image is placed on the transfer drum by the image drum. As black is being put on the transfer drum, the paper feeds through the machine and the image is transferred to the paper. The transfer happens before the black toner gets to the cleaning roller so the only colors that repeat across the page are the yellow, magenta and cyan.

The problem here is obviously the physical contact of the cleaning roller when it is not supposed to be, but why is it doing this? There have been problems with what is called a stopper arm, which works with SL 1 to engage and disengage the cleaning roller. If the stopper arm breaks or the tension spring that keeps the arm in place is missing, the roller stays engaged. To locate these two parts, you need to remove the transfer drum. Open the front door and unlock the transfer drum via the large green handle. Next, open the right door and remove the transfer drum by grabbing hold of two green handles. Looking back inside the machine on the side that the transfer drum was removed, the black cleaning roller sits above and parallel to a yellow sticker (Fig. 3). To the right of the cleaning roller, you should see a white plastic arm with a tension spring attached to it (Fig. 4). If the spring or part of the white stopper arm is missing, you've found the problem. You may be able to push against the cleaning roller and notice it moves in and out about a half-inch or more (if it is working correctly it should not do this). This is not a sure-fire test to see if the cleaning roller is functional, but when these parts are missing or broken, it is often a good clue.

Figure 3

Figure 4

The replacement part numbers are not listed in the service manual, but it might be a good place to write them down for future reference. The stopper arm is RB1-9891-000 and tension spring is RS5-2699-000.

The rest of this article will be dedicated to the procedures to remove and replace the stopper arm and tension spring.

1. Turn off the printer and unplug it. Remove the transfer drum by unlocking it via the large green handle inside the front door and then removing the unit from the right side door.

2. Remove the cleaning roller by pulling on the shaft from the right hand side (Fig. 5). Do not pull the roller out more than one inch as this may break the bushings on the other side. Slide the roller to the right and remove it from the printer.

3. Next, release the coil springs from each side of the cleaning roller holder by taking the spring tip and sliding it toward the center of the printer (Fig. 6).

Figure 5

Figure 6

4. Rotate the white bushings next to the coil springs until the flat spot on the inside part of the bushings is facing down. The bushings are keyed and when rotated into this position, they will allow the cleaning roller holder to slide toward you, (Fig. 7).

5. Locate the tension spring attached to the white stopper arm (Fig. 4), if still attached, and remove it.

6. Remove the stopper arm by first removing the e-clip to the right of it. Carefully unclip it from the shaft, being careful of where it may land. Next, slide the shaft that holds the white stopper arm toward the center of the machine and slide the stopper arm to your right and remove it (Fig. 8). If you damage the e-clip, it is a standard size (part number XD2-1100-502).

Figure 7

Figure 8

7. Installation is the reverse of the procedure.

Note: Be sure to reattach the coil springs from step # 3, or the cleaning roller won't move into place to do the cleaning.