HP 4650 Density/CPR sensor out of range: problem and explanation
HP 4650: during previous three months, error log showed 54.0B Density sensor out of range occurring every 10 pages with occasional 54.10 CPR sensor out of range, but no problems with print quality. Suddenly yellow became light every time machine switched on, but fault cleared by a full calibration. Always yellow and never any other color. Also evidence of poor color plane registration, clearly shown in corners of Configuration Page.
Tried replacing CPR sensor, also cleaned ETB brass contacts to printer with metal polish, and HV PCB wire-link contacts. No improvement in fault. As a last resort, studied service manual more carefully (should have done this first!). Under the fault condition I Disabled Cartridge Check (under Diagnostics menu), then opened front door and exchanged yellow and magenta cartridges. I realised that if the fault migrated to magenta the printer circuitry (e.g. DC controller) would be at fault, but if it remained on yellow (despite its position in the Configuration Page printout changing, as expected), the fault must lie in the yellow cartridge. Result: the latter.
So I replaced the OPC drum in the yellow cartridge, refitted it in its rightful place, re-enabled Cartridge Check and did Full Calibration. Result: Fault has completely disappeared and not returned.
I have found no information on 54.0x Density sensor out of range errors (where x = B, C, F etc.). I believe 54.0B means yellow, since my yellow OPC drum was probably faulty for some time. I have no information on the assignment of 54.0C, 54.0F etc.
I believe the explanation of what is happening here is as follows. I think the DC Controller applies various voltages within a standard range to the cartridge PCRs during calibration, whereupon the density of the pattern generated is read by the CPR sensor assembly. Good OPC drums will deliver patterns of color whose density falls within an acceptable range. Failing this, the DC Controller must reset the voltage to force the OPC drum to generate a pattern for that color of acceptable desity (see Service Manual p158). The need to do this is symptomatic of a failing OPC drum. The same defect undermines color plane registration, a separate but related parameter – hence the accompanying 54.10 CPR sensor out of range error. Replacing the failing OPC drum prevents the recurrence of both errors.
These error messages have nothing whatsoever to do with the drum home-position (phase) sensors.
If my understanding is in any way defective, please someone correct me. However I believe this explanation fits the facts perfectly and accounts for everything I have mentioned.
