800k Floppy Drive Reads 400k But Not 800k

[DontGetHit]

So I have an 800k floppy drive from an SE that has been cleaned and lubed. From my experience, 400k disks work fine but 800k disks do not work about 70% of the time. When the disk is inserted I get the error that the disk is "damaged". So this leads me to believe that whatever triggers the drive into knowing that what size disk it is, does not always work. But what does that? An interesting thing is that I can get the drive to read 800k disks more commonly by manually moving the heads to the front and then inserting the disk. For some reason making the heads return to track 0 to start reading the disk makes it more likely to work. I am stumped.

 

[LaPorta]

Wow this is really old, but in case you still want to know...

The upper head is likely not contacting the surface of the disk.

 

[Franklinstein]

Do 800k disks formatted in that drive work reliably in that drive? If so, you'll want to realign your track-0 sensor using a universally known-good disk. If not, you can try to clean the heads. Ideally you'd use one of those foam-tipped sticks and 100% isopropyl alcohol but cotton swabs and 70% isopropyl will work if you give it time to dry and make sure there are no cotton threads stuck on the heads.
If it's still wonky, try adjusting the head tension. I'm pretty sure there's a special tension gauge you're supposed to use, but I don't know where you'd get one, let alone what the actual tension is supposed to be, so you'll have to experiment. You can try stacking a penny on top of the head once the disk is in and seeing if the disk reads properly. If so, that's your problem and you need to increase tension. Usually the tension spring for the top head has a series of steps and you'd just move the spring to the next lower one. You may have to break some glue to get it to move; they're tacked down at the factory.

 

[bibilit]

yes i agree, the 400k drive has only once head (the lower one) when the 800k drive has two. So when reading a 800k disk you need both.

Yes try cleaning the upper head first, if you don’t get any improvement, move the spring of the upper head a notch down.