How to Fix Faded Black Backgrounds on the Konica Minolta Bizhub

A faded black background defect presents as a solid black fill area—a page header, a reverse-out text block, a full-page black flood fill, or a dense halftone shadow region—that prints as grey, uneven, or visibly washed-out rather than as a deep, uniform black. The defect may affect the entire black fill uniformly (globally reduced density) or selectively (certain zones within the fill are lighter than others, creating a mottled or streaked appearance within the dark area). White specks, voids, or grey patches within what should be a solid black field are characteristic presentations of this defect class.

This symptom is particularly disruptive in professional and production print environments where black-background documents, infographics, presentation covers, and photographic prints with deep shadow regions are common. It is also a frequent source of user-reported complaints on machines that appear to print text and halftone content acceptably but fail on high-coverage black fills.

Faded black backgrounds are caused by failures or degradation across multiple subsystems of the electrophotographic engine: the charge system (which must create a sufficiently high and uniform surface potential on the OPC drum), the laser exposure system (which must fully discharge the exposed areas), the development system (which must supply adequate toner to the discharged latent image), the transfer system (which must efficiently move the developed toner from drum to paper or ITB), and the fusing system (which must fully fix the heavy toner load to the paper surface). A deficiency in any one of these subsystems, or a combination of marginal conditions across multiple subsystems, produces a faded black background.

NOTICE: Always power off the machine and disconnect the AC power cord before accessing internal components unless a specific step explicitly requires the machine to be energized. Allow the fusing unit a minimum of 30 minutes to cool before handling. Never expose the OPC drum surface to ambient room light for more than 60 seconds during service procedures.

2. Affected Model Reference Table

Model Family	Representative Models	Engine Type	K Drum Unit	K Developer Unit	Laser Unit	Fusing Unit Designation	Main Control Board(s)	HVPS Designation
C360 / C280 / C220 Series	bizhub C360, C280, C220	4-drum tandem color	DR-311K	DV-311K	LU-301 / LU-302	FU-P01 / FU-P02 (model-dependent)	PRCB / MFPB	HVPS-1 / HVPS-2
C458 / C368 / C308 Series	bizhub C458, C368, C308	4-drum tandem color	DR-512K	DV-512K	LU-302	FU-P01 / FU-P02	PRCB / MFPB	HVPS-1 / HVPS-2
C554 / C454 / C364 / C284 / C224 Series	bizhub C554, C454, C364, C284, C224	4-drum tandem color	DR-512K	DV-512K	LU-302	FU-P01 / FU-P02	PRCB / MFPB / EXCB	HVPS-1 / HVPS-2
C759 / C659 Series	bizhub C759, C659	4-drum tandem color	DR-612K	DV-612K	LU-302	FU-P03 / FU-P04	PRCB / BASEB / CPUB	HVPS-1 / HVPS-2 / HVPS-3
754e / 654e / 554e / 454e / 364e Series	bizhub 754e, 654e, 554e, 454e, 364e	Single-drum monochrome	DR-614K	DV-614K	LU-204	FU-P01 / FU-P02	PRCB / MFPB	HVPS-1
808 / 958 Series	bizhub 808, 958	Single-drum monochrome	DR-616K	DV-616K	LU-206	FU-P05 / FU-P06	PRCB / BASEB	HVPS-1 / HVPS-2
C3300i / C3800i / C4000i Series	bizhub C3300i, C3800i, C4000i	4-drum tandem color	DR-C4K	DV-C4K	Integrated laser unit	Integrated fusing unit	PRCB / MFPB	HVPS
C250i / C300i / C360i Series	bizhub C250i, C300i, C360i	4-drum tandem color	DR-C4K	DV-C4K	Integrated laser unit	Integrated fusing unit	PRCB / MFPB	HVPS
C650i / C750i / C850i Series	bizhub C650i, C750i, C850i	4-drum tandem color	DR-C5K	DV-C5K	LU-302 / Integrated	FU-P03 / FU-P04	PRCB / BASEB / CPUB	HVPS-1 / HVPS-2
284e / 224e / 164 Series	bizhub 284e, 224e, 164	Single-drum monochrome	DR-512K	DV-512K	LU-204	FU-P01	PRCB / MFPB	HVPS-1

Note: Part numbers listed are representative for each model family. Always confirm exact part numbers against the specific machine serial number, firmware version, and regional service manual before ordering replacement components. Fusing unit designations vary by paper size configuration (A3 versus A4-only models).

3. Understanding the Defect: Black Density in the Electrophotographic Process

To diagnose a faded black background effectively, the technician must understand how maximum black density is achieved through the electrophotographic (EP) process chain and where each subsystem can fail to deliver adequate density.

