I just fixed a 5.25 Mitsumi D509V3 drive I got with some PC scrap parts. It was working, but the problem was that the motor started to spin after the disk was inserted, but it didn't wait until the lever was closed. So the lever had to be closed while the motor was spinning, and that could possibly damage the floppy disk. After the lever was closed, the drive stopped spinning and the data could be read on the PC. But after removing the disk, sometimes the drive started to spin and refused to stop.

I discovered that this issue was caused by broken/bent endstop switch, which was being activated too early. It had a lever which was supposed to be pushed by the floppy disk, after the lever on the front panel was lowered, but it was badly aligned and I found it impossible to adjust the lever angle and height in a proper way - when it was too high, motor started spinning to early, and when it was too low, it didn't start spinning at all.

After looking at the mechanism, I realized that after the disk is insterted and I close the lever, a metal arm goes down. So I decided to desolder the old switch, and make a DIY solution, using three pieces of copper clad, a wire and some cables. I glued one piece of copper clad to the bottom part of the mechanism, and soldered a piece of wire there, which acts as a contact. It is soldered only in one point, in order to have some elasticity. Then I glued another piece of copper clad to that metal arm, soldered some wires, connected that in place of old switch and that's it.

When the lever is completely closed, contact is made between these two surfaces I put there and the motor starts spinning. Problem solved 😁