Brother MFC-J3520 Service Manual page 253

If the machine does not automatically enter the firmware loading mode, perform the
following procedure for switching to that mode and then load the firmware having an
extension .upd.
1) While holding down the power switch, plug the power cord into an electrical
outlet. When the Home key lights, release the power switch and immediately
press the power switch once to display the following pattern.
2) Within three seconds after the above pattern appears, hold down the power
switch for approximately two seconds to display the following pattern.
3) Within three seconds after the above pattern appears, press the power switch
once to display the following pattern.
4) Within three seconds after the above pattern appears, press the power switch
two times to display the following pattern.
5) Within three seconds after the above pattern appears, press the power switch
three times to display the following pattern.
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