First question would be whether the board has multiple clock rates or is stuck at 25 MHz.

It appears to be fixed. There's no config jumper listed, and I can't find any unnoted ones.

Also, it is a socketed processor with no upgrade socket. This is information I intended to provide in the initial post.

A DX4-75 would be the easiest and most compatible upgrade, I think. The Intel DX4ODP75 comes with a built in voltage regulator and heatsink. You'd have to weigh that against buying the three parts separately. You could get a DX4-100 upgrade and just run it underclocked too.

As you mention, the Pentium OverDrive was late. It is expensive and you don't know what compatibility issues you might have.

Different part numbers for different boards with different processors suggests each board was a one trick pony... https://eprm.ardent-tool.com/eprm/f108.htm

jakethompson1 wrote on 2021-07-14, 23:13:

A DX4-75 would be the easiest and most compatible upgrade, I think. The Intel DX4ODP75 comes with a built in voltage regulator and heatsink. You'd have to weigh that against buying the three parts separately. You could get a DX4-100 upgrade and just run it underclocked too.

Might be a miscommunication on my part, but isn't the ODP for an upgrade socket (which I don't have) and I'd need an ODPR instead? And everything else I'm seeing says there's no built-in voltage converter/

jakethompson1 wrote on 2021-07-14, 23:13:

The Intel DX4ODP75 comes with a built in voltage regulator and heatsink.

If you're replacing the existing CPU, make sure you get the DX4ODPR version. The DX4ODP is meant for an overdrive socket.

If you can find an interposer the P24t will be the fastest.

There also should be ways to force other bus speeds no matter what if your willing to dig and mod.

Cyrix and AMD 5x86’s with a voltage regulator will run 4x25=100mhz as well

Your onboard video and cache situation will hold you back even with a better CPU but you might approach DX2-66 local bus performance with a better chip (past experience is the DX4-75 was a tad slower than a DX2-66)

Good Luck

What should I search for to find an interposer? Is there a specific one for each upgrade option?

EDIT: It was suggested elsewhere that configuring the jumper to the "Pentium Overdrive" setting listed in the manual would reduce the voltage to 3.3v. Is that likely?

ultimate386 wrote on 2021-07-14, 23:24:

jakethompson1 wrote on 2021-07-14, 23:13:

The Intel DX4ODP75 comes with a built in voltage regulator and heatsink.

If you're replacing the existing CPU, make sure you get the DX4ODPR version. The DX4ODP is meant for an overdrive socket.

Are you sure? I see that someone posted a picture of the motherboard now, and I thought so too at first, seeing that it's not even a ZIF socket, but if you look at the lower right interior corner it does look like there is a 169th hole.

Gnoman wrote on 2021-07-15, 00:43:

EDIT: It was suggested elsewhere that configuring the jumper to the "Pentium Overdrive" setting listed in the manual would reduce the voltage to 3.3v. Is that likely?

I'am afraid there's nothing on that board that looks like a voltage regulator for 3.3V, even if they went sci-fi and rendered it in Goa'uld crystal tech or Star Trek Voyager bio-neural circuits.

jakethompson1 wrote on 2021-07-15, 01:04:

ultimate386 wrote on 2021-07-14, 23:24:

jakethompson1 wrote on 2021-07-14, 23:13:

The Intel DX4ODP75 comes with a built in voltage regulator and heatsink.

If you're replacing the existing CPU, make sure you get the DX4ODPR version. The DX4ODP is meant for an overdrive socket.

Are you sure? I see that someone posted a picture of the motherboard now, and I thought so too at first, seeing that it's not even a ZIF socket, but if you look at the lower right interior corner it does look like there is a 169th hole.

I'm never 100% sure anymore 😜. BUT, the pictured motherboard does not have a soldered CPU (although it does have the pads for one). That 169th pin is just for disabling an existing CPU.

Right now the jumpers are set for 25mhz CPU.
If you want to add a 33mhz bus CPU you will need to change the jumpers.
Its not worth it to upgrade from a 486dx2-50 to a 486dx4-100
The performance boost will be only about 10%

But if you upgrade to the Pentium Overdrive 80mhz. The performance boost will be substantial.
This is the only CPU upgrade I would recommend as being worth the costs.
But then it will no longer be a 486 computer.

