Manual solution: Unplug all speaker wires. Turn the unit on. If the red light stays off, one of your speakers has a short. Reconnect carefully. Page reference: Troubleshooting flowchart (Page 36).
There is no official standalone “AVR 51 Manual 134” published by Harman. Instead, the complete manual includes a section or schematic number 134. For the end-user, the Owner’s Manual is the only PDF you need for setup, while the Service Manual (which contains page 134) is for repair technicians. Harman Kardon Avr 51 User Manual 134
Here is the practical knowledge usually found in the first 30 pages of the manual. These are the top reasons people hunt for the PDF. Manual solution: Unplug all speaker wires
Understanding the hardware is the first step to utilizing the manual's instructions effectively. Reconnect carefully
Leave at least 30cm (12 inches) of clearance above and to the sides of the unit to prevent overheating.