Picked up a cheap 1040STF described as 'the disk drive is not functioning as it should (sometimes spins up and sometimes doesn't)'. Predicted a dead floppy drive.... not the case, wouldn't read floppies with any drive. Sometimes the floppy drives would not appear on the desktop.
Took it apart and found some strange corrosion on pin 6-7 of the 1772 FDC chip;
Desoldered the chip and underneath the pad for pin7 was corroded and seperated from the PCB trace. Looked heat related as trace was discoloured around the pin... but cause not obvious - possibly a manufacturing defect giving a bad contact with resistance causing heat, but im just guessing.
Anyway I fitted a 28 pin socket and repaired the broken track with kynar wire.
Result - happy ST! 
If anyone can suggest a reason for the corrosion I'd be interested to know. The pinout i found says pin 7 is a data pin a mystery.
Statistics: Posted by grobda — Wed May 27, 2015 6:35 pm