Sony MP-F75W-01G floppy drive problems (Mac Portable)

[4seasonphoto]

My 5125 Portable had been working great since I recapped it back in 2008. Then I started to get "This (floppy) disk is unreadable. Eject | Initialize" error messages. Now it happens regardless of whether I have a disk in the drive or not. And if I do insert a floppy and attempt to initialize it, I get a "Disk locked" message even if it's not.

I made a mess of that original floppy drive, but I had an identical unit on hand. But it gives me exactly the same error messages! Tried it in a different computer with different cable, and the problem appears there too.

So far I've cleaned & lubed the drive, checked ESR of the aluminum and tantalum electrolytic capacitors (they seem OK), verified that the various switches are working as expected when a disk is inserted. All stepper motors spin, though the one which drives the heads never budges unless I manually move the head assembly off it's parked position.

I'm baffled: Has anyone seen this sort of behavior before?

 

[uniserver]

do you have an external floppy drive you can check just to make sure the swim is good?

or just swap the floppy drive out with a known working one from another mac.

its just a standard auto inject sony 2mb drive.

Techknight discovered that swim's in the portable can go...

is your +5 +12 good?

That 12v boost regulator can get weak.

how are you powering it?  with a 6v AGM battery and the 1.5a portable charger?

 

[techknight]

Check the 12V regulation. make sure it isnt falling horribly. Also, there is a MOSFET that switches current on-off to the drive, check it to make sure voltages are at the drive and stable. Pinouts of the drive can be found online. 

Once this checks out, Replace the SWIM.

 

[bigmessowires]

Now it happens regardless of whether I have a disk in the drive or not. 

Interesting. And uni, you were just telling me yesterday about a common failure you've seen being "the OS keeps saying the disk is not initialized… when there is no disk even in the floppy drive."

I was thinking about this more, and I believe it means the RD line on the floppy connector is somehow stuck at GND. I'm pretty sure that would lead to the behavior you're seeing - complaints that a phantom disk is not initialized. That's pin 18 on the external DB-19 floppy port (see http://old.pinouts.ru/Storage/MacExtDrive_pinout.shtml).You can test this theory yourself by checking pin 18 vs pin 1 with a multimeter. With no floppy drives connected, RD should be 5 volts because there's a pull-up resistor on the logic board. If it's anything substantially lower, or 0 volts, then that's the problem. 

If this is due to some kind of logic board failure, then the "disk is not initialized" errors should persist even if there are no floppy drives connected to the Mac. That's something you can test.

If it's a failure with the drive itself, then I would expect you'd see 5 V with no drives connected, but a constant 0 volts with the drive connected (measure the voltage at the internal floppy port if the drive is connected externally, and vice-versa).

Since you swapped drives and it didn't fix anything, I'm guessing it's a logic board issue. Could be a faulty SWIM, bad Bourns filter (does the portable have these?), a fried pull-up resistor, or a broken trace on the logic board.

 

[bigmessowires]

Another possibility, after reading techknight's and uniserver's suggestions, is that a weak 5V or 12V or -12V supply is preventing the drive from operating correctly, so it just outputs a constant 0 on its RD line. I would test this with the drive connected, so you can see the effect of the drive load on the supply. Connect your drive internally, then check the external floppy port 5V, 12V, and -12V voltages at pins 6, 7, and 5.

 

[4seasonphoto]

Thanks for the ideas!

But I investigated further, and it appears that I really do have two floppy drives which produce exactly the same sort of error messages. If I install the suspect drive into a different computer, I see  the exact same problems there too. And if I install a known-good drive into the Portable, it works flawlessly. I guess it's a relief that it wasn't my circa 2008 re-cap job which was causing problems.

 

[techknight]

It might be the microswitches dirty/bad on the front of the floppy drive itself. 

 

[4seasonphoto]

It might be the microswitches dirty/bad on the front of the floppy drive itself. 

That was one of the first things I checked, not just by pressing down on them, but by verifying that they were doing their thing correctly when a floppy was inserted.