Power Up/Down Write Protection - Commodore 1540 Service Manual

1541 CIRCUIT THEORY

Power Up/Down Write Protection

This circuit prevents erroneous data from being written on the disk during power up/down sequences. During a
power up, the 12VDC supply is not applied to the R/W coils and amplifier circuits before the processor has
control of the logic. During a power down the 12VDC supply is removed from the R/w coils and amplifier
circuits before the processor loses control of the logic.
Q1 acts as a series pass transistor, biased to regulate the 12VF output to the R/W coils and
Amplifier circuits. Q2 is a feedback amplifier monitoring the 5VDC supply. CR5 develops a
precise reference voltage for Q2. L8 and C15 delay the 12VDC supply.