HP Color LaserJet CM4730 Service Manual - Page 178

The developing voltage (bias) causes the toner to adhere to the electrostatic image that the laser/ scanner assembly created on the photosensitive drum. Two types of developing biases are used: the developing DC negative bias and the developing AC bias. The high-voltage circuit on the power supply generates both biases. These biases are superimposed on one another and then applied to the primary charging roller, which transfers the biases to the drum. The biased developing cylinder picks up toner particles and deposits them onto the electrostatic image on the photosensitive drum. The image is now visible on the drum. Generation of developing blade bias: The developing blade bias (BL) is the DC negative bias that is generated to charge the toner. This bias is generated for each color in the developing blade highvoltage power supply circuit in the high-voltage power supply PCB. The high-voltage power supply PCB applies the developing blade biases (BL1, BL2, BL3, BL4) to the developing blade in each cartridge at specified timing. The value of the developing blade bias varies in response to the command from the DC controller. Generation of attraction bias: The attraction bias is generated to attract paper to the ETB and to transfer toner onto the paper efficiently. Both DC positive and DC negative biases are generated when attracting paper and cleaning the ETB. The high-voltage power supply PCB applies the attraction biases (ATTS) to the attraction roller in the ETB unit at the specified timing. The value of the attraction bias varies in response to the command from the DC controller. The DC controller changes the value of the attraction bias and regulates it in constant current control in response to the ATTRACTION CURRENT VALUE RETURN signal (HVATTS) sent from the high-voltage power supply PCB. The following are the printing sequences. ● Attraction charging bias: This bias charges the paper positively or negatively and makes the negatively charged toner transfer easily. ● Between-sheets bias: This bias prevents the residual toner on the ETB from adhering to the attraction roller in the between-sheets during continuous printing. ● Cleaning bias: This bias prevents the residual toner on the ETB from adhering to the attraction roller during ETB cleaning. Generation of transfer bias: The transfer bias is generated to transfer toner onto the paper. Both DC positive and DC negative biases are generated in the transfer high-voltage power supply circuit in the high-voltage power supply PCB. The DC positive bias is generated when transferring toner, and the DC negative bias is generated when cleaning the ETB. The high-voltage power supply PCB applies the transfer biases (TR1, TR2, TR3, TR4) to the transfer roller in the ETB unit according to the print sequences. The value of the transfer bias varies in response to the command from the DC controller. The DC controller changes the value of the transfer bias and regulates it in constant current control in response to the TRANSFER CURRENT VALUE RETURN signals (HVTR1S, HVTR4S) and the HVATTS signal sent from the high-voltage power supply PCB. The following are the printing sequences. ● Print bias: This bias transfers toner on the photosensitive drum surface onto the paper during printing. The transfer roller is applied with the DC positive bias. ● Between-sheets bias: This bias prevents the residual toner on the ETB from adhering to the attraction roller in the between-sheets during continuous printing. ● Cleaning bias: This bias cleans the ETB by returning any toner remaining on the ETB surface to the photosensitive drum. The transfer rollers in the yellow and magenta cartridges (first and third colors) are applied with DC negative bias. The rollers in cyan and black cartridges (second and fourth colors) are applied with DC positive bias. 158 Chapter 4 Theory of operation ENWW