to get the Amiga to work with high density disks, the Commodore engineers took a little short cut and just spin the drive at half the normal RPM (150 instead of 300). this keeps the data rate the same, allowing the custom chip to remain unchanged.

this means you can't just use a standard PC floppy drive and expect it to work, even with a little PAL on the back. this drive has been modified.

as is typical in the Amiga community, the mods have been done in such a way to make them hard to reverse engineer. in this case, the rework wire is hair-thin magnet wire covered in silicone. try to remove the silicone, and it shreds the wire.

my guess is that it taps into the BA6986FS spindle motor driver and allows the PAL to slow it down when a high density disk is detected.

simple board.

the schematic is pretty simple. looks like it does the pin swizzle for the disk change signal, creates the READY signal the amiga needs, but then it does some clever stuff with the index pulse. additionally, it uses the two reserved pins and the unused drive select line presumably to plumb the rework wires.

soldered some wires to the rework wires to help figure out where they go.

gotcha!

ok I've replaced the wires with bigger and more visible ones. lol.

so presumably the way this works is that the two bodge wires hook into the clock signal from the controller chip to the spindle motor controller (pin 7 on this example). the motor controller derives the RPM from this signal, so the external PAL sneaks in an additional divide by 2 in order to half the RPM. not too shabby!

@tubetime How did you get from BA6986 to BA6486?

@RueNahcMohr couldn't find the actual datasheet but Rohm makes other FDD spindle motor drivers, so that's just the one I found with a datasheet