Onkyo TX-SR502 Service Manual page 34

OPERATION CHECK-2
OUTPUT LEVEL & THERMAL DETECTION
Regarding the power supply system of this unit.
<Function>
1. High-voltage mode
Usually, the power supply supplies high voltage to the power amplifier circuit when the relay(RL6901) is turned on.
2. Low-voltage mode
When as follows, a power supply changes to the state of supplying low voltage when the relay (RL6901) in turned off.
(1) Excessive output signal is continuously outputted from power amplifier.
(2) Temperature of a heat sink over 80 degrees.
<Purpose>
1. The loss which power amplifier generates is decreased. As a result, the miniaturization of a heat sink is realized.
2. Corresponds to a safety standard.
<Block diagram>
POWER AMP.
THERMAL
SENSOR
THERMAL
R6380
DETECTION
Q6303
<Necessity for a check of operation>
May not notice the fuse (F6901, F6902) broken in the case of repair.
Because the power supply is continuing supplying voltage from the High side of SEC1 at high-voltage mode.
When a power supply Circuit changes to low voltage mode, amplifier does not operate.
Therefore, it is very important to check operation of this power supply system containing a fuse.
<Procedure>
[NOTE2]
Don't connect speakers or any load.
Don't short speaker terminals.
(1) Set the unit is Multi channel input state.
(2) Set the master volume into the minimum.
(3) Apply sine wave signal to MULTI CHANNEL INPUT (FL+FR+C+SL) terminals.
(4) Connect an oscilloscope to speaker terminal of FL channel.
(5) Adjust master volume so that a speaker output may become distortion about 5%.
(6) Check that the RL6901 is turned off after about 5 seconds.
And the power supply changes to low-voltage mode.
(7) Adjust the master volume so that output voltage may become half.
(8) Check that the RL6901 is turned ON after 2 to 3 minutes.
And the power supply returns to high-voltage mode.
+B1
SPEAKER
TX-SR502
OUT
502E//8250
HIGH
+/- 53.0 V
MODE
LOW
OUTPUT
+/- 35.0 V
MODE
LEVEL
DETECTION
-B1
VOLH
Q7008
2
MAIN
MICROPROCESSOR
High-voltage
Low-voltage
RL6901
D6904
HT-R520
D6903
+/- 68.0 V
+/- 33.5 V
RL6902
SEC1H
Q6901
23
TX-SR502/E/8250/HT-R520
SEC1
High
Low
PRI.
F6901
Low
F6902
High
T901 MAIN
TRANSFORMER