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Marantz Model 2285B Receiver

Years Made & Context

Production Years: 1978 - 1980

The Marantz 2285B was part of the final "B" revision series before the more radical 2500/2600 designs. It represents the evolution of the classic Marantz sound with improved specifications and slightly different circuitry than the non-B models. The "B" series features updated tuner sections and other refinements.

Key Specs: 85W/channel | THD: 0.08% | Weight: 24kg (53lbs) | S/N Ratio: 75dB (phono) | Damping Factor: 60

Current Issues & Common Failures

The 2285B shares many issues with other late-70s Marantz receivers but has some specific "B" series characteristics and known failure points.

⚡ Power Supply Issues

Main Filter Capacitors: The 6800µF or 8200µF main filters often show increased ESR and reduced capacitance.
Voltage Regulator Failures: The "B" series uses different regulator circuits that can develop problems.
Thermal Stress Cracks: Solder joints on the power supply board (P700) crack due to heat cycling.
Power Switch Arcing: The main power switch contacts can pit and cause intermittent operation.

🔋 Capacitor Issues

Phono Stage Electrolytics: Critical for low-noise performance; often degrade significantly.
Coupling Capacitors: Throughout the signal path, particularly C001/C002 on the phono board.
Tone Control Capacitors: The "B" series uses different tone circuits that need attention.
Protection Circuit Caps: Timing capacitors in the protection circuit can fail.

🔊 Amplifier Problems

Bias Drift: Bias adjustment pots R763/R764 become intermittent or dirty.
DC Offset Issues: Can drift high due to aging transistors in the differential pairs.
Protection Relay (RY701): Contacts oxidize, causing channel dropouts or no sound.
Output Transistor Issues: While generally robust, outputs can fail if bias is incorrect.

🎛️ Controls & Physical

Dolby FM Board Issues: The 2285B includes a Dolby FM board that can develop problems.
Scratchy Controls: Volume, balance, bass, and treble pots all accumulate oxide.
Push-button Switches: Tape monitor, loudness, filter switches become intermittent.
Dial Light Failures: The 8V 300mA lamps (typically 4) burn out over time.
Wood Case Condition: Walnut veneer often shows wear and may need refinishing.

⚠️ CRITICAL: The "B" series uses different protection circuitry than earlier models. Never bypass the protection relay without understanding the circuit. Always check DC offset (< 50mV) before connecting speakers.

Repair Solutions & Restoration Approach

⚠️ The 2285B is moderately complex but very serviceable. Intermediate electronics skills are sufficient for most restoration work.

Recommended Restoration Protocol:

Initial Safety Check
- Use a dim-bulb tester for initial power-up
- Measure DC offset at speaker terminals (should be < ±50mV)
- Check for obvious shorts, blown fuses, or burnt components
Power Supply Restoration (P700 Board)
- Replace main filter capacitors (typically 6800µF or 8200µF 63V)
- Replace all other electrolytic capacitors on P700 board
- Re-solder all connections, especially high-current paths
- Test and replace regulator transistors if needed
Amplifier Section (P750 Boards)
- Adjust bias using test points J719/J720 (typically 10mV across emitter resistors)
- Adjust DC offset using R765/R766 for 0mV ±50mV
- Replace coupling capacitors C759/C760 (typically 3.3µF)
- Clean or replace protection relay RY701
- Re-flow all solder joints on amplifier boards
Preamp & Control Section
- Deoxit Treatment: All switches and potentiometers
- Replace all electrolytic capacitors on phono board (P400)
- Replace capacitors on tone board (P500)
- Address Dolby FM board (P900) - often can be left unpowered if not used
- Clean selector switch contacts thoroughly
Tuner Section (B-Series Specific)
- The "B" series tuner is different from earlier models
- Clean tuner gang capacitor plates carefully
- Replace electrolytics on tuner boards
- Check alignment - only adjust with proper equipment and knowledge
Final Testing & Cosmetic
- Replace all dial lights (typically 4x 8V 300mA lamps)
- Clean faceplate with mild cleaner (avoid abrasive materials)
- Refinish wood cabinet if needed
- Perform full function test on all inputs and controls

Capacitors That Need Replacing

Complete capacitor replacement is essential for reliable performance and to restore original sound quality.

Power Supply (P700)

Main Filter Capacitors

6800µF or 8200µF 63V. Replace with high-quality, low-ESR audio capacitors.

Amplifier Boards (P750 L&R)

Critical Signal Path Caps

C759/C760 (3.3µF coupling), C757/C758 (47µF feedback). Use film caps for coupling if possible.

Phono Equalizer (P400)

RIAA Precision Caps

All electrolytics plus C001/C002 (1µF coupling). Film caps recommended for critical positions.

Tone Control (P500)

Bass/Treble Circuits

All electrolytics. These directly affect tone control accuracy and frequency response.

Dolby FM Board (P900)

Optional but Recommended

All electrolytics. Even if Dolby FM isn't used, capacitor failure can affect other circuits.

Protection Circuit

P800 Board

Timing capacitors (often 47µF or 100µF). Failure can cause improper protection operation.

2285B-Specific Restoration Notes

Bias Adjustment: 10mV across emitter resistors R771/R772 (measure at J719/J720 test points)

DC Offset Adjustment: Adjust R765/R766 for 0mV ±50mV at speaker terminals with no signal

Bulb Specifications: Typically 4x 8V 300mA fuse-style lamps for dial illumination

"B" Series Identification: Look for "2285B" on front panel and different circuit boards than non-B models

Dolby FM Consideration: The Dolby board can often be disconnected if problematic, but capacitor replacement is still recommended

Value Note: The 2285B is highly collectible. A professional restoration ($500-$800) can increase value to $1,000-$1,500+ depending on condition. DIY restoration is feasible for those with appropriate skills.