3.1 The EP Process Chain for Maximum Black Density

Maximum black density requires all of the following conditions to be met simultaneously:

Sufficient surface charge on the OPC drum: The Primary Charge Roller (PCR) or corona wire must establish a uniform, high-magnitude negative surface potential (typically −600 V to −900 V, model-dependent) across the full drum surface. Any reduction in this potential—from PCR wear, contamination, or a low HVPS charge bias output—reduces the charge differential available for development, which limits toner adhesion and reduces final density.
Full laser exposure of the image area: The laser must fully discharge the drum surface in the exposed (image) area to near zero volts (−50 V to −100 V residual, model-dependent). Insufficient laser power (aging diode, contaminated laser window, or reduced HVPS laser bias) leaves a residual negative charge on the image area that repels toner and reduces density.
Adequate toner development: The developer unit must apply a full, uniform layer of negatively charged toner to the discharged image area on the drum. Low toner concentration in the developer, an aged developer carrier reducing toner triboelectric charge, a worn developer roller, or an incorrect developer bias voltage (HVPS output) all reduce the quantity of toner deposited on the drum image area.
Efficient primary transfer to the ITB (color models): On color machines, the toner image must transfer completely from the K drum surface to the ITB surface under the correct primary transfer voltage (PTR-K bias from the HVPS). Insufficient or excessive transfer voltage, a contaminated PTR-K roller, or a dirty ITB surface reduces transfer efficiency and leaves toner on the drum (reducing density on the output) or produces scatter artifacts.
Efficient secondary transfer to paper: The toner image on the ITB (color) or drum (mono) must transfer completely to the paper surface. Heavy toner loads in solid black fills are particularly susceptible to secondary transfer failure, especially on coated, glossy, or heavy-weight paper stocks, or when the 2nd BTR (Secondary Transfer Roller) or secondary transfer voltage is not optimized for the paper type.
Complete fusing of the heavy toner load: Solid black fills carry the maximum toner mass per unit area of any print mode. If the fusing nip temperature, nip pressure, or nip dwell time is insufficient for the paper type and toner mass, the toner will not fully melt and penetrate the paper fiber. This produces an under-fused black background that appears faded, matte, or shows a mottled texture.

3.2 Defect Sub-Type Classification

Sub-Type	Visual Description	Primary Suspect Subsystem
Global density reduction	Entire black fill area uniformly grey; no zonal variation; text and halftones may appear acceptable but lighter than expected	Developer unit aging / toner concentration; HVPS charge or developer bias; laser power reduction
Mottled / speckled fading	Black fill area shows white specks, voids, or irregular lighter patches randomly distributed; borders of the fill may be denser than the center	Secondary transfer failure (heavy toner load / paper type mismatch); 2nd BTR contamination; fusing under-temperature or nip pressure deficiency
Zonal fading (one side of page)	Black fill is denser on one side of the page width and fades toward the opposite side; may be gradual or abrupt	Developer roller wear (end-to-end sleeve variation); fusing roller thermistor TH1 / TH2 zone imbalance; laser exposure non-uniformity across scan width
Fading only on first pages after idle	Black fill is faded on first 1–3 pages after the machine has been idle; subsequent pages are acceptable density	Toner supply warm-up lag; developer toner concentration drop during idle; fusing unit initial temperature overshoot or undershoot affecting first-page density
Fading that worsens with consecutive high-coverage pages	Black background density is acceptable on first pages but progressively degrades over a long run of high-coverage prints	Toner supply insufficiency (M11 / M29 motor; toner bottle near-empty); developer toner concentration depletion under high demand; fusing unit thermal overshoot reducing fusing efficiency
Fading only in copy mode	Black fill in copies appears grey; same content printed from PC driver produces correct density; scanner illumination or image processing causing underexposed dark areas	Scanner image exposure setting; scanner lamp / LED illumination array output; copy density adjustment in Service Mode
Fading with rough or matte texture in the fill area	Black fill appears partially fused; toner surface appears rough or granular rather than smooth and glossy; toner may flake slightly when flexed	Fusing unit: under-temperature (thermistor TH1 / TH2 fault), fusing roller end-of-life coating failure, nip pressure deficiency

4. Required Tools and Materials

Lint-free dry cloths (IPA-compatible)
Isopropyl alcohol (IPA), 99% purity
Cotton swabs (ESD-safe)
Flashlight or pen light
Digital multimeter with insulated probes rated for high-voltage DC measurement
Printed solid black test pages (full-page 100% K coverage) for density evaluation
Various paper types (standard 80 g/m² office paper; heavy-weight 120–200 g/m² stock; coated paper if used in the customer environment)
Service manual for the specific machine model
Replacement drum unit DR-xxx K (as appropriate to model)
Replacement developer unit DV-xxx K (as appropriate to model)
Replacement fusing unit (as appropriate to model), if fusing is indicated
Replacement toner bottle (TN-xxx K) for the affected model, if toner supply is indicated
ESD wrist strap and ESD mat

WARNING: Fusing unit components operate at temperatures of 160°C–200°C during normal operation. Before any fusing unit access, power off the machine and allow a minimum of 30 minutes cooling time. Confirm the fusing roller temperature using the Service Mode temperature display (Service Mode → Machine → Status → Fusing Temperature) before touching any internal fusing components. Thermal burns from premature fusing unit contact are a significant field injury risk.

5. Preliminary Verification Procedure

These steps must be completed before any component removal or replacement. They establish a precise defect profile and direct all subsequent diagnostic effort to the correct subsystem. A faded black background defect has multiple possible causes; these steps eliminate incorrect diagnostic paths before parts are touched.

