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Hi All, As I was getting a little burnt out on fixing the OSI MPI floppy drive (and I am waiting for an alignment disk to arrive) I decided to shift my focus to getting a spare 502 board working. The board was very hacked up, and the first thing to do was to remove all the wires, and fix the cut tra...

After some offline discussions with Dave, I decided to replace the LM339. The good news is that it now seems all the scope signals are now back on spec, and the drive now spins at the correct speed. I am able to get a dump of the contents of a OS65D floppy, but I am getting spurious errors. I think ...

Just thinking about this a little more and looking into how a comparator (the LM339 @ F4) works. "A comparator is a circuit that accepts two voltages, V1 (in my case it's 5V coming in at 4F-4) and V2 (in my case it's an oscillating input on 4F-5), and outputs zero volts if V1>V2, or outputs a positi...

4F-6 (1V.DIV 2ms) 4F-7.JPG Some quick observations: - 4F-5 is about 12V peak to peak (should be 8V according to the manual). Seems to dip slightly below 0V. Pulsing too fast - 4F-2 is certainly not a square wave (low voltage is about 4V, high voltage is about 6V) - 4F-6 is at about 0.7V but with a v...

Just some more readings from the 4F ic.

4F-5 (5V/DIV 2ms) 4F-2 (2V/DIV 2ms) 4F-6 (1V.DIV 2ms)

Thanks for your write-up on your personal OSI history. Quite interesting reading about all the stuff you made. It's a shame that none of your old Portland Boards stuff exists anymore, as I'm sure Dave would have been excited to add the contents to his page.

Phil

Tonight I had a little time to read some of the MPI 51 manual, mostly focusing on the speed control area. I have to admit to not to understand it very well, but in general, I think I get the basic concept. I took a scope reading at J4-28 which is the tachometer output from the motor. (This was taken...

I just quickly wanted to follow up my last post with something about the motors. Unless my calculations are totally off, the new motor is spinning way too fast. just a little back history. Basically the original motor is dead. It's made by a company called Buehler. I was able to buy two new motors m...

Hi Dave, Thanks for the reply. Yes, I was using the rawdump utility. I wanted to get a better idea as to what the computer we receiving from the drive and I thought this program may give be a clue. Tonight I battled with flakey connections to the back plane but I was able to get some readings using ...

I wanted to start a new thread in the hopes that people here can help me fix my OSI (MPI 51) drive. I've never worked on a disk drive before so please excuse my ignorance. First I should start off by explaining what I have done so far: - Replaced main drive bearings. Originals were shot. - fixed (ho...