Preamp Stage Failures#

Preamp failures are less common than power supply or output stage failures, but can cause complete loss of signal or distortion.

Failure Mode: No ±14V Rails#

Symptoms#

• No audio output (all channels)
• LED may still light
• Power supply otherwise functional

Cause#

The ±14V regulators (Q801/Q802) have failed.

Diagnosis#

1. Measure +14V and -14V rails
2. If missing, check:
   • Q801 (2SC3421) for +14V
   • Q802 (2SA1358) for -14V
3. Test transistors as described in DC/DC Failures

Repair#

Replace Q801 and/or Q802 as needed.

Failure Mode: Op-Amp Failure (IC501-IC518)#

Symptoms#

• One or more channels dead
• Distortion on specific channel
• No signal at preamp output

Diagnosis#

1. Verify ±14V present at op-amp power pins
2. Signal trace through preamp stages
3. Compare voltages to working channel

Op-Amp Specifications (NJM4565M)#

All 18 op-amps are the same part (51E38257S01).

Repair#

Op-amps are surface-mount. Replacement requires:

1. Hot air rework station or fine-tip soldering
2. Verify polarity (pin 1 indicator)
3. Check solder bridges after installation

Failure Mode: Input Protection Diode Failure#

Symptoms#

• No input signal passes
• Input shorted to ground

Components#

• D101/D102, D201/D202, D301/D302, D401/D402 (DAP202/DAN202)
• Located at input stage

Diagnosis#

Check diodes in circuit - should show normal diode readings, not shorted.

Failure Mode: Gain Potentiometer Failure#

Symptoms#

• No sound or very low output on channel pair
• Scratchy sound when adjusting
• Intermittent output

Components#

• VR501, VR502: 10K dual pots (CH1/2)
• VR503, VR504: 50K quad pots (CH3/4)

Diagnosis#

1. Measure resistance across pot (should match rated value)
2. Check for open or intermittent wiper
3. Clean with contact cleaner or replace

Failure Mode: Coupling Capacitor Failure#

Symptoms#

• DC offset at output
• Loss of low frequencies
• Intermittent sound

Common Coupling Capacitors#

• C151/C251/C351/C451 (10µF) - Input coupling
• C122/C222/C322/C422 (0.1µF) - Stage coupling

Diagnosis#

Check capacitors for:

• Leakage (should be very high resistance)
• Open circuit (no signal passes)
• Low capacitance (check with capacitance meter)

Signal Tracing Procedure#

To isolate a preamp fault:

1. Apply test signal to RCA input (1kHz sine, ~0.5V)
2. Trace signal at these points:

3. Signal disappears? Fault is between last good point and dead point
4. Signal distorted? Check that stage’s op-amp and associated components