View Full Version : a 7500 proccesor upgrade gone awry

07-02-2001, 03:21 PM
Heres the deal, I have a Macintosh 7500 thatsh as 200 megs of RAM, 13 gigs across 4 drives, and a adaptec 13940uw scsi caed. For a while i also had a 604e 180 installed, but then i started having problems. My monitor would have all sorts of screen artifacts like neon green and pink dots when selecting items, or vertical lines when scrolling web pages. I also got a lot of static in the sound. Frustrated i put back the 601 100 that it came with and the problems went away. Is this something I am going to have to live with if i want to put it back or is there something else i can do. Someone suggested turning off the "Built-In Graphics Accelerator" extension but that didnt help. PLEASE

07-02-2001, 04:34 PM
With G3/G4 upgrades use to be you needed to turn off "Speculative Addressing" but I think that has been fixed... so you may want to look at G3 upgrade if you have available slot. RAM? video card? monitor aging or something? Zap VPRAM and PRAM with TechTool Lite 3?

I'm suppose to upgrade a freind's 7500/100 and trying to make sure I know what to beware of.


07-02-2001, 08:37 PM
Speculative addressing needs to be off. It is a G-series (and VERY late 604e) processor function. But definitely disable the Graphics Accelerator extension. And try to make sure you have good quality VRAM and RAM. I had similar problems with an 8500 and swapping VRAM solved it totally. Others merely have to disable the extension and it is fixed.

07-03-2001, 03:09 AM
you mention something about speculative addressing, is it possible to turn that off? you said to turn it off, how would i do that if i can? Will the VRAM affect the static in the sound? Or is that the speculative addressing? I would really appreciate any more help.

07-03-2001, 08:51 AM
Most likey the VRAM is not up to spec. It may have been when originally installed and running with the 100Mhz 601PPC, but it does not seem ot be anymore. Often this problem would show up when people isntalled G3 upgrades, since the CPU is so much faster than the stock, this often tested marginal VRAM. Apple generally installed 60ns main memory, but I think the VRAM was 70ns, maybe slower.

VRAM for that system should be cheap, but it may be hard to find. The Gurus may carry it.

You could get PCI video, this is much better anyway.

You could also get a G3/4 upgrade. These are fairly cheap and may or may not do anything. You may still have the same issue, but your system would be much faster. http://macgurus.com/infopop/emoticons/icon_smile.gif You also have more options for the bus speed. Mainly I am suggesting a G3 because its faster. Maybe your 180Mhz604e is having issues?

DSM is right, but here is some more info:
From what I understand there are two versions of 'speculative computing'. The G1/2 boards do support speculative processing (SP), but they do not support speculative access (SA). Speculative access is newer - this came in with the late 604e/Mach5-Kansas and G3 systems. I'm not sure if SA and SP are disabled with the G3 upgrades - they probably are, but its only a difference of 3% or so in performance. Its not a huge deal and it only affects G3 and newer upgrades on vintage PMacs. XLR8 and other upgrade companies has software that disables SA/SP on various systems.

Speculation is disabled in NVRAM on most upgrades, except Newer which disables it via an on board chip. This is done early in the boot process so its fairly safe. If its not disabled the CPU can corrupt data and possibly crash. This is more of an issue with a G4, since your system may not boot up if SP/SA is enabled. Since the G4 is so much faster it can spend a lot of time 'speculating' as it waits for data.

Life in the fast lane leads to:
The Resteraunt At The End Of The Universe

07-03-2001, 03:28 PM
The audio signal on the 7500 gets processed via the Grand Central chip in the I/O subsystem, then gets handled through the PCI controller (Bandit). The Video signals are all routed through the Chaos and Control chips. So the two don't really intermingle until they get to the processor bus. I would pull the VRAM in pairs (first one pair and run the machine, then swap pairs) and see if things work better. It might be a bad module. Have you run any RAM tests on your machine?

Thanks for breaking down the SA/SP issue. I never was totally sure what the deal was with the terms.

Did you try the system without the Graphics extension yet? That might be the fix.

07-03-2001, 04:24 PM
Thank you to all who have helped, i will attempt and post my results afterwards. I only have two VRAM modules though and they have to be in pairs so that might pose a problem to swapping pairs out. THank you anyways.

07-03-2001, 05:44 PM
My guess, based on the video artifacts is that the 180 MHz PPC604e is overheating. The 180's were some of the most power-hungry 604s, using well over 10 watts, if I remember correctly.

So, I think that the lower powered 601 is staying cool and the high power 180 is running hot.

So try a few things. First, make sure that the fan in your 7500's power supply is running. Second, you may wish to put a fan on the CPU or if you have a free PCI slot, perhaps add a PCI slot cooler. This is a fan that exhausts air from your computer case through a PCI slot. However, you'd probably prefer not to spend any money until you've tested this theory.

You might try disconnecting three of your four drives and testing the machine with the 604e/180. That should cut way down on heat production. If you are mechanically inclined, consider unscrewing the heat sink and replacing the heat sink grease between the CPU and the heat sink. You can get a tube of adequate heat sink compound at Radio Shack for about $3. If you want to try that and don't know how, ask.

If you have another 604e card available or a G3 card, test with that. I really think that the combination of four hard drives (assuming they're all internal) and the power hungry 604e/180 plus the change to summer temperatures is probably causing a heat problem.

07-04-2001, 01:58 AM
Thank you trag, this gives me another option to try. Today i took it into an open room where i would be able to work with it easily. I cleaned the contacts on all RAM and VRAM. BTW, the VRAM is 70 ns. I cleaned the contacts on both of my processors and then ran the computer with case completely open, drive bays folded out and everything. I noticed fewer artifacts after this but they were still there, so maybe it was heat seeing as how i got less artifacts when it was very well ventilated.

07-04-2001, 01:43 PM
I know many of my x86 friends run with their cases open and a large fan blowing on it with the AC cranked up! http://macgurus.com/infopop/emoticons/icon_smile.gif

I think those late model 604e chips used about 18 to 25Watts... there were the power monsters of a byegone erra.

Looks like you got this one Trag http://www.macgurus.com/ubb/tongue.gif http://macgurus.com/infopop/emoticons/icon_wink.gif