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Successful Miniscribe 3212 repair

D

dkedrowitsch

Experienced Member
Joined
Dec 27, 2006
Messages
102
Location
Boyertown, PA USA
In case anyone runs into a similar problem, below is a quick recap of a recent repair I just completed.

For reasons of pure nostalgia I've been looking for a clean Miniscribe 3212 5-1/4 HH 10-meg hard drive to use in my XT clone, and couple weeks ago a very nice looking one popped up on eBay for less than the usual fortune most sellers ask for hard to find drives, so I snagged it. Unfortunately after it arrived and I tested it, it would begin to spin up and eventually fail with a flashing 0100 fault code which meant the CPU didn't detect an index pulse.

Without schematics it was a bit of a challenge to troubleshoot, but I eventually traced where the index signal was expected to come from, a single pin on the spindle motor header. Turns out the hall-effect sensor in the motor was bad and was not pulling the index pin low to signal the CPU whenever the motor made a full revolution.

Fortunately this particular drive has an exposed flywheel on the spindle motor, so I was able to fasten a 3mm magnet to the flywheel and mounted a simple unipolar hall effect sensor to the underside of the HDA. I tied the new sensor into the index circuit on the PCB and BINGO, the drive spun right up and has quiet bearings to boot!

Unfortunately it was now flashing a different error code. This time is was 1000, unable to uncover track 0 sensor. This one was an easy fix, a single drop of light oil on the outer bearing of the head positioning stepper motor and the drive would spin up, perform self tests, and was ready to attempt low level formatting.

The LLF didn't go so well. It succeeded, but about 20% of the media was marked as defective. Plus I could hear what sounded like "dragging" as the heads were positioned around the outer 1/3 of the platter.

I decided to open the drive and take a look, and with a heat gun I was able to lift the warranty sticker without damage so the drive still looks untouched. The oxide on the platter appeared to be degraded due to I assume age or the environment, although externally the drive looks brand new. Under a magnifying glass the surface of the oxide film was scattered with tiny black deposits which the heads had been dragged across, hence the scraping sounds. Crap.

Since I have several common Miniscribe 3650s, I stole a "clean" platter out of one and transplanted it into the 3212. The hub, outer diameter, and thickness of the 3212 platter and 3650 platters appeared to be identical so I figured what the hell. Using nothing more than latex gloves and careful patience, I shaped folded pieces of paper into wedges that fit between the steel spring arms to lift the heads away from the disk, and unscrewed the head assembly from the positioners to allow the platters to be removed, and simply removed the 3212 platter and dropped one from the 3650 back in place. Reversing the procedure, I carefully reassembled the 3212 without incident and not a single mark on the surface of the disk.

I fired it back up to attempt another low level format--this time not only did it succeed...not a single bad sector was detected--just like the factory label indicates. Bonus!

I've been using the drive for a few days now and after a battery of tests and heavy use it seems to perform as good as it looks, brand new. :)

Anyway, hopefully this is interesting to some and maybe a little useful too.

Dieter
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Nice job! I had similar problems with an ST412 bought from ebay, the index sensor failed, so the speed sense circuit couldn't count 512 disc revolutions before recalibrating the heads to track 0. I've managed to get a pair of the exact match sensors as seagate uses on the ST412 drives from a retailer that keeps stock of them on ebay and in combination with a bit of WD40 on the spindle it solved my problem.

But after LLFing my drive, I also found that the two platters in the drive were heavily damaged, maybe from prolonged bad storage enviroment. Fortunately, I got some ST225 drives so I decided to sacrifice one for transplanting it's platters to ST412. With a lot of care and patience I experienced the same results as you did, the drive works fine and with no bad sectors. Unfortunatelly, you can't say that my drive is 100% original after losing it's unique dull orange iron oxide platters, but it's perfectly functional.

Thanks for sharing your repair experience!
 
It seems maybe these old hall effect sensors aren't the most reliable! In the case of my Miniscribe 3212 drives, the sensor is actually internal to the motor and out of reach under the flywheel. It seems an external sensor may have been used in some cases because there is an empty mounting boss cast into the housing where one could be mounted similar to your ST-412 uses, and a magnet hole drilled in the flywheel at the perfect location to pass an external sensor. I couldn't find one as shown for the ST412 on ebay, so I used a simple 3 pin radial-lead sensor and attached leads to it. I'd be curious to know which one you used from eBay?
 
WOW

Even at that time, I think these guys were hermetically sealed after being pumped full of Nitrogen (for heat dissipation and corrosion/Air-chemical reaction reasons).

Have you noticed the drives running hotter?

Great job if that is what floats the boat!!!
 
I agree, index sensors seem to be sensitive. I was ready to do the same DIY fix with the diodes/resistors modem7 posted (ingenious trick to revive the sensor) but I came across the same spare on ebay and so I bought a couple.

I've found the exact new spare sensor for the ST412 on ebay here: http://www.ebay.com/itm/MAGNETIC-PI...103?pt=LH_DefaultDomain_0&hash=item3caa49dcaf .They come in lot of twos and even with some schematics about their function. Seems though that there is a difference between hall effect sensors and variable reluctance sensors (transducers) but I'm not sure which is more robust and if a hall effect sensor can replace an older reluctance transducer sensor in old hard disk implementations easily. Reading further on the net, seems that hall effect sensors are miniaturized enoug to be mounted internally to the motor/spindle assembly as dkedrowitsch mentioned.

As for the platter transplant, the truth is that although my drive has zero bad sectors now I only power up my ST412 drive about once in every two moths and the maximum time it stays on is about half an hour. I didin't notice excessive heat or other trouble but I can't say for sure that the transplanted platter trick makes a robust and reliable drive for frequent/ordinary use. The ST506/412 service manual (http://www.minuszerodegrees.net/st506_st412_interface.htm) states that purified air flow ensures reliable performance with a 0.3 micron filter, but only prolonged use could answer griffk's suspicion. Afte all, theese components have long exceeded their design life expectancy...
 
They are open to atmosphere through a simple filter element. I've never seen a hermetically sealed consumer ST-412 type hard drive. The reason I had to swap the platter in my Miniscribe 3212 was because something had reacted with the oxide coating and ruined the disk. I assume heat and humidity was the culprit... Being sealed would have prevented that but I think maintaining a perfect seal on consumer disks wouldn't have been feasible.

Last night I stole thin film coated platters from a Seagate 277R that had developed a bunch of media defects and transplanted them into the Miniscribe 3650 which was the donor for my 3212. I was curious to see if a thin film disk could be used in place of oxide. The drive seems to be working fine so far haha. We shall see. If it continues to work long term, it's nice to know it's possible to use the more rugged thin film media to repair an aging drive with deteriorated oxide.
 
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