Generate a machine-internal 100% K solid fill test print.
Access Service Mode → Machine → Test Print. Select or generate a full-page solid black test page (100% K density coverage) and print it from the machine’s internal controller, not from a host PC. This eliminates any possibility that the density deficit originates in the host driver, color profile, or document preparation software. Examine the output under strong, consistent lighting.
Compare print mode versus copy mode output.
Place a printed solid-black reference sheet on the scanner platen and make a flatbed copy. If the copy output is denser than the original print defect suggested, the defect is genuinely in the print path. If the copy output also shows a faded black background on a document that is itself correctly black, the defect may be partially or wholly in the scanner exposure path. Proceed to Section 6.8 for the copy-mode diagnostic path.
Check the toner level for K (black) in the machine UI and Service Mode.
Access Service Mode → Counter → Toner Level (K). A critically low or empty black toner bottle is the single most common cause of globally faded black output and takes approximately 30 seconds to diagnose. If toner level is at or below 10%, replace the toner bottle and run 10–20 warm-up prints before evaluating output density.
Record drum and developer counter values for K station.
Access Service Mode → Counter → Life Counter. Record the K drum counter (DR-xxx K) and K developer counter (DV-xxx K). Compare these values against the rated replacement intervals listed in the service manual. Components at 80% or more of their rated service life are high-probability contributors to reduced density.
Check the Auto Gradation Adjustment (AGA) last-run status and result.
Access Service Mode → Adjust → Image Quality → Auto Gradation Adjustment → Last Result (model-dependent). If AGA has not been run recently, or if the last AGA result shows a significant correction applied to the K channel, execute a new AGA cycle (see Section 7) before replacing any component. A machine that has never had AGA run after a drum or developer replacement will consistently show reduced black density because the tone reproduction curve is not calibrated to the current component state.
Classify the defect sub-type using the classification table in Section 3.2. Record the sub-type, as it directs the diagnostic sequence order. For example, a mottled / speckled fading sub-type should proceed to fusing unit diagnosis (Section 6.6) and secondary transfer diagnosis (Section 6.5) before developer unit diagnosis. A global density reduction sub-type should proceed first to developer unit diagnosis (Section 6.2).
Test with a known-good paper stock.
If the defect is being reported specifically on heavy-weight, coated, or specialty paper, print the same 100% K solid fill test page on standard 80 g/m² office paper. If the density is acceptable on standard paper but faded on the heavy or coated stock, the defect is a media-type transfer or fusing mismatch rather than a component failure. Proceed to Section 6.7 (Paper Type and Transfer Optimization) before replacing any hardware.

6. Diagnostic and Repair Procedures

6.1 Calibration Baseline: Auto Gradation Adjustment (AGA)

Applies as first corrective step when: Global density reduction is observed; drum and developer counters are within normal range; no obvious hardware defect is yet identified; AGA has not been run recently or has not been run since the last component replacement.

Auto Gradation Adjustment (AGA) is the machine’s primary image density self-calibration routine. It prints a multi-step gradation target page, scans the output, and recalculates the tone reproduction curve (TRC) for each color channel to compensate for component aging, environmental drift, and toner characteristic changes. When the K channel TRC drifts due to developer aging, drum surface wear, or ambient humidity changes, the machine underestimates the toner required for maximum black density. AGA recalibrates the TRC and restores correct density. This is not a workaround—it is the designed compensation mechanism for gradual component aging.

Load A4 or Letter paper (standard 80 g/m²) in the primary paper tray. AGA requires paper in the tray to print the calibration target.
Access Service Mode → Adjust → Image Quality → Auto Gradation Adjustment → Full Adjustment → Execute.
Allow the full AGA cycle to complete. Do not interrupt. The machine will print a gradation target page, pause briefly to scan it, and then apply the computed TRC corrections. This cycle takes approximately 2–5 minutes depending on the model.
After AGA completion, print a fresh 100% K solid fill test page and compare its density against the pre-AGA output. If density has recovered to a satisfactory level, document the repair (AGA recalibration) and return the machine to service. No component replacement is required.
If AGA does not improve density, or if AGA applies only a small correction but density remains unacceptable, the underlying hardware is outside the compensation range of AGA. Proceed to Section 6.2.

NOTICE: AGA must always be run after any drum unit, developer unit, fusing unit, or laser unit replacement. Returning a machine to service after component replacement without running AGA will result in incorrect density output and generates unnecessary callback visits. AGA is mandatory, not optional, after imaging path component changes.

6.2 Developer Unit (DV-xxx K) Toner Concentration and Aging

Applies when: Global density reduction across the full page; AGA did not fully correct the density; developer counter shows high usage (above 70% of rated interval); or faded density is accompanied by slight background fogging in light areas of the same print.

The developer unit (DV-xxx K) contains the two-component developer mixture (negatively charged toner particles and ferrite carrier beads), the developer roller (magnetic roller sleeve), the doctor blade, and the Toner Concentration Sensor (TCS). The TCS monitors the toner-to-carrier ratio in the developer mixture and signals the PRCB to activate the toner supply motor (M11 for K) when toner concentration falls below the target level.

As the developer unit ages, carrier beads lose their ability to impart sufficient triboelectric charge to toner particles. Under-charged toner adheres less selectively to the latent image, reducing development efficiency and producing globally lower black density. Additionally, a failing TCS may misread the toner concentration and either under-supply toner (reducing density) or over-supply toner (causing background fogging). A doctor blade that has degraded in flatness or compliance delivers a non-uniform toner layer to the drum, contributing to both density reduction and mottle.

