In case anybody else is looking for information on this, I took a chance and tested the Pentium MMX 233 and it worked!

However, the Wikipedia article is wrong about using the 75 MHz setting on the motherboard. You must use the 100 MHz setting for the correct speed. The BIOS will display the incorrect speed (ie. it will show "Pentium MMX 100 MHz") but it will be running correctly according to setmul.exe and benchmarking.

I've tested each motherboard setting using 3DBench2 and setmul.exe (to see the report processor speed):

[Motherboard Setting - Setmul Reported Speed - Benchmark Result]
75 MHz - 179 MHz - 123.2
90 MHz - 214 MHz - 143.1
100 MHz - 238 MHz - 153.9
120 MHz - 122 MHz - 111.9
133 MHz - 136 MHz - 121.2
150 MHz - 153 HHz - 124.6
166 MHz - 171 MHz - 134.7
200 MHz - 205 MHz - 145.5

Hope somebody finds this helpful!

Hi !

I recently acquired a PC 350 (Model 6587-9BT) that also come with a P54C-166, and I'm seeking do to the MMX upgrade as well.

My findings led me to know that this motherboard doesn't seems to deliver the dual-voltage the Pentium MMX was designed for, so you may fry your CPU on the long run.

The regular Pentium (P54C) needs 3.3V for it's core and 3.3V for I/O, whereas the Pentium MMX (P55C) needs 2.9V and 3.3V respectively.

AFAIK, you need a voltage regulator board to deliver the correct voltage for your Pentium MMX.

There is a diagram of the motherboard that shows that the PC 350 that come with a Pentium MMX out of the factory has a connector for such a board (See 14)

http://ps-2.kev009.com/pcpartnerinfo/ctstips/f5aa.htm

The non-MMX ones does not have said connector, but maybe you can add it yourself ?

The FRU for the voltage regulation board is 76H3658

http://ps-2.kev009.com/pcpartnerinfo/ctstips/db52.htm

I hope that it helps.

Aww, you just ruined my day 🤣.

You are probably right… I don’t have anything soldered at that spot either.

I’ve also been having instances where the PC won’t POST but I didn’t extensively test with the non-MMX CPU. However I think it might related to the CMOS battery or ide/floppy cables. After it boots it’s very stable but the chip does get hot so I’m ordering a fan/heat sink.

Is that VRM available anywhere to purchase?

Sorry to disappoint.

I'm still trying to find one, but they seems pretty rare unfortunately.

I also need to find out whether or not the connector can be soldered on the motherboard before.

Is there a way to confirm the motherboard CPU voltage? (ie. any DOS tools?)

I would have not expected the system to POST let alone be stable (30+ consecutive runs of 3DBench2) at the higher single-supply voltage?

After doing a lot more research, I agree with your conclusion. It appears that you can often run 2.8v Pentium MMX CPUs for a short while but it's a bad idea and will eventually cause problems.

I see you found the IBM FRU for that VRM (76H3658) but do you know what chip that was? It possible to source this elsewhere?

As I'm searching I'm seeing references to "30 pin VRM" which is exactly the size on this motherboard...?

I can’t even find a picture of the IBM part…

You can also look for one of the Kingston or evergreen socket 7 upgrade cpus or even a socket 7 interposer to use with a mmx cpu

Thanks for the suggestion. The overdrive CPUs are extremely expensive.

Do you think it’s possible to modify this motherboard for 2.8v maybe?

EDIT: I can’t find any interposers either but that sounds interesting.

It should be possible but sadly that's outside my wheelhouse.

Hey, I've got an IBM PC 350 in storage somewhere. I haven't seen it work properly in years, but it came as a pair with a PC 340. It was my little brother's bedroom PC for a few years until he upgraded to a Sempron.
I'm sure adding the VRM socket would work to add the capability, but the biggest concern would be the BIOS - apparently it recognizes it already, though.

I've also been having trouble finding the VRM itself! I have an HP Vectra XM which I would like to add an MMX or K6 to, but no VRM = no go! Don't want to cook any of my CPUs.

If anyone finds out about the VRM, do post please. I'd love to get my machine out and working again but as I recall it had some minor problem that prevented it from being useful. Don't recall exactly what.
The 340 also suffers from "I won't POST anymore"-itis...

It sounds like the MMX version of the IBML 330/350 was more rare (which makes sense because it would have been more expensive).

I suppose I'll have to make a choice to use a slower chip (Pentium 166) which is also worse because it doesn't fully support SETMUL like the MMX chips do, or I can run the MMX chip with over-voltage. I've ordered a Cooler Master DP5-6I31D heatsink fan and it's very large so I don't think cooling would be a problem.

I'm also surprised at the prices of Socket 7 motherboards on eBay. This hobby is quickly becoming expensive! I'm now also somewhat considering DosBox + CRT Monitor.

OK - So the IBM 76H3658 (VRM) seems to be the Semtech MP55C-3.3-2.8 VRM. Here is the datasheet.

I'm not an electrical engineer (or even close!) but it seems quite possible to build a compatible adapter?

That's great. This is what it looks like

Doesn't seems very complicated. I have no skills, but I bet it can be reproduced if all the components could be identified.

Unless I'm missing something, this looks incredibly simple. I think all we need to know is the voltage regulator (U1) . I've contacted Semtech but I doubt they will get back to me.

If we can figure it out, I would even be willing to design some PCBs and build a few for anybody that needs them.

Unfortunately I haven't been able to find them for sale for under $100 anywhere. An HP model number is on eBay but it has burnt tracks.

It probably wouldn't be difficult to replace such a device entirely with a modern voltage regulator or the ability to set a voltage - You could use one VRM at 3.3V pass thru, 2.8V MMX, and 2.2V for the later K6-2 and K6-3s.

Dug her out of storage to see how she was doing, just as poor as I remember it

Well ... Here's my solution for using the Pentium MMX 233:

(Yes, it's just running the CPU at the higher/single voltage)

It's a Noctua 80mm fan with a 3D printed 80mm-to-50mm adapter and a heatsink and another Noctua case fan in front of it. The CPU stays extremely cool to the touch even running benchmarks, etc. I'll see how long it lasts.

Hopefully oneday I can find the proper voltage regulator (or figure out how to build one!).

Let us know 😀

What's the best way to stress test the system to check for stability?

I've tried 3DBench2 and it locked up the system (once), but Duke3D and Memtest86 ran for many hours without any issues. However, installing Windows 98 SE is giving me problems. (freezing)

All of my hardware is new (to me) so I'm not sure if the motherboard is defective or the compactflash-to-ide adapter, etc.

Should I maybe run something like Quake overnight and see if it crashes or is there something better?