Yamaha YST-RSW300 Service Manual page 8

YST-RSW300
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3 7 6 3 1 5 1 5 0
I CHECKS AND ADJUSTMENTS
G Confirmation of power amplifier operation
For the power amplifier which has been repaired, it is absolutely necessary to confirm that a correct waveform is obtained
at points indicated by A and B in the schematic diagram according to the following procedure.
Devices required
Signal generator
8-ohm or 6-ohm load resistor
Oscilloscope (dual trace type)
Connection
1) Connect the output signal from the signal genera-
tor to the input terminal of the main unit.
2) Disconnect the connector terminal connected to
the speaker unit and reconnect it to the load resis-
tor.
3) Connect the HOT side of the oscilloscope CH1
probe to the point A or B indicated in the figure and
the GND side to the GND of the main unit. (Fig. 1)
4) Connect the oscilloscope CH2 input to the red side
of the connector cable, which is connected with the
load resistor.
At this time, the GND terminal of CH2 must be left
unconnected.
Point A (Cathode of D29/D30)
T E
L 1 3 9 4 2 2 9 6 5 1 3
V: 20 V/div H: 2 msec/div
DC range 1 : 1 probe
26-28 Vp-p
w w w
8
Setting
1) Set the signal generator to the sine wave, 100 Hz
and minimum output level settings.
2) Set the volume of the main unit to the minimum po-
sition.
3) Turn on the power to the main unit.
4) Adjust the output level of the signal generator and
the volume of the main unit so that the output level
observed at oscilloscope CH2 is 26 Vp-p to 28 Vp-p.
Waveform observation
With the settings made as described above, observe
the waveform obtained at CH1 for judgment.
Point B (Anode of D27/D28)
V: 20 V/div H: 2 msec/div
DC range 1 : 1 probe
x a o
y
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G Idling adjustment
To stabilize operation of the amplifier, turn ON the power with no input signal and wait for 2 to 3 minutes in non loaded
condition before the adjustment.
Confirm that the voltage across the terminals TP1 and TP2 is 20 mV to 350 mV.
If it exceeds 350 mV, open (cut off) R136.
Attention:
If the idling current exceeds 350 mV after a power amplifier repair, check for a defective component before cutting the
resistor.
MAIN (1) P.C.B.
R136
POWER
DRIVE
Q Q
3 7 6
POWER
DRIVE
26-28 Vp-p
DC 20 mV to 350 mV
u 1 6 3
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2 4
8 9 9
PS AMP
A
PS
AMP
D29 D29
D30 D30
D28 D28
3 1 5 1 5 0 8 9
TP2
PS AMP
D27 D27
PS
B
AMP
DC 20 mV to 350 mV DC 20 mV to 350 mV
TP1
TP2
B A
TP1
m
R136
c o
.
Fig. 1
2
8 9 9
DRIVER
WOOFER
2 4 9 8 2 9 9
GND