Access Service Mode → Machine → Developer → Toner Concentration (K). Read the current TCS output value. Compare against the nominal target value and the acceptable range listed in the service manual for the model. A TCS value significantly below the target indicates genuine toner depletion (low toner concentration). A TCS value above the nominal range indicates that toner concentration is high but density is still low, which confirms that the carrier beads have lost their charge-imparting capacity (carrier aging) and the developer unit must be replaced regardless of toner level.
Power off the machine. Remove the K drum unit (DR-xxx K) and set it aside on a covered surface, shielded from room light.
Slide the K developer unit (DV-xxx K) out horizontally from its station, keeping it level to prevent carrier bead spillage.
Inspect the developer roller sleeve surface under oblique flashlight illumination for: longitudinal scoring, embedded carrier beads, toner clumping, or visible doctor blade damage (bending, separation, or packed toner under the blade edge).
Slowly rotate the developer roller by the drive gear. Observe whether the developer layer visible on the sleeve surface appears uniform in thickness and color (a consistent dark grey) or shows zones of variation (lighter patches indicating depleted toner concentration, or darker bands indicating toner clumping).
If carrier aging is confirmed by TCS reading, or if physical damage is found on the developer roller or doctor blade, or if the developer counter is at or beyond the rated service interval:
Replace the developer unit (DV-xxx K). After installing the replacement, perform Developer Unit Initialization: Service Mode → Machine → Developer → Initialize → K → Execute. Allow the full initialization cycle to complete (approximately 3–5 minutes). Do not print until initialization is complete.
After developer unit initialization, run AGA (Section 7, Step 3) and print a 100% K solid fill test page. Evaluate density. If density has recovered, proceed to Section 7 for the full post-repair calibration sequence. If density remains insufficient, proceed to Section 6.3.

NOTICE: Never operate the machine without running Developer Unit Initialization after a DV-xxx replacement. Operating the machine before initialization causes the TCS to apply incorrect toner supply commands, which will either starve or flood the new developer unit within the first few hundred prints, causing immediate re-failure or heavy toner contamination inside the machine.

6.3 OPC Drum Unit (DR-xxx K) Coating Fatigue and Surface Filming

Applies when: Global or zonal density reduction that is not corrected by developer unit replacement; drum counter is above 60% of rated interval; faded density may be accompanied by a slightly matte or veiled surface appearance on the printed black fill; or density reduction is accompanied by light ghosting of previous images in the black fill area.

The OPC (Organic Photoconductor) drum surface must achieve maximum charge potential when charged by the PCR and discharge to near zero when exposed by the laser. As the drum coating ages, both of these properties degrade simultaneously: the charge acceptance (how much charge the drum surface will hold) decreases, and the residual voltage after laser exposure increases. The result is a narrower contrast ratio between the charged (non-image) and discharged (image) zones, which reduces the quantity of toner attracted to the image area and produces lower black density.

Toner filming—a thin, uniform deposit of toner that the cleaning blade cannot remove, typically caused by under-charged toner from an aging developer or by a cleaning blade operating at the limits of its service life—creates an additional dielectric layer on the drum surface that further reduces charge acceptance and development efficiency.

Power off the machine. Remove the K drum unit (DR-xxx K). Shield it from room light immediately.
Using oblique flashlight illumination, rotate the drum slowly through at least two full revolutions by the end flanges. Inspect the drum surface for:
- Overall dulling or uniform loss of surface gloss compared to a new drum reference (coating fatigue)
- Toner filming: a thin, continuous brownish or dark grey coating that does not wipe off with a single dry cloth pass and gives the drum a slightly opaque appearance
- Cleaning blade polishing marks: fine lateral tracks across the drum surface indicating the cleaning blade is operating at the limits of its effective life
- Physical scratches, pits, or abrasion marks (these produce lines, not global fading, but may coexist with coating fatigue)
Attempt a gentle wipe of one small zone of the drum surface with a dry, clean lint-free cloth using a single pass. If a thin toner deposit lifts off, toner filming is present and the drum and cleaning blade are both at or past service life.
If coating fatigue, toner filming, or cleaning blade polishing marks are confirmed:
Replace the K drum unit (DR-xxx K). Before installing the new drum unit, inspect the drum station slot:
- Clean the guide rails with a dry lint-free cloth.
- Check the drum grounding contact spring (leaf spring or pin at the rear of the drum station slot) for bending, corrosion, or breakage. A degraded grounding contact prevents the drum substrate from establishing a stable electrical ground reference, which impairs charge uniformity and reduces density. Clean or replace the grounding contact as needed.
- Check the PCR bias contact (the contact that delivers the PCR charging voltage from the HVPS) for corrosion or poor seating. Clean with an IPA-dampened swab and allow to dry.
Install the new drum unit. Run Developer Unit Initialization (if the developer unit was also replaced in Step 6.2; skip if developer unit was previously confirmed good), then run AGA, and print a 100% K solid fill test page. If density has recovered, proceed to Section 7. If density remains insufficient, proceed to Section 6.4.

6.4 Primary Charge Roller (PCR) Wear and HVPS Charge Bias Deficiency

Applies when: Drum and developer replacement did not restore density; global fading is uniform across the full page width and length; fading may be accompanied by uniform background fogging (indicative of insufficient charge potential, which reduces the contrast between image and non-image areas); or Service Mode HVPS charge bias reading is below specification.

