The Macintosh 840av was the big brother to the 660av. This Mac used the same case as the Quadra 800 allowing it to be able to hold to 1/2 height hard drives in it's internal bays. All the wiring was already installed so other than obtaining a second SCSI hard drive sled installation was a breeze.
There is a considerable amount of space at the bottom of the case to allow clearance for long nubus cards such as another video card.
This machine isn't quite as easy to pop open and work on as the 660av but not the hassel that people will discover the IIvx/IIvi/Centris/Quadra 650/PowerMac 7100 case was. The only trick is to remove the single screw holding the motherboard in place and then unplugging all cables up top before sliding the motherboard out of the case.
Though you can put 128 mb 72 pin simms into the four slots on the motherboard these particular memory chips will not allow you to reinstall the motherboard because they are too tall and contact the case so I was not able to test these chips to see if the Mac can even read them. My guess is that they will not work because they do not work in the 660av Macs.
Not much you can do upgrade wise with the motherboard other than nubus cards and swapping in larger vram simms.
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