8" Floppy Disk Drive - Head not returning

[Pickelhaube]

This feels like a question better asked 40 years ago but I have run into a wall and feel like at least ONE person on this forum has experience with it.

 

 

The drive is the Shugart 801 with an LSI PCB, supposedly meaning it does not need -5VDC compared to earlier models.

 

The main problem is that the stepper motor works FORWARD but not backward, meaning the head can go all the way out but must manually be pushed back in. This means a disk cannot be read/written to effectively.

 

The stepper motor seizes up when trying to go backwards.

 

Everything else seems right to me based on the fact it is mostly working properly in DOS, minus the said problem. I have it going from a 34 pin floppy cable in a PIII system going to 50 pin shugart and Windows 2000 can even send signals to it.

 

I found the service manual here (39025-1_SA800_801_Service_Manual_Mar82.pdf) which has been pretty helpful so far but does not state anything about the stepper not stepping backwards.

 

If I had to guess, it is either a bad servo, a bad track 0 detector (making it impossible to go backwards) or possibly a failed component on the PCB, though every component looks to be in perfect shape.

 

Hope someone can help, I have a lot of data I need to recover!