Apple III / Disk III not reading floppies

Hi there,

I'm a bit lost troubleshooting this quirky machine, and I figured that this would be the best place to discuss with tech wizards.

I got my hands onto a regular Apple III, 128k. Nothing particular with this one.

I had to change a 74S257N for it to boot, and I raised the 128K 5V card to 256k.

However I cannot boot on anything at all, either using the internal drive or an external Disk III plugged in the internal port.

I've cleaned heads, but I keep getting the dreaded "RETRY" prompt.

I'm testing a Confidence disk written from an Apple II (using a Floppy Emu & Copy II Plus 9.1, Auto Bit Copy in Try Standard mode), having none of the original software.

I've noticed that on the internal drive, the belt falls pretty easily from the bottom, although it does not feel stretched nor desintegrating.I have never attempted surgery on a Disk II/III before.

What is the best course of action?

Help is much appreciated.