Brother checks the filters associated with the power source, especially that of the + B, if it is dry it will not turn on.
If it is one of those that use a pulsating power source, they do not turn on if the vetical is short, the protection circuit does not let it turn on.
How much do you have from + B? ... If there isn't that voltge, check the diode from the shopper transformer (the one from the source)
check on the main card if the voltage of 1.8 volts appears even without turning it on, if so it is because the epron is damaged and you will have to reprogram it