🤣, try to make sure you didn't leave a standoff underneath your motherboard like I did. I have the EXP-4045, brilliant board tho, I like it a lot.

5x86 has an internal multiplier. Make sure to disable whatever multiplier settings you have, and use the base clock, otherwise you could fry the chip (5v can DEFINITELY fry the chip, so it's likely you have already cooked it)

Ensure it has proper cooling. Heatsink and fan is a MUST for anything over 66mhz, and anything P5 and over.

Hi Ampera

I built this 486 last year and been using it as a DX2/66 for a long time, so I know the system is stable. No standoffs where they shouldn't be or anything like that 😀 It is a really great board, just wish it would boot the 5x86 133!

From what I've read, the 5x86-133 uses the DX4 multiplier setting? So if it's set at 2x it'll be 4x internal multiplier (4x33 =133), if it's set to 3x it'll use that (3x33=100). I found some info said that certain boards will only run it at 3x multi as it's basically a DX4-100 at that point, which my board should support, but it doesn't help me 🙁 I am using regular 33mhz clock

I do have a heatsink/fan attached, I thought I'd try 5v as a last ditch effort but that didn't help either! Perhaps the chip was a dud to begin with?

Yea, then I don't know, it's possibly DOA. They are fairly robust chips, but this can happen.

Try using the jumper settings for AM486DX/DX2 instead of DX4, but leave the voltage at 3.3V. I have a 486 PCI motherboard which uses the exact same jumper settings for the Am5x86 and the enhanced 3.3V Am486DX2. The DX4 multiplier jumpers are ignored in that case and the chip internally sets it to 4x and runs at 133MHz.

I did try that also, for my motherboard only one jumper is different between 486DX and 486DX4 (JP43) so I did give that a try, no dice sadly 🙁

I've ordered myself one of those ISA/PCI diagnostics cards with a hope it might shed some light, otherwise looks like I'm going to have to shop for a DX4-100

Just to be sure, have you put back the DX-2 to see if it will still work with that chip?
If that's the case you can be pretty sure that everything except for the 5x86 is working and nothing broke while you fiddled around with your rig trying to get the 5x86 to work.

Yep, back running with the DX2/66 and currently reinstalling Windows 95 (upgraded to a larger hard drive as well) it is indeed just the 5x86 that won't boot. I've got a DX4/100 on the way which is on the official support list for my motherboard, so fingers crossed that one works!

I really just wanted to get a maxed out FPS in Doom, my DX2/66 on the Doom benchmark hits 28fps currently. So close!

I've been using my AMD 486 DX4 100 MHz (L1 write though version) as my main DOS PC for years and it runs Doom at 35 fps. If you got the Intel version, then you should see sightly better performance.

My early revison Asus PVI-486SP3 won't run the AMD 5x86 wthout issues because of its old BIOS and the newer BIOS with full AMD 5x86 p75 support won't work with the flash chip my board is equipped with.

I settled for an Intel DX4-100 running at 120 MHz, the Intel DX4 at 3x40 MHz performs near identical to an AMD 5x86 p75 running at 4x33 MHz.

I don't know if every Intel DX4-100 wll do 120 MHz but I do know that not all AMD DX4-100 CPUs are stable at 120 MHz so there are no guarantees.

Most DX4-100s are fine at 120Mhz. SV8B above and your golden for AMD and Intel.

I currently am running a Socket 3 AM486-DX4-100 and OC'd it to 120Mhz (It was actually by the guy who sold me the board/chip/RAM combo) It has ran stable ever since with 0 problem.

I actually wonder if your 3.3v regulator is busted? Do you have a meter you can test it with?

Skyscraper wrote:

My early revison Asus PVI-486SP3 won't run the AMD 5x86 wthout issues because of its old BIOS and the newer BIOS with full AMD 5x86 p75 support won't work with the flash chip my board is equipped with.

