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the slow Performa 640 CD


Well-known member
Recently I acquired a Performa 640 CD DOS Compatible, after not owning a 68k mac for over a decade. I got it set up, put 8.1 on it because that's what I had handy. Felt really slow, like, were 68040s really this slow? You can see windows draw piece by piece.

Started system profiler, it says it's a 20 Mhz 68LC040...huh? Eventually I burned a system 7.5 restore disc and after some false starts managed to get TattleTech and Speedometer on it. TattleTech says I have a MC68040 with a 33 MHz bus running at 7.09 MHz (1772). Speedometer benchmarks put it about 1/8 as fast as a Quadra 605. Markings on the chip are XC68LC040 at 33 Mhz as expected. So, what would cause a 68040 to run slower than its bus speed? Have any of you seen anything like this before? I assume the ROM sets up the clocks

What I have tried:

- Replaced the fan. (I didn't even know it had a fan until after some disassembly I saw it in there, not spinning. So I don't know how long it has cooked itself in the previous owner's possession)
- Resetting PRAM (which is really just disconnecting power, the PRAM battery is dead)
- Removing RAM, reinstalling OS, etc.

What I have not tried yet:

- Replacing the dead PRAM battery. Waiting for a 3-AAA holder to arrive
- Taking the DOS compatibility card out
- Replacing the logic board caps
- Replacing the 68LC040. I do plan to put a full 68040 in it, but I was hoping to verify the logic board works correctly before I buy one, they're getting pricey
- Measuring the clock output with a scope. I'd have to haul one home from work, and figure out how to run the logic board on the bench
- 40 Mhz resistor mod. Only after I get everything else working, and even then, I hate surface mount tiny thing soldering.

Any ideas?


Well-known member
That’s very odd. According to EveryMac, the CPU should indeed be running at 33 MHz:


It doesn’t look like these Macs used any SW/ROM configuration of their clocks. They have a clock generator IC that outputs the desired frequency based on the resistor settings, which is what the 40 MHz resistor mod changes:


It does look like this logic board has electrolytic caps that should be replaced, and the board may need a good cleaning if they leaked:


I’d take care of that before doing anything else invasive.


Well-known member
Update: I took the DOS card out, put the '040 in the main socket, and cleaned the board a bit. It boots and shows 33 MHz, and is now much nicer to use.

I have replacement capacitors on order. I don't see any leakage, but I may as well replace them before they do. Then I'll tackle the DOS card.


Well-known member
I replaced the capacitors (fortunately no major leakage) and reinstalled the DOS card, and it’s all working at proper speed. A lot of nostalgia in a small package :)