Why on earth would anyone want to repair a floppy disc drive? It’s quite a while since most of us bade them good riddance and started using USB sticks and Flash memory cards. However, I still use floppies from time to time, mostly with my trusty BBC Micro which still sits in the corner of the workshop.

Recently I was asked to recover some documents from some old 5.25″ BBC Micro floppy discs. The documents themselves were in an unusual format, about which more another time, but the first step was to simply get the data off the discs. The discs were 80-track ones, and I have a pair of Chinon FZ-506 80-track drives for the Beeb.

Out of all the various Beeb drives I’ve had over the years, I’ve kept these two because they’re housed in a compact casing with a mains power supply, and they have handy 40/80 track switches on the front, not hidden round the back. One drive has always been a bit reluctant to start spinning, but for occasional workshop use that wasn’t a problem. I’d got in to the habit of just opening and shutting the door a little which would kick the motor into action. However, when it came to intensive use backing up these old discs, which needed both drives, the failure to start became a real pain. I didn’t have a spare drive, and finding another one (especially in Poland) isn’t easy these days.

My curiosity got the better of me and I decided to open up the drive and find out what was wrong with it. It wasn’t hard to take apart and I soon had the motor revealed. Here’s a photo of it sliced into its component parts.

It’s a ‘pancake’ motor, so called because it’s (nearly) flat. The shiny silver bit on the left is the turntable which drives the disc, and it sits in a bearing. The next layer is the circuit board containing the windings and controller circuitry, and below that is the rotor which is a multi-pole magnet on a steel disc.

The motor is controlled by a Mitsubishi M51785P motor controller chip. The chip’s data sheet revealed that the motor has three phases, each of which has a coil to drive the rotor round and a hall effect sensor for feedback. This particular one is arranged with two coils per phase, but occupying 6/7 of a revolution, so the motor goes more slowly than the chip is driving it. At least, I think that’s what’s going on. Here’s a closeup of the circuit board. You can see the six coils, and the coloured wires I soldered on to measure things while the motor was running. Because of the way it’s built, it’s impossible to access most of the circuit board while the motor is assembled.

The controller chip seemed to be doing all the right things: its oscillator was running, and the outputs to the coils were doing sensible things. The coils themselves were all undamaged and measured the same resistance as each other. But I noticed something odd about the hall effect sensors. There are three of them, HG1, HG2 and HG3. I noticed accidentally that if I shorted together the two wires taking the output of HG1 to the controller, the motor still ran but sounded very rough. Not surprising. The same happened if I shorted the output from HG3. But shorting the output from HG2 had no effect at all. Aha! Only HG1 and HG3 seemed to be having any effect on the motor. I swapped HG1 and HG2 just to see what would happen, and the fault moved to HG1. That proved to me that I had a faulty sensor, not a faulty chip.

Where to get a replacement sensor, though? This drive was made some time in the late 1980s, and I couldn’t find hall effect sensors in today’s electronics catalogues which would fit mechanically and electrically. I had a rummage around the workshop and found a scrap 3.5″ floppy drive. A squint at the circuit board revealed a suspiciously hall-effect-looking device of the right shape and size nestled next to the spindle rotor, used for index sensing. Well, it had to be worth a try. I extracted it and fitted it to the 5.25″ drive in place of the faulty one.

Success! The motor now ran more smoothly, and shorting each of the hall sensors in turn had roughly equal effects, so they were now all working. Best of all, the motor started reliably every time. Interestingly it wasn’t as quiet as the other drive, but I suspect the scavenged hall sensor is optimised for magnetic fields from the side rather than the front, given how it was mounted, so it’s probably not perfect.

The last job was to realign the head slightly, because this drive was a bit fussy about reading some discs. I found a disc that it struggled with but that the other drive would read every time, and tweaked the position of the head stepper motor each way a little until this drive read that disc reliably. You can see in this photo that the stepper motor has elongated mounting holes, so it’s possible to loosen its screws (there’s another one just out of shot to the right) and turn the motor a few degrees to adjust the position of the head.

After all this work, the drive read all the discs I asked of it without any problems. I hope it’ll be OK for the next decade or two.