I settled for an Intel DX4-100 running at 120 MHz, the Intel DX4 at 3x40 MHz performs near identical to an AMD 5x86 p75 running at 4x33 MHz.

I don't know if every Intel DX4-100 wll do 120 MHz but I do know that not all AMD DX4-100 CPUs are stable at 120 MHz so there are no guarantees.

Interesting... Does the size of the newer BIOS exceed the capacity of your board's flash chip? AFAIK the latest BIOS is 1Mbit in size and 1M flash chip isn't that expensive.

My board (acer ap43, pci) does not support the amd 133. It does post but is extremely slow and doesn/t understand it so it switches caches off and even the harddisk is not found.

Running it as a normal dx4 and it is fine, using the 3x multi. It now runs at 3x50mhz and is pretty fast. The goal was 4x40mhz (better timings) but nomatter what i do the 2x=4x multi gives problems.

So i think a 5x86 chip will always boot as long at it is set as a dx4 (3x multi).
I think yours is just defective.

I have a replacement on the way to try out, I'll check the voltage before I pop the chip in to make sure it's a steady 3.3v. Thanks for the replies guys. I also have a DX4-100 coming as insurance 😀

I also just picked up the same model of computer I had back in 1996-1998, an IBM Aptiva (Pentium 120) which is now making me quesiton whether I really need the 486 at all. The Aptiva is more powerful, and has significantly more nostalgia value for me. This hobby seems to send me in constant circles!!

lazibayer wrote:

Skyscraper wrote:

My early revison Asus PVI-486SP3 won't run the AMD 5x86 wthout issues because of its old BIOS and the newer BIOS with full AMD 5x86 p75 support won't work with the flash chip my board is equipped with.

I settled for an Intel DX4-100 running at 120 MHz, the Intel DX4 at 3x40 MHz performs near identical to an AMD 5x86 p75 running at 4x33 MHz.

I don't know if every Intel DX4-100 wll do 120 MHz but I do know that not all AMD DX4-100 CPUs are stable at 120 MHz so there are no guarantees.

Interesting... Does the size of the newer BIOS exceed the capacity of your board's flash chip? AFAIK the latest BIOS is 1Mbit in size and 1M flash chip isn't that expensive.

If I remember correctly it isn't a BIOS size issue it's just that the new BIOS won't work correctly with the older model of flash chips. You can flash the BIOS and the board sort of works but with alot of bugs. Replacing the BIOS chip with one of the models later PVI-486SP3 boards use do fix the issue but I could not be bothered as I have other 486 boards that recognizes and runs the AMD 5x86 P75 perfectly. There are plenty of information about this BIOS update issue with the Asus PVI-486SP3 spread over a few threads here on Vogons if you want to know more.

It made sense to use the Intel DX4 for the Asus 486 build which is a remake of my first fully custom PC build back in 1994. The original build had an AMD 486 DX2 CPU though but later got upgraded with an AMD DX4 running at 120 MHz.

Link to my Asus PVI-486SP3 build thread. AMD DX5-160, Asus PVI-486SP3 desktop system

Well, don't I feel like a bloody numpty. I stripped my case bare today to test out the DX4-100, so I could see what I was doing. Sure enough, connected to the pins in the manual for changing voltage.. is nothing. No transistors.

I'm sure I would have noticed before had I not crammed 2 VLB cards 3 ISA cards several IDE cables etc. all into a mid tower ATX case.. But enough excuses.

It looks like 3 parts are missing, Q1, Q1A and Q1B, all the other resistors etc. seem to be there. I'm pretty handy at soldering but have no idea what values should go in, anyone more electrically minded than me that could tell me how I could figure it out?

ADD:
Also in my defense.. The board came to me installed with an AMD DX4-100, I can only assume the seller had it running at 5v

You can try to find a socket VRM module that will turn 5v into 3.3v.

But any DX4+ chip you install into 5v, you fry pretty much.