The PCR applies the initial negative charge to the drum surface before laser exposure. If the PCR surface is worn (reduced contact resistance), contaminated, or if the HVPS charge bias output voltage is below the nominal specification, the drum surface will not reach its designed maximum charge potential. This reduces the charge-to-development contrast and directly limits achievable black density on solid fills.

On most current bizhub models, the PCR is integrated within the drum unit (DR-xxx K) and is replaced as part of the drum unit. If the drum unit has already been replaced in Section 6.3 and global fading persists, the PCR is not the issue (since it was replaced with the drum unit). Proceed to HVPS charge bias verification below.
HVPS Charge Bias Voltage Verification:
With the machine powered on and in standby mode, access Service Mode → Machine → Input Check / Output Check → HVPS (model-dependent path). Check for any reported HVPS fault flags or out-of-range voltage indications for the K charge bias output.
Using a digital multimeter with insulated probes, measure the HVPS charge bias output voltage at the PCR bias contact test point specified in the service manual for the model. Compare the measured value against the nominal specification (typically −600 V to −900 V DC depending on model and process speed). A reading more than 10% below the nominal specification confirms an HVPS charge output deficiency.
Before condemning the HVPS board, inspect and clean the PCR bias contact path:
- Locate the PCR bias contact spring or pin in the drum station slot (the electrical contact that mates with the PCR conductor on the drum unit end plate).
- Clean the contact surface with an IPA-dampened swab. Inspect for corrosion, bending, or fatigue that would cause high contact resistance.
- Inspect the HVPS harness connector at the HVPS board and at the machine frame contact block for corrosion, bent pins, or loose seating. Reseat all connectors in the charge bias circuit.
Re-measure the charge bias voltage after cleaning and reseating all contacts. If the voltage is now within specification, the contact path was the cause. Run AGA and print a test page.
If the charge bias voltage remains below specification after contact cleaning and reseating:
Replace the HVPS board (HVPS-1 or model-equivalent). On models where the charge bias is generated by a circuit integrated into the PRCB rather than a separate HVPS board, consult the service manual to confirm the board architecture. After HVPS replacement, re-measure the output voltage to confirm correct output before reassembling.

WARNING: HVPS charge bias outputs operate at −600 V to −900 V DC. Developer bias and transfer bias circuits operate at −200 V to −600 V DC. Always use insulated probes rated for the measurement voltage. Never probe HVPS test points with bare hands. Never short HVPS output contacts to machine frame ground or to each other. Discharge any capacitors in the HVPS section by leaving the machine powered off for a minimum of 60 seconds before accessing the HVPS board.

6.5 Laser Exposure System: Reduced Laser Power

Applies when: Global density reduction is present in print mode; drum and developer units are confirmed good or recently replaced; density reduction is uniform across the page width (not zonal); laser power correction values in Service Mode are elevated, indicating the PRCB is compensating for reduced diode output.

The laser diode(s) in the LU-xxx unit degrade gradually over their service life. As output power decreases, the laser can no longer fully discharge the drum surface to the target residual voltage in the image area. The result is a higher residual charge in the exposed (image) zone, which repels toner and reduces the developed toner mass on the drum. In solid black fills—where the laser must continuously discharge the full drum width for the entire page length—even a marginal reduction in laser output power is directly visible as reduced density.

Access Service Mode → Machine → Laser (or equivalent path for the model). Check the laser power correction value for the K beam (on multi-beam models, check all K beam channels). On most bizhub models, the laser power correction is expressed as a percentage or as a correction step count. A correction value at or near the maximum of its compensation range (typically indicated by a warning flag or a correction value above 80% of the maximum) confirms that the PRCB has been progressively increasing laser power to compensate for diode aging, and the diode is near the end of its compensation window.
Inspect the laser window(s) for the K station (the narrow transparent strip at the base of the laser unit housing, through which the laser beam exits toward the drum surface). Using oblique flashlight illumination, look for: toner dust deposits, paper fiber strands, or smears. Clean the laser window with a lint-free cloth lightly dampened with IPA, followed by a dry cloth pass.
After cleaning the laser window, print a 100% K solid fill test page. If density has improved, laser window contamination was the cause. Run AGA and return the machine to service.
If the laser window is clean and laser power correction is at or near the compensation limit:
Replace the laser unit (LU-xxx). The laser diode is not a field-replaceable sub-component on LU-302, LU-204, or LU-206 units; the complete laser unit assembly must be replaced.
After laser unit replacement, perform the following calibrations in order: (a) Auto Color Registration / Laser Beam Alignment if required by the model (Service Mode → Adjust → Machine → Laser Registration), (b) Auto Color Registration (ACR) on color models, (c) Auto Gradation Adjustment (AGA). Do not print production output before completing all required calibrations.

6.6 Fusing Unit Deficiency

Applies when: Black fill area appears physically rough, matte, or granular rather than smooth; toner in the black fill flakes or smears slightly when the paper is flexed; defect sub-type is mottled / speckled fading; density is acceptable on standard 80 g/m² paper but severely faded on heavy-weight or coated paper; or fading appears only on the trailing edge of the black fill area (the last portion of the fill to exit the fusing nip).