Personally , I would recommend you build a new computer with a Socket 7 motherboard either Intel 430vx or 430tx chipset.
Then you can use faster RAM, More cache and PCI slots.
Also support for larger harddrives and more RAM in bios.

A Newer bios will have more options available too.

I have a PS/1 with a 486dx2-50 CPU.
Its a PS/1 Essential and MultiMedia edition.
Its a Standard 168-pin Intel 486 CPU.
Any 168-pin 486 CPU will work. You just need to change the jumpers for a 33mhz BUS CPU in order to use a 168-pin Overdrive CPU.

Only the IBM PS/1 with 486sx33 CPU need the 169-pin Overdrive CPU upgrade.

See my thread for info and Manual:
IBM PS/1 Multimedia ( max restore ).

Manual: http://ps-2.kev009.com/pccbbs/aptiva/63g2028.pdf

I change the speed on my AcerAross 486DX2/50 cpu from 25mhz to 33mhz and notice quite a significant increase in speed running OS/2 v3 Connect. The 32megs of ram help it run a lot smoother than my original v3 Warp as well. The system is stable a hell with that increase.

I don't think that's the case with this machine - I have a completely different motherboard than the one in your thread, and none of my jumpers are labeled as a 25/33 bus switch.

If I could get a 10% performance boost, that would be fine - my original target was a DX2-66, but the only one of those I could find appeared to have critical hardware errors.

Gnoman wrote on 2021-07-15, 20:42:

I don't think that's the case with this machine - I have a completely different motherboard than the one in your thread, and none of my jumpers are labeled as a 25/33 bus switch.

If I could get a 10% performance boost, that would be fine - my original target was a DX2-66, but the only one of those I could find appeared to have critical hardware errors.

See the manual for your motherboard and jumper settings.
There are about 5 or 6 different motherboard on the 2155.
It all depended on what store you purchased it form. It was an IBM marketing thing.
Different stores sold a different version of the PS/1 2155

Jumper setting are NOT labeled on motherboard you have to see the manual for settings.
Just scroll down to the 2155 motherboard section. Its about half way down the document.
The best you can do is a 486dx2-66 for a Standard 486 CPU.
If you go with an Intel Overdrive CPU you still have to change the CPU BUS settings to 33mhz.
Because right now with the 486dx2-50 the BUS setting is set to 25Mhz time two. ( 25x2 = 50mhz)
The Intel Overdrive CPU requires that the BUS setting be set to 33mhz for it to work
Thats if you use a 168-pin CPU.
If you use the 169-pin Intel Overdrive CPU you can leave the motherboard alone as this works on 25mhz BUS.
Check on the Bottom of the CPU. The 169-pin Intel Overd drive CPU has an extra pin in the center edge of the CPU.
The IBM Blue Lighting CPU will work in your computer too.
It is basically a Cyrix dx2-66 CPU.
The motherboard bios will recognize it as a Cyrix CPU.

See photos, There is a:

IBM Blue lighting 486dx2-66
Intel Overdrive 486dx4-100 ( 168-pin )
Intel Overdrive 486 with 169-pin on bottom so you can see what it looks like. ( But don’t use this type ).

So you can use any Standard 486 ( 5-volt CPU ) with 168-pins
From speeds 33mhz thru 66mhz.

If you use the Intel Overdrive CPU then you can use the 486dx4-100 or Pentium Over drive CPU.

But like I said I would leave your computer alone unless you perform the Pentium Overdrive upgrade.
Going from 50mhz to 66mhz is a minimal improvement.
And its a waist of money to purchase a $100 Intel overdrive CPU unless you get the Pentium.

Seems like we need some photos of the ops motherboard/cpu socket in case “stock imagery “ has failed

I have the manual, and it doesn't list a 33/25 mhz switch for any of the jumpers. Just a CPU type select. The options listed for that select are "486DX", "486DX2", and "Pentium Overdrive". It may be that the "Pentium Overdrive setting would set it to 33mhz, but the manual does not say that. The manual is quite explicit that it is a 168-pin socket, with NO pin-169 hole. I'm currently thinking that an ODPR75 is my best option.

The picture posted by rmay635703 yesterday IS my exact board.

Not sure but it looks like one of these two motherboards.
See pages on manual.

One more photo. These two look almost Identical.
Check to see which one is yours.