5.25" Floppy drive motor problem

[bonzer35]

I'm trying to repair an old 5.245" floppy drive to work with my old Acorn BBC computer. There is a problem with the spindle motor where it sometimes requires a 'nudge' to get it going. Once spinning, the drive reads perfectly well.

You'll see from the first photo that I have put marks on the rotor (that match a mark on the chassis) at the positions where the motor requires a push to get started. I also took the rotor off and activated the motor to see if the stators were all magnetizing. The stators that have a mark on are the only ones that seem to get magnetized. I'm not sure if this is a problem though, as there are hall sensors under the rotor and I'm guessing that they inform the controller IC which field to turn on so it may be that with the rotor missing, no other fields are getting activated.

BTW, the dodgy jumper wire is something that I had to add as a capacitor had leaked and removing it from the PCB took off the pad.

Any idea what should I test for next in order to repair this?

Thanks

[KevinA]

Any help to you?

[bonzer35]

Ah, thanks but the drive covered in this manual has a spindle with a belt-driven dc motor whereas mine is a different type of drive motor.

Interesting manual though - I wish you could get this type of repair/service info with modern machines!

[amyk]

These are usually 3 phase motors; your symptoms line up with a dead phase.
Check resistance between the phase leads. They should all be identical.

[bonzer35]

I have measured the resistance between each winding and I get 8.5 ohms between all of 3 of them

[fzabkar]

List of Measuring Instruments

Oscilloscope
Brian
Frequency Counter

5-9 Write Protect Sensor Check

a) Load and unload a blank diskette which is not write protected, and check that write protect sensing is definitely made using Brian.
b) If the write protect sensing does not work well, exchange lever frame assembly according to 4-7.
c) Repeat

Who or what is/was "Brian"?

[fzabkar]

I have measured the resistance between each winding and I get 8.5 ohms between all of 3 of them

Is it possible that one of the phase drivers in the motor controller has failed or become weak??

[CaptDon]

I'd be more tempted to seek a new or used drive from the junked units cluttering up local computer stores. It is getting hard to find older hardware and sadly millions of the old 5.25" units are in the landfills. Savey computer store owners keep a couple old drives around for 'transferring' old disks for their valued customers. I still have a ZIP-250 (can also read ZIP-100) and several 3.5" floppy drives. Sadly I sold the last 5.25" in my junk box.

[Gyro]

I see a small electrolytic cap at the edge of the PCB, right up at the top of the photos. In the absence of any other obvious faults, you might as well try changing it. There's always the chance that it is decoupling something important in the driver chip (or involved in the feedback from the FG sense track around the edge of the poles). Check that it isn't a bipolar one.

[MarkT]

Perhaps the leaking cap took out more traces?  Could be a hall sensor or a phase trace...