CBM900 hard disk timeout

Hello!

I am back home, and I tried to fix the broken C900. Three RAM chips
needed replacement, I exchaged them for new ones and the computer again
boots normally.

I disassembled the hard drive and there were no fuses inside, but after
I rotated the drive by hand a few times and reseated all the connectors
it started to spin again. After I assaembled it back it stopped
spinning, so I repeated the process and it now spins properly. It looks
like some kind of mechanical problem.

Anyway, the computer is still not able to communicate with the drive.
All operations result in timeout. I tried these solution:

* Select all four "drive select" DIP switches.
* Use another data connector on the controller.
* Play with the undocumented first DIP switch.
* Play with the "Set drive type" option in BIOS; apparently the option 2
is for 20MB disk.
* Use "Park drive 0" and "Park drive 1" commands from BIOS.

All I get is only a bunch of timeouts on every oepration (seek, reset
drive etc.)

Here are the theories I came up with:

* The controller is dead. That doesn't seem to be the case, as removing
the controller gives additional error message "Controller/drive
initialization failure" which is not present when the controller is
inserted. So there must be at least some communication between the
computer and the controller.

* The controller is partially dead. Maybe exchanging the ASIC chips with
another WD1003 would help in such case, but all WD1003 PC controllers I
found on eBay have the chips soldered. I wasn;t able to find a
WD1003-CMD anywhere for sale, only WD1003-WA? ISA cards.

* The drive is partially dead. I guess timeout issues would rather
indicate errors on the PCB and not the drive mechanics. On the other
hand, the drive is able to perform self-test and flash the LED, so at
least it must be partially working. Maybe some chips related to the MFM
interface are dead. Exchaning the PCB with another Miniscribe 3425 mught
help here. If MikeS' offer to do that still stands, I would be glad to
take it.

* The computer is able only to communicate with select types of MFM
disks and the 3425 is not one of them. This seems unlikely, as the
WD1003 controller is pretty standard ans there should be no reason for
some special kinds of disks to generate timeouts with it.

Any other ideas would be welcome :-)

Regards,
Michau.


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