Diagnostic Test Plan#

This test plan was developed for a specific failure case but applies to general DC/DC converter and power supply troubleshooting.

**Fresh Start Approach:** This plan is structured to verify everything from scratch, setting aside previous assumptions. Let the measurements guide the diagnosis.

Symptom Summary#

• Smoked twice (original failure + after FET replacement)
• No visible damage on PCB
• IC920 (PWM controller) verified producing pulses
• With FETs installed: 12V input dropped to 8V, current climbed to 1.2A before smoke
• Output transistors tested OK
• B+ to GND resistance unstable (rises then falls)

Pre-Test Setup#

• Remove Q903, Q904, Q905, Q906 (switching FETs)
• Disconnect any speaker connections
• Disconnect RCA inputs
• Ensure remote turn-on wire is connected (or jump REM to B+)

Phase 0: Fresh Start - Visual & Basic Verification#

**START HERE** - Before any electrical tests, verify the fundamentals with fresh eyes.

0.1 Visual Inspection#

Examine the entire board carefully under good lighting (magnification helps).

0.2 Basic Continuity (Power Off, FETs Removed)#

0.3 Dead Short Check (Power Off, FETs Removed)#

Quick check for obvious shorts before applying power.

Phase 1: PWM Controller Verification#

**RE-VERIFY FROM SCRATCH** - Previous tests indicated IC920 was working, but let's confirm with fresh measurements.

1.1 Unpowered IC920 Checks#

1.2 Powered IC920 Checks (FETs Removed)#

Setup: Bench supply at 14.4V, 200mA current limit, REM jumpered to B+

1.3 PWM Output Verification (Oscilloscope Required)#

Setup: Same as 1.2, probe on IC920 outputs

**Note:** If PWM outputs are absent or abnormal, stop here - the controller or its support components need attention before proceeding.

Phase 2: Gate Driver Transistors#

**HIGHEST PRIORITY** - If damaged, FETs receive improper gate drive and run in linear mode causing thermal runaway.

Q901, Q902 - 2SB1132 (PNP)#

Test Method: IN-CIRCUIT (preliminary), OUT-OF-CIRCUIT (if suspicious)

Location: Near transformer T901, power supply section (page 8, area D4-D5)

Failure Indicators:

• Low resistance in any “OL” test = shorted junction
• No reading in forward tests = open junction
• In-circuit readings may show lower than expected due to parallel components

Part Number: 48E39095S01 (2SB1132)

Phase 3: Secondary Rectifier Diodes#

**HIGH PRIORITY** - Shorted rectifier = dead short on transformer secondary = massive primary current

Test Method: IN-CIRCUIT (preliminary), OUT-OF-CIRCUIT (if suspicious)

D802 - FCH10A15 (Fast Recovery)#

D803 - FRH10A15 (Fast Recovery)#

Location: Near transformer T901, secondary side

D801, D808 - 11EFS2 (Fast Recovery)#

Failure Indicators:

• Reverse direction shows ANY voltage reading = shorted
• Forward shows OL = open
• Forward shows very low (<0.2V) = likely shorted

Phase 4: Transformer T901#

Test Method: IN-CIRCUIT

Location: Large component, center of power supply area

Transformer Ratio: 5:8:1 (Part: 25E39082S01)

Failure Indicators:

• Any winding reads 0.0Ω = shorted turns
• Any winding reads OL = open winding
• Reading between isolated windings ≠ OL = insulation breakdown

Phase 5: Secondary Rail Load Test#

Test Method: IN-CIRCUIT (FETs removed)

Procedure: Measure resistance from each rail point to GND. Readings will drift as capacitors charge - note the final settled value.

Failure Indicators:

• Very low resistance (<10Ω) that doesn’t rise = dead short on that rail
• Resistance rises then falls continuously = leaky component

Phase 6: Voltage Regulator Transistors#

Test Method: IN-CIRCUIT (preliminary)

Q801 - 2SC3421 (NPN) - +14V Regulator#

Q802 - 2SA1358 (PNP) - -14V Regulator#

Location: See page 8, near C806/C807

Phase 7: Control/Protection Circuit Transistors#

Test Method: IN-CIRCUIT (preliminary)

Note: DTC/DTA series have internal bias resistors - readings may vary from standard transistors

Phase 8: Support Components Quick Check#

Test Method: IN-CIRCUIT

Gate Resistors#

Timing Components (IC920)#

Phase 9: B+ Input Path (Detailed)#

Test Method: IN-CIRCUIT

Test Results Summary#

Components Requiring Replacement#

Components Verified Good#

**Note:** Starting fresh - populate this table only with components verified during THIS test sequence.

Power-Up Test Procedure (After Repairs)#

**Only proceed after all above tests pass**

1. Reinstall FETs (Q903-Q906)
2. Set bench supply to 14.4V, 100mA limit
3. Connect and observe:
   • Current draw should be low (<50mA) initially
   • LED should illuminate
4. Slowly raise current limit while monitoring:
   • Voltage should stay at 14.4V
   • Current should remain reasonable (<500mA at idle)
5. If voltage drops or current spikes - STOP IMMEDIATELY
6. Check for heat on any components

Post-Repair Verification#

**Only proceed after Power-Up Test passes** - These tests verify the amplifier is fully functional before connecting to real speakers.

Phase 10: Power Supply Rail Verification#

Setup: Bench supply at 14.4V, 2A limit, no load connected

Idle Current Draw:

Phase 11: Signal Path Verification (No Load)#

Setup:

• Bench supply at 14.4V, 2A limit
• Signal generator: 1kHz sine wave, 200mV RMS
• Oscilloscope on outputs
• No load connected

Input to Output Test:

Gain Control Test: (VR503/VR504)

• Turning gain down reduces output amplitude
• Turning gain up increases output amplitude
• No distortion or oscillation at any gain setting

Filter/Crossover Test: (if applicable)

• HPF switch attenuates low frequencies
• LPF switch attenuates high frequencies

Phase 12: Dummy Load Testing#

**Dummy Load Resistors:** Use non-inductive power resistors. Mount on heatsink or allow air cooling.

Recommended Dummy Loads:

12.1 Low Power Load Test#

Setup:

• Bench supply: 14.4V, 5A limit
• Signal: 1kHz sine, adjust for 1W output (~2V RMS into 4Ω)
• Dummy load: 4Ω per channel

12.2 Medium Power Load Test#

Setup:

• Bench supply: 14.4V, 10A limit (or battery)
• Signal: 1kHz sine, adjust for 10W output (~6.3V RMS into 4Ω)
• Dummy load: 4Ω per channel

12.3 Full Power Test (Use Car Battery or High-Current Supply)#

**Caution:** Full power testing generates significant heat. Monitor continuously. Have fire extinguisher nearby.

Setup:

• Power: Car battery or 14.4V supply capable of 20A+
• Signal: 1kHz sine, adjust for rated power (40W = ~12.6V RMS into 4Ω)
• Dummy load: 4Ω per channel
• Duration: Brief bursts only (5-10 seconds)

Phase 13: Thermal Verification#

Setup: Run at medium power (10W/ch) for 5 minutes with dummy loads

Phase 14: Protection Circuit Verification#

Test each protection feature:

**Short Circuit Test:** Place a 1A fast-blow fuse in series with the short. This protects both the amp and your test setup.

Phase 15: Final Acceptance Checklist#

Sign-off:

• Date: _____________
• Tested by: _____________
• Ready for installation: [ ] Yes [ ] No - needs: _____________