6.2. How do I open the case and get to the motherboard?
[Courtesy of Norman Yarvin:]
Three sets of screws hold the case together. The first set
consists of two screws underneath the keyboard retainer posts, and
three screws in the rear panel. (The keyboard retainer posts hold the
keyboard to the base -- one is next to the socket for the keyboard
plug. Note the seam around the top of the retainer posts. Remove the
covers from the posts, and you'll find a screw inside each.)
When this set of screws is removed, the top (plastic) part of
the case is attached to the bottom only by some cables, which have
enough slack to let you take the top of the case off, tilt it
backward, and rest it on its rear behind the machine. However, this
is quite a difficult task. This is because it is hard to get the very
front of the case up; the plastic is shaped in such a way as to catch
on to the metal bottom somewhere. A bit of wrestling with the case is
usually necessary here, and some people prefer to take the top part of
the top part off (the next paragraph) before taking off the bottom
part of the top part.
The second set of screws holds the top part of the case
together. It consists of six screws, pointing upwards, which are
found under the overhang of the case. The only time they really need
to be removed is when replacing the fan. In addition to these screws,
two or three plastic hook-and-tab latches in the front of the machine
need to be released. The top of the case must be moved outward
slightly (usually with a screwdriver or finger pressure) along the
joint to the right of the floppy drive in order to release the
catches. [The three plastic catches in the front are in the middle,
and at about the 7th-to-10th louvers from the left and right of the
Taking off the top of the case exposes only the floppy drive,
the hard disk (underneath shielding) and the power supply. To get to
the motherboard, you then have to unscrew the third set of screws --
three screws at the very front of the machine. The two latches on
either side of the motherboard cover become obvious when one tries to
lift up the panel, and are easily released. The motherboard cover
pivots at the rear; it can be lifted up. To lift it up more than an
inch, the power supply cable and the 10-pin video cable must be
unplugged. After that the metal shielding can be lifted and slid
along the tracks and then flipped up or removed. This exposes the
motherboard. If you need to remove the motherboard shielding
completely and the monitor assembly, you need to disconnect the floppy
and hard disk cables from the motherboard (note the direction of the
connectors, and when replacing them be very sure that the 20-pin and
34-pin connectors are seated correctly on the associated pins). If
you have a PC7300 power supply and motherboard, your floppy drive
power cable might also be connected to the motherboard. To remove the
whole assembly, in that case, you'll need to remove the power
connector from the back of the floppy drive as well.
Be careful when putting the top of the Unix PC/3B1 back on; a
common cause for the machine not powering up after the cover is
replaced is the 120VAC connectors (brown and blue wires crimped on)
falling off the power input, or being pulled off by the hard drive as
the case is closed. The green ground wire has a tendency to get
caught in the fan blades (causing the fan not to start spinning when
the machine is turned on) or caught in the case itself. Check that
the fan is working after closing the case.
As always, use your head. Be careful when exposing any
component of the Unix PC. There are many static sensitive components;
ground yourself before poking around inside.
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