SD-AT50H (serv.man10). To check and cancel protect circuit detection line - Sharp Audio Service Manual (repair manual)

– 10 –

1: In this model the microcomputer (IC301 or IC701) detects

the following malfunctions once power is supplied, and cuts
off power from all the parts except for the circuit driving the
microcomputer.

1

 Abnormal drop in the output voltage of each regulator

2

 Output offset over voltage of the 1-bit amplifier

(above approx. DC 2 V)
Power is cut off after “ER_AP02” is displayed.

3

 Mains overvoltage of the 1-bit amplifier

4

 Defects in the cooling fan or its drive circuit

(when stopped)
Power is cut off after “FAN LOCK” is displayed.

2: Criteria for abnormality, and checking and cancelling the

detection line
The criteria:

1

 The voltage of pin 91 on IC301 (PROTECT) or pin 19

(PROTECT) on IC701.

2

 The voltage of pin 29  (OFFSET) on IC701: Low (below

approx. 1 V) (abnormal) and High (above approx. 2.5
V) (normal) .

3

 The voltage of pin 19  (PROTECT) on IC701.

4

 The motor rotation is controlled when the voltage of pin

20 (FAN CHK) on IC701 is below approx. 1 V.
The motor stops when it is above approx. 1 V.
If the voltage of pin 21 (FAN LOCK) on IC701 is not
between approx. 0.21 V and 0.9 V whilst the motor
rotation is controlled, it is judged as abnormal.

Checking and cancelling the detection line:

Caution: If the PROTECT/OFFSET detection line is

disconnected in case of 

1

, 

2

 or 

3

 above, a

protective operation does not function. The
disconnection by output short-circuit of each
regulator may cause the parts or PCBs to burn
out and therefore do not disconnect the detection
line when the cause is not identified.
(Power supply can be detected with an
oscilloscope or like during approx. 0.5 seconds
after an abnormality is found. Note that the
detection would be difficult with a tester as the
voltage rises and falls rapidly.)

1

1

1

1

1

Connect a measuring device (such as an oscilloscope) to
the stabilizing output of the regulator, and turn on the unit.
If the detected voltage is extremely lower than the voltage
on the circuit diagram (below approx. 1.5 V), the regulator
itself or the circuit is defective.

2

2

2

2

2

Adjust the voltage of the output offset. If a protective
operation is still activated, QA100 ~ QA103, QA200 ~
QA203 or QA300 ~ QA303 on the 1-bit amplifier could be
defective (the detection circuit of the speaker output DC
voltage).

3

3

3

3

3

The unit functions properly if power is supplied after removing
BIA111 (1-bit amplifier main power supply connector). (No
sound is heard from the speaker.)

* Adjust the output offset voltage after repair.

If a protective operation is still activated even after removing
BIA111, QA413, QA414, QA415, QA410, QA411 or QA412
could be defective (the detection of power supply voltage).

4

4

4

4

4

Connect a measuring device (such as an oscilloscope) to
pin 21 (FANLOCK) on IC701 terminal, and turn on the unit.
Connect the collector of Q822 to GND and make sure the
fan motor rotates properly. If not, check if the motor or fan
is obstructed and the motor itself is defective. If all the above
are fine and the voltage of pin 21 is below approx. 0.2 V with
the collector of Q822 connected to GND, check Q830, its
peripheral circuits or the connection of the fan motor. The
fan motor could be defective if the voltage of pin 21 exceeds
approx. 0.91 V right after connecting the collector of Q822
to GND and remains above approx. 0.91 V.