Performa 638CD upgrades for fun

[Macdrone]

My original 638CD has been a great machine since 93 and been everywhere and back again.  I figured I would show it some love.

First did the 40 mhz upgrade shown here.. http://lowendmac.com/2014/overclocking-the-mac-quadra-series/

Put in a real 40 mhz full fpu cpu with heat sink, put a heat sink on the video chip as suggested.

Put in a 2 gig hard disk.

Replaced the 300i with a 600i to keep it good and bootable.

Added a comm slot ethernet.

Loading original software now.

Its a 2 ram slot board.  FYI when they silk screened one slot 32 meg and the other 16 meg (lower sizes also listed on same slot) they meant it here.  I put in 2 32 meg simms, no boot chime no start.  Had to put in exactly what it showed.  52 megs of ram is better than the 20 that I had in there.

 

[register]

Might an 128 MB RAM work along with the 16 MB RAM?

 

[Macdrone]

i put higher than what the slot said and no boot, before I even did the resistors.

 

[johnklos]

These machines will take a 128 meg SIMM and a 64 meg SIMM for a total of 128 + 64 + 4 megs, or 196 megs.

 

[Macdrone]

mine won't.  I put in a 64 and a 32 and it would not boot.  According to the overclock page there was a rev a and a rev b so maybe my rev a wont take it.

 

[Macdrone]

Looks like it needed new caps.  Cleaned ram slots, that didnt do it, replace those cans and got 2 64's to work.

which is plenty for a 68K mac

 

[Bunsen]

IIRC you have to have the right kind of 128MB SIMM, but I forget the details.  The other slot should take either 32 or 64.

 

[CC_333]

Having a split arrangement like that strikes me as being rather strange. Why couldn't Apple just have both slots accept equal amounts of RAM?

I'm sure that somehow there's a good reason for it (cost-cutting, architectural deficiencies, etc.), but I still don't get it.

c

 

[rsolberg]

CC_333, it has to do with the onboard ram- it behaves as one side of a two sided module, limiting what can be installed and recognized in the "16MB" slot. I've had a few 486 PCs with a similar configuration involving a 32 bit bus and 72 pin SIMMs. 4MB onboard, four empty slots (rather than the two in this case). Three slots will accept 16MB SIMMs, the fourth is wired to the onboard ram and will only accept a 4MB module- no larger or smaller, and it must be single sided. When installed, the memory controller reports an 8MB SIMM is installed in that slot, a 4MB when it's empty. I think it's ultimately a cost saving measure, as the RAM controller would have to address a third "module" if the onboard RAM weren't sharing that slot.

Pentiums and PPCs are a different story - they typically use a 64-bit memory bus, so 72-pin SIMMs must be installed in matched pairs in order to fulfill the 64-bit requirement. Common DIMMs are 64-bits wide, so they can be installed individually.

*This is a generalisation, I know there are some exceptions.

 

[CC_333]

That makes sense. But, provided the needed address lines are there, couldn't one like run those extra lines from the onboard RAM (by either piggybacking it, or eliminating it entirely) to the "16 MB" slot and make it work like a proper slot comparable to the "32 MB" slot?

Maybe I'm missing something, as I don't know much about this stuff (yet!).

c

 

[Macdrone]

at 136 mb now I dont think I would need more.  I dont know anything on a 68k that uses that much.  I dont do photoshop, i guess it could.

 

[Floofies]

Add a resistor to the system fan to make it quiet! I did this on my Quadra 630 (essentially the same) and it's nice. Even with the 040 running at 40MHz you don't need the fan running at full blast, so a 120 or 150 Ohm resistor inline on the fan cable will make it nice and quiet.

 

[Macdrone]

I dont mind the normal fan speed, pretty quiet really.

 

[Floofies]

Really? Must be a different fan than mine... it's incredibly loud, even moreso than my MDD G4 (The "loudest mac ever").

 

[Macdrone]

The desktop performa 638 is really quiet.  Might want to replace yours if its noisy.  Ill start up a few of mine and find a quiet one if you want one.