(818) 244-4440 as@tekmg.com

Experts on Marantz Model 2250B Receiver Repair

50-watt "B" series workhorse (1978-1980) | Comprehensive restoration guide

➜ Start a Repair Process

Years Made & Context

Production Years: 1978 - 1980

The Marantz 2250B is part of the popular "B" revision series, offering 50 watts per channel of classic Marantz sound. It sits in the middle of the lineup and is highly regarded for its excellent performance-to-size ratio. The "B" series featured updated tuner sections and other circuit refinements over earlier non-B models.

Key Specs: 50W/channel | THD: 0.08% | Weight: 22kg (48lbs) | S/N Ratio: 74dB (phono) | Damping Factor: 60

Current Issues & Common Failures

The 2250B is generally reliable but shares the typical age-related issues of all vintage Marantz receivers. Its moderate power output means it experiences less thermal stress than higher-powered models.

⚡ Power Supply Issues

Main Filter Capacitors: Typically 4700µF or 6800µF main filters that dry out, causing hum and power supply ripple.
Regulator Circuit Problems: Transistors and capacitors in the voltage regulator circuits degrade over time.
Power Switch Arcing: The mechanical power switch contacts can pit and cause intermittent operation.
Cracked Solder Joints: On power supply board (P700) due to thermal cycling.

🔋 Capacitor Issues

Phono Stage Capacitors: Critical for RIAA equalization and low-noise operation.
Coupling Capacitors: Throughout the signal path, particularly between stages.
Tone Control Capacitors: Affect bass and treble response accuracy.
Protection Circuit Timing Caps: Can cause delayed or improper relay operation.

🔊 Amplifier Problems

Bias Drift: Bias adjustment pots (R763/R764) become dirty or intermittent.
DC Offset Issues: Can drift due to aging transistors in the differential input stage.
Protection Relay (RY701): Contacts oxidize, causing channel dropouts.
Thermal Paste Deterioration: Original thermal compound dries out on output transistors.

🎛️ Controls & Physical

Dirty Controls: Volume, balance, bass, treble pots all accumulate oxide.
Push-button Switch Problems: Tape monitor, loudness, filter switches become intermittent.
Dial Light Failures: Typically 4x 8V 300mA fuse-style lamps burn out.
Wood Cabinet Condition: Walnut veneer often needs refinishing or repair.
Selector Switch Issues: Can cause crackling or loss of signal on certain inputs.

⚠️ CRITICAL: Always check DC offset before connecting speakers. Should be less than ±50mV. High DC offset indicates failing coupling capacitors or transistor issues that could damage speakers.

Repair Solutions & Restoration Approach

⚠️ The 2250B is an excellent candidate for DIY restoration due to its straightforward design and good documentation. Basic to intermediate electronics skills are sufficient.

Step-by-Step Restoration Process:

Initial Safety & Assessment
- Use a dim-bulb tester for safe initial power-up
- Measure DC offset at speaker terminals (target: < ±50mV)
- Check fuses and look for signs of previous repairs or damage
Power Supply Restoration (P700 Board)
- Replace main filter capacitors (typically 4700µF or 6800µF 63V)
- Replace all other electrolytic capacitors on the P700 board
- Re-solder all connections, especially transformer and rectifier connections
- Test regulator transistors and diodes
Amplifier Section (P750 Boards)
- Adjust bias (typically 10mV across emitter resistors R771/R772)
- 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
- Replace dried thermal paste on output transistors
Preamp & Control Section
- Deoxit Treatment: All potentiometers and switches
- Replace all electrolytic capacitors on phono board (P400)
- Replace capacitors on tone control board (P500)
- Clean selector switch contacts thoroughly
- Address any Dolby or filter board capacitors if present
Tuner Section (B-Series)
- Clean tuner gang capacitor plates carefully (do not bend!)
- Replace electrolytic capacitors on tuner boards
- Check alignment - only adjust with proper equipment and knowledge
- The "B" series tuner is generally more stable than earlier versions
Final Assembly & Testing
- Replace all dial lights (typically 4x 8V 300mA lamps)
- Clean faceplate and knobs with mild cleaner
- Refinish wood cabinet if needed
- Perform comprehensive function test on all inputs and controls
- Verify power output and distortion characteristics if possible

Capacitors That Need Replacing

Complete capacitor replacement is recommended for optimal performance and long-term reliability.

Power Supply (P700)

Main Filter Capacitors

4700µF or 6800µF 63V. Use high-quality, low-ESR audio capacitors for best performance.

Amplifier Boards (P750 L&R)

Signal Path Capacitors

C759/C760 (3.3µF coupling), C757/C758 (47µF feedback). Film capacitors recommended for coupling positions.

Phono Equalizer (P400)

RIAA Critical Capacitors

All electrolytics plus C001/C002 (typically 1µF input coupling). Use low-leakage types for best performance.

Tone Control (P500)

Frequency Response Caps

All electrolytics. These directly affect the accuracy of bass and treble controls.

Protection Circuit

P800 Board

Timing capacitors (often 47µF or 100µF). Critical for proper relay operation timing.

Control Amplifier

Preamp Section

All electrolytics in the line-level signal path between input selector and power amp.

2250B-Specific Restoration Notes

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

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

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

Identifying B vs Non-B: "2250B" on front panel, different tuner board, circuit refinements

Common Upgrade: Replacing ceramic disc capacitors in phono stage with film types improves sound quality

Thermal Management: Less critical than higher-power models but still replace dried thermal paste

Value Consideration: The 2250B is very popular. A complete restoration ($400-$700) can increase value to $700-$1,200 depending on condition. It's an excellent balance of power, size, and classic Marantz sound.