Hi,

> >Would it not be a good idea to swap a Q840AV chip with the Q800?
> >The Q840AV also has 4 VRAM chips which I presume are the VR256K SIMMS,
> >but I'll check. [No Q840AV ever got 4 512K VRAM chips, or other, did it?]
>
> OK, I think that you're "mixing metaphors" here. To switch chips
> would mean to pull the 80Mhz 68040 out of the 840AV's motherboard and
> shove it into the 800's motherboard, trying to essentially overclock
> the machine. The "chip" has no VRAM or VRAM sockets...what it sounds
> like you're trying to do is swap *motherboards*, essentially using
> the 800's case and power supply to drive a Quadra 840AV.

To expand on this a little, every Quadra 650 / 800 I've ever set up runs
fine at 40 MHz. The Quadra 650 / 800s use a 33 MHz 68040 generally with a
manufacture date newer than the middle of 1993, and run much cooler than,
say, a 33 MHz 68040 from a Quadra 950, which are generally older. The
older chips may not work at 40 MHz, but every newer one I've tried runs
fine at 40 MHz.

This includes all seven of my Quadra 605 / Performa 475 motherboards which
have newer 25 and 33 MHz '040s running at 40 MHz.

In order to do this on a Quadra 650 / 800, find the oscillator next to the
innermost NuBus slot (16.667 MHz) and replace it with a 20 MHz oscillator.

If you have a Quadra 840AV, it can usually be safely overclocked to 50
MHz. Don't remember the location of the oscillator on that. But if you
don't have a case, a milk crate with a bunch of zip ties will do. I have
two Quadras set up this way.

No matter what you overclock, though, make sure you take off the heat
sink, wipe off the old and usually dry compound and put on new heat sink
compound. Keeping the chips cool is important for stability.

John
Sixgirls Computing Labs