Time to bring this topic upon these forums!
Last year I attempted my first NeXT MO-drive repair (https://www.nextcomputers.org/forums/index.php?topic=922.msg34314#msg34314). I bought a DOA one from Rob@BlackHoleInc. Following the guide by khashoggi (NeXT Magneto Optical Service Manual.pdf, found online) I replaced the caps (my old Mouser-cart (https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=2f481d35ea) some parts might be unavailable by now and some would benefit being lower height). There was quite a bit of corrosion from the cap leakage on the motor control PCB (rev A), but it still seemed to function fine with a bench PSU. However the drive, once connected to a Cube, did not work: it did take a disc in, spin it up and make some seek-like noises but no disc would ever mount in the OS. I put the drive aside.
Fast forward a year, I found another MO-drive pop up on eBay for a resonable price. I might regret this, but now I have two of these drives!
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I've so far taken this second drive apart and done the following:
- cleaned and recapped the motor control PCB (revision B this time, quite a bit of corrosion in this one too), verified that it, too, runs with a bench PSU and spins at a reasonable-looking speed (I don't have tools to verify the RPMs)
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- cleaned and recapped the Digital A board. No damage there, and not even any real fishy-cap-juice -smells here.
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I am somewhat worried about these surface-mount potentiometers:
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These don't look too bad, but some on the other board (and in the previous drive) do look a bit corroded. In the first drive I sprayed these with IPA and ever so slightly turned them back-and-fro. This time I'll try not to touch them at all.
TODO:
- clean and recap the analog board
- clean and lubricate drive mech
- reassemble and test
I am not having too high hopes on this drive either, but one never knows! As we have no service manuals on how to adjust the drives, it might be a good idea to take measurements of the potentiometer values (especially from a working drives) for comparison.
For someone qualified in reverse-engineering, perhaps it would be possible to re-create the motor control PCB? I had a quick look that both ICs (Hitachi HA13403 and HA17324) have datasheets available, and there were several of both types for sale on eBay. Also these might not always be damaged on old boards and might be re-usable. These boards seem very prone to have physical damage due to cap leakage so a replacement would make life much easier. Also having some sort of test-bed for these would be great.
I am not sure how the wavy circular drive-coil could be re-created: perhaps a thick enough traces on the PCB surface would suffice? These original ones, made from plastic layers and thin traces on them might also suffer from cap-juices and have their resistances altered which, I'd guess, might impede the correct RPMs or cause irregular rotation.
I wonder if having a working version to take measurements from would be helpful. Regardless kudos and good luck!
Well, I finished the recap and assembled the drive. Powering it up with a 12+5V bench supply, it didn't blow and made a small click, sounds reasonable. Won't take a disc when not connected to a computer.
Assembled my Cube and tested the drive... no dice. The Cube boots ok and I see the drive detected in /usr/adm/messages. When I insert a disc, it gets pulled in Ok. The drive spins up and sounds like it gets up to speed, but then almost immediately it spins back down. And then it repeats. Between attempts I think I can hear the laser tracking moving back to the home position and back.
I can't remember exactly if this differs from the previous drive, but basically the same. Not sure if the computer froze this badly with it. I guess I might need to try swapping the motor PCB from the previous drive, just to see if this is an RPM problem.
Oh well.