DISCLAIMER: This information is provided for educational purposes only. While we strive for accuracy, we cannot guarantee all information is correct or complete. Always consult official Akai documentation when available. Proceed at your own risk.
Note: The Akai GX-635D is a professional-grade reel-to-reel deck famous for its glass & crystal ferrite (GX) heads. Capacitor replacement should be performed carefully to preserve its exceptional audio quality.
Critical Capacitors to Replace
Power Supply Section
- Main filter capacitors - C1, C2 (large electrolytic cans)
- Voltage regulator capacitors - C3, C4, C5
- Rectifier circuit smoothing capacitors - C6, C7, C8
- DC protection circuit capacitors - C9, C10
Audio Signal Path
- Input coupling capacitors - C101-C104 (Record amp), C201-C204 (Playback amp)
- Equalization circuit capacitors - C105-C108 (Record), C205-C208 (Playback)
- Bias oscillator capacitors - C301, C302
- Output coupling capacitors - C209-C212
- Dolby noise reduction circuit capacitors - C401-C408
Motor Control Circuits
- Capstan motor run capacitors - C501, C502
- Reel motor capacitors - C503, C504
- Speed control timing capacitors - C505, C506
- Braking circuit capacitors - C507, C508
Capacitor Replacement Reference Table
| Location/Board |
Capacitor Codes |
Type |
Recommended Replacement |
| Power Supply Board |
C1-C10 |
Electrolytic |
Low-ESR, 105°C rated |
| Record Amplifier |
C101-C120 |
ElectrolyticFilm |
Audio-grade for signal path |
| Playback Amplifier |
C201-C220 |
ElectrolyticFilm |
Audio-grade, precise values |
| Bias/Oscillator Circuit |
C301-C310 |
FilmCeramic |
High-voltage, stable values |
| Dolby NR Circuit |
C401-C410 |
TantalumCeramic |
Precise values, low leakage |
| Motor Control Board |
C501-C510 |
ElectrolyticCeramic |
Stable temperature characteristics |
Common Symptoms of Failing Capacitors
- Hum or buzz in audio output (especially 60/120Hz)
- Weak, thin, or distorted sound
- Inconsistent tape speed or wow/flutter
- Motor not starting or running erratically
- Functions not working properly (play, record, rewind)
- Complete failure to power on
- Bias oscillator not functioning (affects recording)
- Dolby NR not working correctly
Warning: Always discharge large capacitors before working on the unit. The power supply section can hold dangerous voltages even when unplugged. The GX-635D contains several large capacitors that can retain charge. Use appropriate safety equipment.
Replacement Recommendations
- Use high-quality replacement capacitors from reputable brands (Nichicon, Panasonic, Elna)
- For electrolytics, use 105°C rated capacitors for longer life
- In audio signal paths, consider using film capacitors instead of electrolytics where possible
- Match or slightly exceed the original voltage rating
- Note polarity when replacing polarized capacitors
- For the bias circuit, use capacitors with stable values over temperature
- For Dolby circuits, use capacitors with precise values and low leakage
Board-Specific Capacitor Locations
Power Supply Board
- C1, C2: Large main filter capacitors (usually 2200-3300μF/50V)
- C3, C4: Voltage regulator filters
- C5, C6: Secondary filter capacitors
- C7-C10: Regulation/filter capacitors for various voltage rails
Record/Playback Amplifier Board
- C101-C104: Input coupling capacitors (Record section)
- C105-C108: EQ network capacitors (Record section)
- C201-C204: Input coupling capacitors (Playback section)
- C205-C208: EQ network capacitors (Playback section)
- C209-C212: Output coupling capacitors
Bias/Oscillator Board
- C301, C302: Main oscillation capacitors
- C303-C306: Bias adjustment and filtering capacitors
- C307-C310: Erase oscillator capacitors
Dolby Noise Reduction Board
- C401-C404: Coupling capacitors in encode circuit
- C405-C408: Coupling capacitors in decode circuit
- C409, C410: Power supply filtering
Motor Control Board
- C501, C502: Capstan motor run capacitors
- C503, C504: Reel motor capacitors
- C505, C506: Speed control timing capacitors
- C507, C508: Braking circuit capacitors
- C509, C510: Logic control capacitors
General Maintenance Tips
- Clean all switches and potentiometers with contact cleaner
- Lubricate moving parts with appropriate lubricants (not on brakes or belts)
- Check and replace worn belts and tires
- Demagnetize the tape heads regularly (Akai GX heads require careful handling)
- Adjust tape head alignment after any significant maintenance
- Check and adjust bias current after capacitor replacement
- Clean capstan and pinch roller with isopropyl alcohol
- Check reel motor torque settings
Service Manual: For precise adjustments, obtain the Akai GX-635D service manual which contains detailed schematics, alignment procedures, and calibration specifications. The GX-635D is a complex machine that requires proper calibration after capacitor replacement.
