Floppy controller tantalum

What I tried

In the morning the floppy read fine. By afternoon DIR worked sometimes. By evening freshly formatted disks wouldn’t boot. By night the machine went straight to Cassette BASIC with no non-system disk message, meaning the BIOS never read sector one.

I chased the drive: head contamination, belt, spindle speed, alignment, a capstan on the stepper shaft that looked like it might have slipped. I also found one disk that had been sitting on a MacBook lid and was partly erased by the magnets, which muddied things further.

Then I pulled the floppy controller card and put the meter across the +5 V edge pins.

What I measured

Continuity across +5 V and ground on the controller. I didn’t want to desolder every cap to measure them, so I found the shorted tantalum with continuity checks on the board, replaced it, and the floppy read again.

What broke or surprised me

A tantalum failing progressively looks like several unrelated things getting worse over a few hours. When multiple subsystems degrade together, look for one common factor first. Also, on a tantalum the stripe marks positive, which is backwards from an aluminum electrolytic.

Next

Figure out why disks written on one machine won’t read on the other.