Solid black fills are the highest toner-mass-per-unit-area output mode that a bizhub engine produces. This maximum toner mass imposes the greatest thermal demand on the fusing unit. If the fusing roller temperature, nip pressure, or nip dwell time (determined by paper speed through the nip) is insufficient for the toner mass and paper type, the toner will not fully melt, coalesce, and penetrate the paper fiber. The resulting under-fused toner appears as a faded, matte, or mottled black background because the partially fused toner particles scatter incident light rather than absorbing it as a fully fused, dense black toner layer does.

Verify the fusing temperature:
Access Service Mode → Machine → Status → Fusing Temperature (or equivalent). Observe the current fusing roller temperature reported by thermistor TH1 (main thermistor, center of fusing roller) and TH2 (sub-thermistor, edge zone or pressure roller, model-dependent). Compare against the nominal operating temperature range listed in the service manual for the current paper type setting. A TH1 reading below the nominal set temperature by more than 5°C–10°C indicates a fusing temperature control fault. A significant discrepancy between TH1 and TH2 readings indicates a zonal temperature imbalance in the fusing roller, consistent with the zonal fading sub-type.
Inspect TH1 and TH2 connector seating:
Power off the machine (minimum 30-minute cool-down). Open the fusing unit access panel. Locate the thermistor connector harness (TH1 connects to the PRCB via a 2-pin or 3-pin connector; refer to the model service manual for exact connector designation). Disconnect and reseat both thermistor connectors. Inspect the connector pins for corrosion, bending, or pushed-back contacts. A high-resistance thermistor connector causes the PRCB to read a false elevated temperature, which causes the fusing heater controller to under-drive the fusing heater lamp and produce genuinely reduced fusing temperature.
Confirm the paper type media setting:
Access the machine’s media type setting for the paper tray being used. Confirm that the media type (Plain, Heavy 1, Heavy 2, Coated, etc.) matches the actual paper stock installed in the tray. An incorrect media type setting causes the machine to apply the fusing temperature and transfer bias optimized for a lighter paper when actually printing on a heavy or coated stock, producing under-fusing and faded density on the heavier stock.
If thermistor connectors are confirmed good and media type is set correctly but under-fusing persists:
Measure the thermistor resistance at the thermistor connector with a multimeter (with the machine powered off and fully cooled). Compare the measured resistance against the specification in the service manual at room temperature (typically 50–200 kΩ at 25°C, model-dependent). A thermistor reading out of specification at room temperature confirms thermistor failure. Replace the fusing unit thermistor(s) (available as separate service parts on most bizhub models) or replace the full fusing unit assembly.
Inspect the fusing roller and pressure roller surfaces:
With the fusing unit removed and cooled, inspect the fusing roller (upper heat roller) surface for: coating separation, glazing, cracking, or permanent compression deformation. Inspect the pressure roller for: hardening, surface cracking, or flat spots. A fusing roller with a compromised PFA coating delivers inconsistent thermal contact with the toner, producing an uneven fusing temperature across the paper surface that manifests as mottled fading in solid black fills. If surface damage is confirmed, replace the fusing unit assembly.
Inspect the fusing nip pressure:
Verify the fusing nip pressure by performing a nip width check: insert a strip of paper of known thickness into the fusing nip (with the machine powered off and cooled), close the nip by manually moving the pressure roller cam to the engaged position, then withdraw the paper strip. The nip impression on the paper strip should be uniform in width across its full length (from left edge to right edge of the paper path). A narrower nip width on one side indicates uneven pressure roller spring tension or a deformed pressure roller. If nip pressure is uneven, replace the fusing unit assembly or the pressure roller spring set as directed by the service manual.

6.7 Secondary Transfer System Optimization for Heavy Toner Loads

Applies when: Faded or mottled black background appears specifically on heavy-weight paper (above 120 g/m²), coated paper, or textured paper stocks; the same print on standard 80 g/m² office paper shows correct density; or mottled white voids appear within the black fill area (random, not patterned).

On color bizhub models, the secondary transfer nip (the 2nd BTR pressing against the ITB) transfers the full composite toner image from the ITB surface to the paper. Heavy toner loads in solid black fills create a relatively thick toner layer on the ITB that must transfer completely to the paper in a single pass. Heavier or coated paper stocks have lower surface tack and higher rigidity, which reduces intimate contact between the paper surface and the toner layer at the transfer nip. This causes partial or incomplete toner transfer, resulting in white voids or reduced density in the black fill area.

Check and adjust the secondary transfer bias for the current paper type:
Access Service Mode → Adjust → Transfer → Secondary Transfer Bias (terminology varies by model). The secondary transfer bias is adjustable per media type. Increasing the secondary transfer bias by 1–2 steps for heavy or coated paper stocks improves transfer efficiency for heavy toner loads. Make one adjustment step at a time and print a 100% K test page after each adjustment before making further changes. Do not increase the bias beyond the range specified in the service manual; excessive bias causes charge reversal at the toner layer and worsens transfer.
Inspect and clean the 2nd BTR (Secondary Transfer Roller):
Power off the machine. Access the 2nd BTR. Inspect its surface for: embedded toner, paper fiber deposits, surface crazing, or hardening of the foam or rubber surface. Wipe the 2nd BTR surface with a dry lint-free cloth. If contamination is embedded in the roller surface or if surface hardening or crazing is present, replace the 2nd BTR. A hardened 2nd BTR fails to conform to the paper surface contour at the nip, reducing transfer efficiency at the edges and in the interior of the paper sheet.
Inspect the 2nd BTR bias contact:
Check the electrical contact that delivers the secondary transfer bias from the HVPS to the 2nd BTR roller. Clean the contact with a dry lint-free cloth or IPA-dampened swab. A corroded or poorly seated 2nd BTR bias contact reduces the effective transfer field and directly reduces toner transfer efficiency.
After any secondary transfer adjustment or 2nd BTR replacement, run AGA and print 100% K solid fill test pages on both standard and the problematic heavy or coated stock. Confirm that density is acceptable on all paper types used in the customer environment before returning the machine to service.

