Thanks you very much for this "hard to find" service manual.
Will help a lot in repairing this receiver.
Text excerpt from page 7 (click to view)
Power Amplifier
Reference 7520a/7521a
TROUBLESHOOTING
� PROBLEM: No audio (POWER LED is off). CAUSE and SOLUTION: No voltage at BATT+ and/or REM terminals, or bad or no ground connection. Check voltages at amplifier terminals with VOM. � PROBLEM: No audio (PROTECT LED flashes every 4 seconds). CAUSE and SOLUTION: DC voltage on amplifier output. Amplifier may need service; see enclosed warranty card for service information. � PROBLEM: No audio (PROTECT LED is on). CAUSE and SOLUTION: Amplifier is overheated. Make sure amplifier cooling is not blocked at mounting location; verify that speaker-system impedance is within specified limits. � PROBLEM: No audio (PROTECT and POWER LEDs flash). CAUSE and SOLUTION: Voltage less than 9V on BATT+ connection. Check vehicle charging system. � PROBLEM: No audio (PROTECT LED is on). CAUSE and SOLUTION: Voltage greater than 16V or less than 8.5V on BATT+ connection. Check vehicle charging system. � PROBLEM: Distorted audio. CAUSE and SOLUTION: Input sensitivity is not set properly, or amplifier or source unit is defective. Check INPUT LEVEL setting, or check speaker wires for shorts or grounds. � PROBLEM: Distorted audio (PROTECT LED flashes). CAUSE and SOLUTION: Short circuit in speaker or wire. Remove speaker leads one at a time to locate shorted speaker or wire, then repair. � PROBLEM: Music lacks �punch. � CAUSE and SOLUTION: Speakers are not connected properly. Check speaker connections for proper polarity.
STATUS LEDs Clip: Indicates the amplifier is being overdriven, and your speakers may be in danger. This should blink only on musical peaks, and not be on constantly. Power: Indicates the amplifier is on. Protection: Refer to �Troubleshooting� for specific indications. Figure 15. LED status.
