Here is a unit which would not turn on, or would turn on intermittently or after a long warmup. Applying the scope onto the diodes, where they go to the filter caps, exposed a 60 cycle wave. It was not the result of a leaky diode, but rather of failed filter capacitors in the power supply.
The power supply appeared clean without a load on it, but when the board was drawing current the wave was visible in the outputs of the voltage regulators.
I also replaced the disk drive. Since the newer PC floppy drive uses a new interface, I had to redirect pin 10 to pin 12, and connect pins 2, and 34 together completely. The drive needs the disks to have the tape put over the hole that marks them as double density.
This is a method I have developed for using PC floppy drives in applications where the older Shugart interface was used. It often works... but sometimes you have to also play around with shorting the "disk ready/ disk change" pins. In a few situations I have had to install a button or switch on this pin, so that the unit would know a new disk was inserted.