VSX-74TXVi
242
1
2
3
4
1
2
3
4
C
D
F
A
B
E
7.1.12 FAILURE DIAGNOSIS OF THE POWER AMP BLOCK
If "AMP ERR" flashes on the FL display immediately after the
power is turned on, then the unit is shut down, the power
amplifier block is damaged, and DC voltage is output. In this
case, either or both of the current and voltage amplifier block(s)
is(are) damaged. Herein is described how to diagnose which
block is in failure.
Most cases of damage in the power amplifier block are damage
on the power transistor caused by excess power consumption,
such as short-circuiting of the speaker terminals and use of low-
impedance speakers that are not guaranteed. The failure diagnosis
is therefore conducted on the current amplifier block first, then
on the voltage amplifier block.
Notes:
• During diagnosis of the amplifier block with the power on, if
you touch a high-impedance circuit (in particular, both ends of
100-ohm base resistance of the power transistor) with the probe
of an oscilloscope or a tester, the circuit may be damaged by
oscillation of an oscilloscope or a tester.
• While a failure diagnosis is being conducted with the power on,
do NOT touch parts other than the power, input, and output
blocks.
• Only the points with CP (check point) marks in the schematic
diagram can be diagnosed while the power is on.
1. Damage on the current amplifier block
Check of the power transistor and collector resistance
of each channel, using a tester
Tips:
In most cases where the current amplifier block is damaged by an
excess power consumption of the power transistor, damage is
caused by short-circuiting between the collector and emitter. The
collector resistor is open because of large collector current.
In this case, the channel in failure can be identified by checking
on which channel the resistance between C and E of the power
transistor is 0
Ω
(some hundreds of ohms or less) and the
collector resistance is
∞
Ω
. As a damage scar appears on the
surface of the part when the collector resistance is open, you can
also check it by visual inspection.
Note:
Even if the result of the measurement of resistance with the tester
shows that the power transistor is not short-circuited, that power
transistor may be damaged if the paired push-pull power
transistor has been damaged. Therefore, it is recommended to
replace both parts when either needs to be replaced.
2. Damage on the voltage amplifier block
Check of the base resistance (100
Ω
) and voltage (
±
VH)
resistance (4.7
Ω
) of each channel, using a tester
Tips:
In most cases, the voltage amplifier block is damaged by
oscillation generated by a special load connection, a large-
amplitude output at higher frequencies out of the audible range,
or an influence of damage on the current amplifier block (power
transistors). In many cases, the base resistance (100
Ω
) is
damaged. It is necessary to check each part of the voltage
amplifier block, as damage may have been extended to other
parts.
In most cases, the resistors are open/damaged, or the transistors
or diodes are short-circuited/damaged.
• The resistors must not be open (the resistance must not be
higher than the indicated value).
• The terminals of transistors or diodes must not be short-
circuited (the resistance must not be extremely low).
(If the resistance is low but you are not sure if it is extremely
low, compare the value with that of the normal channel.)
If the voltage amplifier block is damaged, the voltage (
±
VH)
resistor (4.7
Ω
) may also be damaged.
3. Other damage
Check of the bias protection diodes
Tips:
If the power amplifier is damaged, the bias protection diodes
may be deteriorated or open.
For a channel that has been diagnosed to be in failure, the Vf
(forward descending voltage) of the four diodes must be checked
to confirm that they are not deteriorated or open, using a tester,
after repair of the amplifier. If these diodes are deteriorated, its
distortion/frequency characteristics may be affected. If these
diodes are open, the scale of damage on the power transistors
may be extended if any abnormality is generated in the amplifier
next time.
+VH
-VH
-VL
POWER
TRANSISTOR
(NPN)
POWER
TRANSISTOR
(PNP)
OUTPUT
INPUT
Bias protection
diode
Power supply
resistor
Power supply
resistor
Base
resistor
Base
resistor
Collector
resistor
Collector
resistor
Voltage amplifier block
Current amplifier block
+VL
