Overclocking Pentium-75s

Last Updated on July 15, 2017 by Dave Farquhar

I had an overclocking conversation at work today. A coworker wanting to overclock a laptop. I told him I didn’t think that was a good idea. Then this was waiting for me at home:

From: Curtis Horn
Subject: Pentium-75

Hello, I’m one of your readers and I check your view for the tips you sometimes put up. I’m working on a compaq pentium 75 also and maybe you can do what I did. Overclock the chip to 90Mhz. the way I did this is by changing the bus speed to 60Mhz, from 50. this has speed it up significantly I think because the memory is also speed up. You’ll also get a laugh out of this, it’s a Compaq 972 and it has 8MB of memory — ON the Mother board!! I could not believe it. but it’s there. Luckily this leaves 4 simm slots open, so i can add 4 8MB SIMMs. (16MB SIMMs are way to expensive and I have some 8MB laying around and can buy some more for 10$ each) I convinced the person to buy a 5Gig quantum drive, so they have something they can use when they upgrade. Well hope the P75 you’re working on OCs as easily as the one I have here.

Curtis

http://homes.arealcity.com/cis/

Compaq used to put a fair bit of memory on the motherboard itself. My Presario 660 (a 486/66) has 4 MB on the board. There are a couple of Compaqs from the 900 series still floating around at work that have 8 MB on the motherboard as well. But it’s not a common practice anymore, and I don’t recall any other manufacturer who did that regularly–I remember Compaq doing it because I used to sell Compaqs by the truckload and frequently I ended up adding upgrades to them.

Bus speed isn’t nearly as important in the Pentium Pro/II/III/Celeron and AMD Athlon arena, but in Socket 7 and earlier, you’re right, it makes a huge difference. Remember, the bus speed determines the speed at which the CPU can access the memory and the cache, and as the Mendocino Celeron illustrated, cache speed is more important than CPU speed or cache size. In the early days of Tom’s Hardware Guide, Tom Pabst revealed that a Pentium-150 running at 75 MHzx2 outran a Pentium-166, and a Pentium-166 running at 83MHzx2 outran a P200. So what was the point of buying a P200 if you weren’t going to overclock it, right? Ah, the good old days…. This, of course, was one reason Intel decided to start locking CPU multipliers.

The speed of the PCI bus was also tied to the bus speed. A good Pentium-100 could outrun a Pentium-120 because the Pentium-100 had a full 33 MHz PCI bus while the Pentium 120’s PCI bus ran at 30 MHz. The Pentium 75’s PCI bus ran at a pokey 25 MHz. Nobody wants to slow down their video and disk performance by 10 percent, let alone 25 percent.

Overclocking P75s is risky business though. Intel never intended to make a P75. The problem was, they had terrible yields initially on their P90s, but they found a good percentage of the bad chips would run reliably at 75, so they created the P75 and phased out the P60 and P66. (The P66 was actually a better performer because of the bus speed.) The P75 sold like crazy, and Intel wasn’t going to can a best-seller, so once they got over the yield problems, they still marketed P75s. I’ve heard of people going as high as 133 MHz with P75s. I experimented once with a P75 and took it as high as 120 MHz, but couldn’t get 133 (I suspect people getting to that level may have been increasing the voltage). It didn’t run reliably at 120 MHz for long, though I know of people who swear up and down they got 75’s running at that speed reliably with no special tricks.

Overclocking an old chip like that is fine, as long as you’re aware of the risks and willing to live with them. I’d definitely put a heavy-duty CPU fan on it (like a PC Power and Cooling fan for a high-end K6-2). In my case, I’m more interested in having a PC that’s as reliable as possible. Her life’s plenty complicated enough without having and overclocked P75 to deal with.

And we now have better ways to measure overclocking’s effects. Microsoft doesn’t have a dog in this fight but they see the weirdness.

But thanks for the idea, and for the stroll down memory lane, definitely.