6.8 Toner Supply System: Toner Motor (M11) and Hopper

Applies when: Fading is intermittent; density varies between consecutive pages in a long run; fading appears or worsens after extended print runs of high-coverage content; toner bottle is confirmed not empty but toner supply appears insufficient; or Service Mode toner concentration reading for K is persistently below the target value even after toner bottle replacement.

The toner supply motor (M11 for the K station on most bizhub models, M29 or model-equivalent on smaller models) drives the toner supply auger that conveys toner from the toner bottle receiver to the developer unit inlet. A failing M11 motor, a worn auger conveyor, or a toner bottle receiver flap that sticks intermittently causes irregular toner supply to the developer unit. This produces intermittent toner concentration drops in the developer, which present as intermittent or run-dependent reductions in black density.

Access Service Mode → Machine → Output Check → Toner Supply Motor (M11 or model-equivalent designation for K). Execute a manual drive test for M11. Confirm that the motor energizes and runs smoothly. Listen for: intermittent running (stuttering), abnormal noise (grinding or rattling), or failure to start. A motor that fails the manual drive test or shows abnormal running noise must be replaced.
If M11 runs normally in the drive test but toner concentration remains low during printing, inspect the toner supply auger. Remove the toner bottle receiver assembly as described in the model service manual. Inspect the toner supply auger (the helical screw conveyor in the supply path) for: worn or broken auger flights, a cracked auger shaft, or toner compaction in the auger channel that blocks toner flow. Replace the toner supply auger or the complete toner supply assembly if damage or blockage is confirmed.
Inspect the toner bottle receiver seals and the toner bottle cap interface for proper seating and toner flow. A partially blocked bottle inlet reduces toner flow to the auger even when the motor and auger are functioning correctly.
After motor or auger replacement, run Developer Unit Initialization for K (Service Mode → Machine → Developer → Initialize → K → Execute) to re-establish the TCS baseline with the corrected toner supply. Then run AGA and print a 100% K solid fill test page.

6.9 Copy Mode Density Deficiency: Scanner Exposure and Image Processing

Applies when: Faded black background is present in copy mode output; machine-internal test prints (print mode) show correct black density; the defect is consistent with the scanner failing to detect that the original contains a dense black area.

When a document with a solid black background is placed on the scanner, the scanner must correctly measure the near-zero reflectance of the black area and produce image data at the maximum density signal level. If the scanner illumination is weak (aging fluorescent lamp or degraded LED bar), if the white reference plate is contaminated, or if the copy density / exposure setting has been incorrectly adjusted, the scanner will underestimate the black density of the original and produce copy output that is globally lighter than the original.

Check the copy density setting:
Confirm that the copy exposure mode is set to Auto (the machine measures original density and adjusts) or to the Darker manual setting if the original is a high-density document. Access the machine panel → Copy → Exposure / Density and verify the setting. An accidentally applied lighter exposure setting is a common user-induced cause of this symptom.
Clean the scanner white reference plate:
Open the ADF and scanner cover. Locate the white reference strip on the underside of the scanner cover or ADF. Wipe the white reference plate with a clean, dry lint-free cloth. A contaminated white reference plate causes the scanner shading correction to calculate an artificially low maximum white reference, which compresses the density range of scanned image data and reduces the apparent density of dark originals in the copy output.
Run Scanner Auto Shading: Service Mode → Adjust → Scanner → Auto Shading (or equivalent for the model). This resets the shading correction profile against the cleaned white reference plate. After completion, make a test copy of a solid black original and compare density.
Adjust the copy gamma / density correction for the K channel:
If density remains insufficient after shading correction, adjust Service Mode → Adjust → Copy → Image Density (K) by increasing the density setting one step at a time and making a test copy after each step. Do not adjust more than 2–3 steps from the factory default without verifying that lighter-toned originals are not over-darkened by the adjustment.
If density correction does not resolve the copy-mode defect:
Suspect scanner lamp / LED bar aging. On models with a replaceable fluorescent exposure lamp, check the lamp for dark end zones (indicating end-of-life). On LED illumination models, the LED bar does not show visual aging symptoms; measure the scanned output signal level using the scanner diagnostic function in Service Mode if available. Replace the scanner lamp or LED illumination bar as indicated.

7. Post-Repair Calibration Sequence

After any component replacement or adjustment in the imaging, transfer, or fusing path, perform the following calibrations in the exact order listed. For faded black background defects specifically, the Auto Gradation Adjustment is the most critical step: skipping or performing it out of order will result in residual density deficit that may be mistaken for an unresolved hardware defect.

