Brother LX-300 Service Manual page 156

2
Nothing happens when switches are pressed.
a. Defective switch or push switch on the Switch PCB.
b. Defective LED on the Switch PCB.
Problem
a. Defective Is Switch PCB correctly
switch or push mounted?
switch on the
Is switch harness assy
Switch PCB
correctly connected?
Is the white lead connector
connected to CN1 on the
Switch PCB?
Is the red lead connector
connected to CN2 on the
Switch PCB?
When switch SW1 to SW6
is pressed on the Switch
PCB, does the switch
become conducting?
When push switch
RSW1•RSW2 is shorted,
does the connector on the
push switch assy become
conducting?
When switch SW1 to SW6
is pressed on the Switch
PCB or the push switch
assy is shorted, can the
conductivity be checked at
CN16 on the main CPU?
If conductivity can be
checked at CN16 on the
main CPU when switch
SW1 to SW4 is pressed or
the push switch assy is
shorted, can the
conductivity be checked at
the main CPU ports?
If conductivity can be
checked at the main CPU
ports when switch SW1 to
SW4 is pressed or the push
switch assy is shorted, is
this recognized by the main
CPU?
Check Item
Defective Switch PCB
mounting.
Defective connection
of switch harness
assy.
Push switch assy not
connected.
Push switch assy not
connected.
Conductivity problem
in switch SW1 to
SW6.
Defective push switch
assy.
Defective switch
harness assy.
Conductivity problem
in Main PCB.
Defective series
resistor on main CPU.
Defective main
CPU#3.
Cause Remedy
Correctly mount
Switch PCB.
Correctly connect
switch harness
assy.
Correctly connect
the push switch
assy.
Correctly connect
the push switch
assy.
Replace the Switch
PCB.
Replace switch
harness assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
Replace Main PCB
assy.
7-6
See
Repair
Section
3.3.3
3.3.4
Replace
3.3.5
switch SW1
3.3.7
to SW6.
3.3.8
3.3.9
3.3.10
Replace
push switch
assy 1.
Replace
push switch
assy 2.
Replace
series
resistor.
Replace
CPU#3.
3.3.3
3.3.4
3.3.5
3.5.1