Sharp SD-AT1000, Service Manual

Page: 10 / 72

SD-AT1000
– 10 –
TO CHECK AND CANCEL PROTECT CIRCUIT DETECTION LINE
1: In this model the microcomputer (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
(below approx. 1.5 V)
Power is cut off instantly without display.
2
Output offset over voltage of the 1-bit amplifier
(above approx. DC 2 V)
3
Mains overvoltage of the 1-bit amplifier
(above approx. DC 5.5 A)
Power is cut off instantly without display.
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 2 (PROTECT) on IC701.
2
The voltage of pin 7 (OFFSET) on IC701: Low (below
approx. 1 V) (abnormal) and High (above approx. 2.5
V) (normal) .
3
The motor stops when it is above approx. 1 V.
If the voltage of pin 1 (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
Abnormal drop in the output voltage of each regulator
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
Output offset overvoltage of the 1-bit amplifier
Adjust the voltage of the output offset. If a protective
operation is still activated, ICA401 on the 1-bit amplifier
could be defective (the detection circuit of the speaker
output DC voltage).