Marantz 2275 - Power Amplifier Protection and Troubleshooting

The temperature sensitive biasing components of the dynamic circuit are thermally coupled to
the heat sink which mount the power transistors.
6. POWER PROTECTION CIRCUIT
A 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 transistors cause an increased current flow through R740. This increased
voltage potential is applied to the base of H708 through resistor R736 and H705 turning H708
on. Since the emitter of H708 is connected through R727 to the base of H713, the base of H713
is by-passed to the common cneter point through the emitter-collector path of H708. Thus, the
input signal to H713 is restricted to the value which maintains the operation of the output stage
within the safe area. Resistors R730 and R729 with H717 works as a sensing network. When the
center voltage (collector voltage of power transistors) is excessively increased to a positive value
by certain malfunctions, the voltage applied to the base of H705 turns H708 on, thus removing
the drive from H713 and the power transistor. For the other half cycle of the driving signal, the
same operating principle is applied as described above, through H706 and H707.
7. SPEAKER PROTECTOR RELAY CIRCUIT
The speaker protection circuit consisting of H805, H806, H807, and associated parts protects
the speaker systems against turn “ON” and “OFF” transients. This circuit is so designed that no
sound is heard for the first three to five seconds after the power switch is turned on by the time
constant circuit consisting of C809 and R813. This circuit also protects the speaker system
against difficulties due to poor DC balance between the speaker system terminals by instantly
operating the relay and cutting off the speaker system from the circuit. When a positive DC off
balance voltage is developed between speaker terminals by possible defects such as defective
power transistors, short-circuits, or a broken potentiometer .R741 protection is instantly
available. Since the base of H805 is connected to the speaker terminal, it is turned on by this
offset voltage developed, turning transistors H806 and H807 off, thus cutting off the relay and
disconnecting the speaker from the output circuit. When a negative offset voltage is developed,
this voltage directly turns H806 and H807 off, giving the same protection as above.
The circuit also protects the speaker system from possible damage when the amplifier is
over-driven by very low frequencies such as 7Hz or less.
8. SUGGESTIONS FOR TROUBLESHOOTING THE POWER AMPLIFIER
8.1 Excessive line consumption
a. Check for shorted rectifier HOO5; also check CO06 and C007.
b. Check for shorted transistors H713 and H714, HOO1, HOO2, HOO3, and HO004, and check
H709. Check bias diode H710. Check L004 for shorts.
CAUTION: BECAUSE THE DRIVER AND OUTPUT STAGES ARE DIRECT COUPLED,
SEVERAL COMPONENTS MAY FAIL AS A DIRECT RESULT OF A SINGLE
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
SHORTS OR OPEN CIRCUITS BEFORE REENERGIZING THE AMPLIFIER.
8.2 No Line Consumption or Zero Bias
a. Check line cord, fuse, transistors H709, HOO1, HOO2, HOO3, and HO04, and bias diode H710.
b. Check for open rectifier HOO5 or open L004.