Troubleshooting Tables - Brother LX-300 Service Manual

7.6 Troubleshooting Tables

1
Nothing happens when power switch is pressed. (LED indicator does not light.)
a. Abnormal input voltage.
b. Abnormal +5V DC supply.
c. Defective main CPU.
Problem
a. Abnormal input Are AC adaptor, Jack PCB,
voltage power harness correctly
connected?
Is fuse F1 on Main PCB
blown?
Is AC adaptor output
voltage normal (about 21V
DC at rated input)?
Is AC adaptor voltage
applied to connector CN1
on Main PCB?
b. Abnormal +5V Is power supply IC#1 (Pin
DC supply 10) output Low or
switching?
When power supply IC#1
(Pin 10) output is Low, is
transistor Q4 ON?
When transistor Q4 is ON,
is transistor Q3 ON?
c. Defective main Is Reset (Pin 4) of the main
CPU CPU #3 High?
Is oscillator waveform
output from XIN (Pin 3) and
XOUT (Pin 2) of the main
CPU#3?
Check Item
Defective harness
connection.
Blown fuse.
Defective AC adaptor.
Defective Jack PCB.
Defective power
harness.
Defective power
supply IC#1.
Defective transistor
Q4.
Defective transistor
Q3.
Zener diode ZD1 is
shorted.
Defective reset IC#2.
Defective oscillator
XT1.
Defective main
CPU#3.
Cause Remedy
Connect harness
Replace fuse.
Replace AC
adaptor.
Replace Jack PCB.
Replace power
harness.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
7-5
See
Repair
Section
3.3.3
3.3.4
3.3.5
3.5.1
−−−
3.3.3
3.3.4
3.3.5
3.5.2
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
3.4.1
Replace
IC#1.
Replace Q4.
Replace Q3. 3.3.3
3.3.4
3.3.5
Replace
3.5.1
ZD1.
Replace Q3.
Replace
IC#2.
Replace XT1.
Replace
CPU#3.