Good source of info on 8” diskettes and OS65D

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lowrybt1
Posts: 98
Joined: Sun Mar 08, 2015 3:42 pm

Good source of info on 8” diskettes and OS65D

Post by lowrybt1 » Sat Mar 31, 2018 12:52 am

I’m experiencing the joys of working with 8” floppies with a C8P. Every time I try to copy a diskette, I get an ERR #3 that the destination disk is write protected. In fact, I see no write protect/unprotect tab or hole anywhere. Anyone know the trick? Do I need to cut out a tab on the disk?

Also, with two drives tested so far (Siemens FDD-100-8), the head motor appears to lack the strength to move/reset the head to track zero, so (with power off), I manually move the screw so the head is positioned over track zero. With that, the disk boots every time. But I’m going to have to find some source for replacing that motor/screw.

[Update: So, after flip through some Siemens manuals, I understand how the write protect works and I’m guessing the sensor and associated logic is faulty -- resulting in a default to write inhibit. I think there is a trace I can cut to break the inhibit circuit. As for the stepper motor not moving the head to track 0, either the motor is bad or the track 0 sensor is bad (drive thinks its always in track 0 position) or both. ]

Thanks for any help you can provide. TB
C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

Mark
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Joined: Tue Sep 16, 2008 6:04 am
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Re: Good source of info on 8” diskettes and OS65D

Post by Mark » Sun Apr 01, 2018 2:52 am

The Siemens FDD-100-8 manuals are available online various places including http://osi.marks-lab.com/reference/reference.html.
There are various test points indicated which you can use to test the logic of the sensors.
I found it useful to wire up a drive exerciser, where I could toggle various drive control signals (STEP, DIR, HLOAD, SEL etc.) and watch the results.

The steppers are driven with +24V DC ! So you need 115V AC, +24VDC @ 1.6A, +5vDC @ 1.0A to run these. Maybe the 24DC is weak?

Also as you've discovered 8" floppies are backwards from 5.25". You need an open slot to make a disk write protected.

One of my FDD 100-8s developed a fault with the LSI chip on the drive (part of the drive select logic, if I remember correctly). I replaced that section with a pair of TTL chips that mimicked the functional block of the LSI based on the schematics. The rest of the chip seems to work fine, but these things do fail occasionally. Most of the foam head load pads have failed on my drives. I've replaced them with similar consistency foam rubber bits to restore head contact.

Good Luck!
-Mark

lowrybt1
Posts: 98
Joined: Sun Mar 08, 2015 3:42 pm

Re: Good source of info on 8” diskettes and OS65D

Post by lowrybt1 » Wed Apr 11, 2018 3:29 am

Thanks Mark. It looks like you diagnosed it. The 24VDC peaks at 0.9A. I’m guessing that’s an issue with the large axial electrolytic capacitor.
C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

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