Developer Unit Initialization (required if DV-xxx K was replaced):
Service Mode → Machine → Developer → Initialize → K → Execute.
Wait for the full initialization cycle to complete (approximately 3–5 minutes). Do not interrupt. On color machines replacing all four developer units, initialize each color station individually in the order K → C → M → Y (or follow the sequence specified in the service manual).
Auto Color Registration (ACR) (required on color models if drum unit, ITB, or laser unit was replaced or repositioned):
Service Mode → Adjust → Image Quality → Auto Color Registration → Execute.
Verify result values are within ±0.3 mm for C, M, and Y planes in both scan directions. Repeat ACR if values are outside this range. If large errors persist, inspect the ITB registration sensor (PS59 or equivalent) before proceeding.
Auto Gradation Adjustment — Full Adjustment (mandatory after any imaging path, fusing unit, or developer unit change):
Service Mode → Adjust → Image Quality → Auto Gradation Adjustment → Full Adjustment → Execute.
Ensure standard 80 g/m² paper is loaded in the designated calibration tray. Allow the full cycle to complete without interruption. After AGA completion, verify K channel TRC correction by checking the AGA result log (model-dependent; Service Mode → Adjust → Image Quality → AGA → Last Result). A K correction value near the maximum of the correction range indicates the K channel was significantly out of target; this is normal immediately after a drum or developer replacement and confirms that AGA was necessary and has been successfully applied.
Image Density Verification Print:
Print a 100% K solid fill test page (full page) and evaluate the output under strong, consistent lighting. The black fill area should be dense, smooth, and uniform. No white voids, speckles, mottling, or grey zones should be visible. The fill should appear equally dense from the leading edge to the trailing edge of the page and from left edge to right edge.
Multi-stock verification (required if the customer uses more than one paper type):
Load each paper type used in the customer environment into the appropriate tray. Print a 100% K solid fill test page from each tray. Verify that black density is consistent and acceptable across all paper stocks. If density is acceptable on standard paper but faded on heavy or coated stock, adjust the secondary transfer bias for the heavy/coated media type as described in Section 6.7, Steps 1–4, and re-verify.

8. Escalation Criteria

Escalate to Level 2 support or escalate to unit replacement discussion if any of the following conditions apply after completing all procedures in this article:

Global black density reduction persists after confirmed replacement of both the K drum unit (DR-xxx K) and K developer unit (DV-xxx K), with a confirmed clean laser window and a successful AGA cycle. This pattern indicates an HVPS fault or PRCB fault affecting the charge bias, developer bias, or laser power control circuit for the K station.
HVPS charge bias or developer bias voltage is confirmed below specification on multiple channels simultaneously after HVPS board replacement, suggesting a PRCB output fault driving the HVPS reference voltages.
Fusing unit replacement does not resolve a mottled or under-fused black background defect, and nip pressure, thermistor readings, and roller surface condition are all confirmed normal. This pattern may indicate a paper path speed anomaly affecting dwell time in the fusing nip, requiring mechanical drive train inspection.
The laser unit (LU-xxx) has been replaced but laser power correction values return to the compensation limit within fewer than 50,000 impressions of the new unit, suggesting an HVPS laser power supply output instability causing premature diode stress.
The machine produces correct density on individual test prints but not during production runs, suggesting an intermittent high-voltage breakdown or a thermal condition in the process unit that cannot be reproduced in standard test mode.
Physical damage to the drum station guide rails, developer unit lock mechanism, or fusing unit nip pressure cam system prevents correct component seating or engagement.
The PRCB, MFPB, BASEB, or CPUB is suspected based on failure of multiple independent subsystems to resolve the defect after confirmed-good component replacements.

9. Technician Summary

Faded black backgrounds are a high-visibility defect that customers notice immediately and report urgently. The diagnostic approach must be systematic and efficient: begin with the lowest-cost and highest-probability interventions (toner level check, AGA calibration) before moving to component replacement. A large proportion of faded black background service calls in the field are resolved by AGA recalibration alone, with no component replacement required. Always run AGA before and after any component replacement in the imaging path.

The most common root causes, in approximate order of field frequency, are: (1) toner bottle near-empty or empty, (2) Auto Gradation Adjustment not run or out of date, (3) developer unit carrier aging (DV-xxx K), (4) OPC drum coating fatigue (DR-xxx K), (5) fusing unit deficiency on heavy or coated paper stocks, (6) laser window contamination, and (7) HVPS charge or developer bias deficiency. Scanner exposure calibration drift is the cause when the defect is isolated to copy mode only.

For faded density defects that are media-type-specific (correct density on standard paper, faded on heavy or coated stock), always address secondary transfer bias optimization and media type settings before replacing hardware components. These adjustments resolve media-type-specific density issues without component cost and are frequently overlooked by technicians who focus immediately on hardware replacement.

The fusing unit is a frequently overlooked cause of faded black backgrounds. A mottled or speckled fading pattern—distinct from the uniform fading caused by developer aging—is almost always fusing-related. The technician must rub the print to confirm whether toner is fully fused (does not smear) or under-fused (smears or flakes). This single tactile test is the fastest way to distinguish a fusing-path defect from an imaging-path defect and directs the repair to the correct subsystem.