Output Stage Failures#

The output stage handles the highest power levels and is subject to thermal and electrical stress.

Failure Mode: Output Transistors (Q161/Q162, etc.)#

Symptoms#

• Blown fuses
• DC voltage at speaker output
• One channel dead
• Smoke from output stage area
• Distortion on one channel

Root Causes#

Diagnosis#

Test each output transistor (with power off, FETs removed):

Q161/Q261/Q361/Q461 (2SC5100 NPN):

B-E: Red on B, Black on E = 0.5-0.7V
B-C: Red on B, Black on C = 0.5-0.7V
E-C, C-E: OL

Q162/Q262/Q362/Q462 (2SA1908 PNP):

B-E: Red on E, Black on B = 0.5-0.7V
B-C: Red on C, Black on B = 0.5-0.7V
E-C, C-E: OL

Output Transistor Specifications#

Repair Procedure#

1. Test both complementary transistors (Q161 AND Q162)
2. Replace as matched pairs - Even if only one is failed
3. Check driver transistors (Q159/Q160)
4. Verify bias voltage after replacement
5. Check DC offset at output (<50mV)

**Always replace complementary pairs together.** Mismatched transistors cause bias drift and reduced performance.

Failure Mode: Driver Transistors (Q159/Q160, etc.)#

Symptoms#

• Distorted output
• One half of waveform clipped
• Output transistor failure
• Excessive heat in driver area

Driver Transistor Specifications#

Diagnosis#

Same testing procedure as output transistors. Check all junctions.

Failure Mode: DC Offset at Output#

Symptoms#

• Speaker cone pushed in or out at idle
• Speaker runs warm with no audio
• Protection circuit triggers
• Gradual speaker damage

Acceptable Range#

DC offset should be less than 50mV at the speaker terminals.

Causes#

Diagnosis#

1. Measure DC voltage at speaker output (should be <50mV)
2. If high, measure voltages in that channel per Terminal Voltages
3. Compare to a known-good channel
4. Identify which stage has incorrect voltages

Failure Mode: Bias/Thermal Runaway#

Symptoms#

• Output stage runs very hot at idle
• Idle current increases over time
• May eventually smoke or fail

Causes#

• Failed bias transistor (Q158)
• Damaged thermistor
• Incorrect replacement parts

Diagnosis#

1. Measure quiescent (idle) current
2. Normal: ~50-100mA per channel
3. If much higher, bias circuit has a problem

Channel-by-Channel Component Reference#

All four channels are identical. Use this table to find the equivalent component: