Apple LaserWriter 12/640 PS - Manual (Page 183)

Check the Phillips screw holes on the back of the printer. The holes should
be unobstructed. (You shouldn’t replace the Phillips screws until the side
cover is properly positioned.)
If the hole is obstructed, remove the side cover and try again. It may take a
few tries.
3 With the back cover properly positioned, replace the Phillips screws. Do not
overtighten the screws.
4 Next, configure the printer software to use the additional memory.
For configuration instructions, see “Configuring Your Printer Software for
New Options,” next.
If you want to check that you have properly installed the additional printer
memory, turn the printer back on and check the startup page. The amount of
printer memory should be listed there.
The printer does self-diagnostic tests each time it is turned on. During this
testing, the printer verifies the amount of memory installed. If the diagnostics
test determines that the installation is not correct, the Paper Out and the
Paper Jam lights will flash alternately. Check that you have installed the
SIMMs properly in the correct slots. If this does not resolve the problem,
contact an Apple-authorized dealer.
158 Chapter 5