Power Protection Circuit - Marantz 2270 Manual

transistors that automatically maintains the proper base voltage with temperature change. The
temperature sensitive biasing components of the dynamic circuit are thermally coupled through
a heatsink to the power amplifier transistors.
6. Power Protection Circuit
Protection circuit for the amplifier is provided by sensing resistor networks and two switching
transistors. When the output transistors are over-driven, the current increase through the power
Output transistor causes an increased current flow through R789 (or R788) and the potential
across the R789 will be increased. This increased voltage potential is applied to the base of H755
through the resistor R783 and turns on the H755. Since the collector of H755 is directly
connected to the base of H757, this means that the base of H757 is by-passed to the ground
through emitter-collector path of H755. Thus the input signal to the H757 is restricted to the
value which
maintains the operation of power transistor with in the safety area. A resistor
network R777 and R781 also works as a sensing network. When the center voltage (collector
voltage of power transistors) is excessively increased to a positive value by certain troubles, the
voltage applied to the base of H755 makes the H755 turn on, making bypass circuit, and protects
the power transistor. For the other half cycle of driving signal, the same operating principle is
applied provided.
7. Speaker Protector Relay circuit
The circuit also protects the speaker systems from the possible damage when the amplifier is
over-driven by very low frequencies such as 7 or lower cycles.
8. Suggestions for Trouble Shooting of Power Amplifier
8.1 Excessive line consumption
a. Check for shorted rectifiers HOO5; also check COO7 and C008.
b. Check for shorted transistors H758 and H759, HOO1 and HOO2, or check H760. Check for
open control R763, and bias diode H761. Check L004 for short.
CAUTION: BECAUSE THE DRIVER AND OUTPUT STAGES ARE DIRECT COUPLED COM.
PONENTS MAY FAIL AS A DIRECT RESULT OF AN INITIAL COMPONENT
FAILURE. IF A SHORTED TRANSISTOR OR ZENER DIODE IS FOUND, OR
CONTROL OR BIAS DIODE, BE SURE TO CHECK THE REMAINING DRIVER
AND OUTPUT COMPONENTS FOR SHORT OR OPEN CIRCUIT BEFORE RE.
ENERGIZING THE AMPLIFIER.
8.2 No Line Consumption or Zero Bias
a. Check line cord, fuse, transistors H760, HOO1, HOO2, HOO3 and H004, bias diode H761.
b. Check for open rectifier HOO5, or open L004.