pictures are worth a 1000 words. also finding model numbers helps.

Pentium Overdrive, AMD or Cyrix 5x86 are probably going to be the best choices. The Pentium OD and Cyrix chips are usually more expensive than the AMD if budget is an issue.

Model numbers are:

PC Chips M912 ver 1.7 aka DX-6900
UM4980 v2.1

I'll get some pics up asap.

According to the documentation I've been able to dig up, both will support the DX2 (with the UM4980 supporting just about every 486 under sun!). I guess my real question is - do I need to find procs with the same pin count as the socket 3 or do all the different pin counts of the proc listed in the documentation fit?

Of course these boards are still untested so it all may be for nothing. At a dollar for both boards, I had to try! 😉

Make sure that the board says it supports the P24T (Pentium Overdrive) before you bother with that CPU. Even then you may have weird problems with it. Support is very spotty. Some boards will claim to support it but it will still be trouble because of spec changes during the CPU's development.

Physically, Socket 3 will take any 486 class CPU. But some CPUs have features that need specific support BIOS and/or chipset support. Like writeback L1 caching for example. Some chipsets even need to be a later revision in some cases.

Yep socket 3 will physically take any 486 CPU- most CPU's will leave an empty row of holes along each side of the socket. The Pentium Overdrive will use all holes.

Just gotta watch out for the voltage, most 486's up to DX2 were 5V but a lot of later ones like DX4, 5x86, etc were 3.3V only- confirm that your board supports the correct voltage and you have jumpered it correctly before using a 3.3V CPU.

Those are all good points. If your main intent is to test motherboard functionality, I'd start with one of the more stable, yet fast-ish CPUs, perhaps an Intel DX4-100 a 3.3 volts. Once you've figured out that CPU works well under what ever OS or test you desire, move on up the latter to something more fancy like an AMD X5-133, then AMD X5-160, and perhaps a Cyrix 5x86-120 if you can find one. I would be surprised if a PC Chips motherboard works well with a POD-83 in WB mode.

You can refer to the Ultimate 486 Benchmark Comparison for the fastest socket 3 CPU to fit your needs.

All good points made, the only thing I will add is that Intel Overdrives (you will know if you have one) are 5v only, including the DX4 variant (which is internally 3.3v but runs at 5v externally).

Oh, and try to find a manual first before attempting to replace any CPU.

And watch the orientation!

Just an update:

The PC Chips board fired right up using a DX2-66 and corresponding jumper settings. Also, the ISA Trident video card and the VLB IDE interface card I pulled from the same "junk" bin work! Not bad for a couple of bucks.

Question: What kind of cooling does the DX2-66 need? I just have the chip in the socket right now with an external fan blowing air on the mobo.

I couldn't get video from the UM board but it gives me a couple of beeps at POST. It has a Phoenix BIOS so, from what I've read, this could be a video, memory or motherboard problem. 😜

a dx/66 I would highly recommend having a heatsink on. All of the OEM systems I ever opened had one. very few had hs+fan, but it wouldnt hurt.

humanoid wrote:

Question: What kind of cooling does the DX2-66 need? I just have the chip in the socket right now with an external fan blowing air on the mobo.

I prefer to use a wire clip to mount a Socket 7 cooler. That is, if the Socket 3 socket actually has the thingies on the sides

I think I have a couple of socket 7 coolers. I'll see if they fit. Thanks!

socket 7 coolers are probably half an inch too big, both ways. Once upon a time, when places like comp-usa existed, they had cheap $10 "northbridge" coolers that attached with thermal tape, and I used them a lot.

Hmmm... Couldn't find any hs/fans that appeared to fit. Here's the socket:

Are the circled tabs what I should be able to attach a clip to?

yes

One of Zalman's little northbridge coolers could be fit with some ingenuity / thermal tape.
http://www.zalman.com/ENG/product/CategorySec … orySecond=CAS63

Alternatively, visit your local electronics store and they should stock some thermal adhesive paste. You can stick a heatsink on with that and forget about the clips.
I think Arctic Silver makes a thermal epoxy as well but once its stuck on, you won't be able to remove the heatsink easily if you use that.

Lemme look up a pic of how I did it in my own 486...

Edit: Couldn't find a pic of the insides of the rig itself, but heres the cooler that will fit Socket 3 with tabs

You can even exchange the heatsink with the largets that will fit, provided the heatsink isn't too "thick" or the metal clip needs too much force to bend downwards

Edit2:

humanoid wrote:

Are the circled tabs what I should be able to attach a clip to?

Yup! 😀

Yup that is my favorite 486 heatsink attachment design. But it's probably more typical to find heatsink fans that snap over the CPU itself or use thermal glue.

I think OEMs liked glue and enthusiasts got these:

common old snap on type w/ fan that dies.

although some people were getting into Peltier junctions back then for no good reason 🤣

Thanks for the tips, guys. I was at Fry's Electronics yesterday and, believe it or not, they still stock 486 fan/heatsink units (like the one swaaye posted). I picked one up and ordered a bit of thermal tape to try out (the little clips don't do it for me). If the tape doesn't hold up, Fry's also had the thermal adhesive on the shelf.

Anyway, the 486 is in a holding pattern as I think the VLB IDE interface card may be faulty (wouldn't recognize the serial mouse). I ordered another one, so we'll see if the card was the problem.

In the meantime, I've almost completely assembled my MMX 166 machine. 😁