720K Floppy drive issue

[fatwizard]

I have an Epson SMD140A, 720K 3.5" floppy drive that exhibits a symptom I hadn't seen before. It works just fine until it gets about half way to the center, after which it starts re-seeking. This gets progressively worse, and will error out before it gets to the innermost track. It has this same behavior when you try to format a disk as well as when just reading from it. That suggests to me it isn't an alignment problem.

I have cleaned the heads, cleaned and lubed the relevant components, replaced the cans on the logic board, and have substituted a know good power supply. I can use a HD drive, but I can't make one look right.

Anyone run into something like this? I'm stumped at this point.

 

[Chuck(G)]

I've fun into similar problems with Samsung drives. The grease on the leadscrew gets hard with age. The solution was to clean the leadscrew completely and then apply some light lube. Maybe that's what's wrong with your drive.

 

[fatwizard]

Well, this old thing uses a big old style stepper and a split band to move the heads, but I should give the stepper the WD40 treatment. I have had that bring a couple of old Tandon 5.25" drives around, and I don't think I tried that on this one yet.

 

[g4ugm]

It happens. As the drive head moves in the tracks get shorter, the bit denisty goes up, and things get a lot more critical. Try cleaning the heads as well... Might even be worth replacing any electrolytics (caps) as it could be noise. Early Apple MAC floppy disks used a variable RPM so they have more data on the outer tracks. Some one wrote a program to pulse the motor on an Atari floppy so they could read Apple Mac disks.

https://en.wikipedia.org/wiki/Spectre_GCR

CDs are recorded at a constant bit rate so for reading Audio the disk also slows down as the head move inwards

http://www.osta.org/technology/cdqa5.htm

CD's don't have tracks either. Its a long spiral like a Vinyl Disk...

 

[SomeGuy]

I have a system with a problem like that, and in my case it seems to be a controller issue. Have you tested the drive on different machines?

It would be interesting to know if the problem is reading/writing or both. In my case the problem seems to be only with reading. If I format a floppy, the formatter will claim it is full of bad sectors. But if I throw it under my Kryoflux all of those "bad" sectors decode fine.

 

[fatwizard]

The system this drive goes with is an Atari ST, of which I have but one example. What I do have are other drives to help verify the controller. The internal drive works flawlessly, as does the two PC HD drives I have converted for use with the ST. I discovered last night that if I format a disk in this drive as single sided, it works like a charm. There seems to be something up with the top head. I wish I could find a test disk for the ST.