Marantz 2270 - Power Amplifier Protection and Troubleshooting; Power Protection Circuit Explanation; Speaker Protector Relay Function; Power Amplifier Troubleshooting Guidance

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
speaker
protection
circuit
consisting
of
H808,
H809,
H810,
etc
protects
the
speaker
systems
against
any
loud
“‘pop”
sound
developed.
This
circuit
is
so
designed
that
no
sound
is
heard
for
the
first
three
or
five
seconds
after
the
power
switch
is
turned
on
by
the
time
constant
circuit
consisting
of
C807
and
R816.
This
circuit
also
portects
the
speaker
systems
against
some
troubles
due
to DC
off
balance between
the speaker
system
terminals
by
instantly
operating
the
relay
and cut off
the
speaker
systems
from
the circuit.
When
DC
off
balance
voltage
(positive)
is
developed
between
speaker terminals
by
possible
defects
such
as broken
power transistor,
short:
circuits,
or
broken
potentio
meter
R762,
as
the
base of
H808
is
connected
to
the speaker
terminal,
the
transistor
H808
is
turned
on
by this offset
voltage
developed
and
this
makes the
_
transistor
H809 and H810
turns
off,
thus
cutting
off
the
relay and
disconnecting
the
speaker from
the
output
circuit.
When
negative
offset
voltage
is
developed,
this
voltage
directly turns
off
the
H809
and H810, thus
speaker
is
cut off
from
the
circuit and
protected.
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.