Xref: utzoo comp.sys.mac.hardware:10234 sci.electronics:19251
Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mit-eddie!bbn.com!gonzalez
From: gonzalez@bbn.com (Jim Gonzalez)
Newsgroups: comp.sys.mac.hardware,sci.electronics
Subject: Mechanical Problem On 400K Drive
Message-ID: <63691@bbn.BBN.COM>
Date: 12 Apr 91 16:02:29 GMT
Sender: news@bbn.com
Reply-To: gonzalez@BBN.COM (Jim Gonzalez)
Followup-To: comp.sys.mac.hardware
Distribution: usa
Lines: 43

I'm attempting to repair what appears to be a common problem on Mac 400K
drives.  I've managed to identify the cause, but I need advice on the
best approach to correcting the problem.  Our chief technician says that
they buy new drives rather than attempt to repair them.  800K drives
aren't an option, since the machine in question has the ancient
firmware, 400K drives are no longer available, and I anticpate this 
failure recurring, so I want to learn how to fix them.

The drive fails to eject under computer control, and ejects manually
only with considerable effort. In addition, once a disk has been
ejected it is all but impossible to insert a new disk because the
mechanism is no longer aligned with the cabinet opening.  

The way the mechanism works is that a tiny motor controlled by the CPU
actuates a cam via a set of gears.  The cam, in turn, draws a slide
back along the right side of the drive.  It is this same slide that you
are pushing back when you push a paper clip through the hole in the
front of your Mac.  Anyway, the slide travels back about a quarter
inch, at which point what appears to be a small arm with a button
swings down to lock the slide in place, allowing the diskette to be
ejected by spring force.  My feeble attempt at a picture follows:

 paper				   arm with button, poised above notch
 clip			   O==== <-  in slide (obscured by other parts)
 pushes	.----------------\___|----------------\____.
 here ->|					   |    cam pulls from
        |__________________________________________| <- here

The problem is caused by this arm sticking, apparently due to dried or
dust-ladden lubricant (there may also be a worn=-out spring involved).  
Instead of snapping into place, it drifts down over the course of several 
seconds.  Unfortunately, this is too slow for the spring-loaded slide, 
which snaps back and prevents the button from dropping into place.

I have two questions.  First, what is the best way of relubricating the
arm?  I was thinking of swabbing the area with TF and then adding a bit
of fresh silicone grease.  Second, has anyone studied this mechanism
enough to know if there is a spring that I might need to replace as
well?  The whole thing is held together with snap rings, and I don't
relish the though of pulling this stuff apart.  It's sort of like the
guts of a huge  wristwatch.

				-Jim.