The three different models of the Model 25 all have very different upgrade options.
The only upgrades common to all Model 25 models (except the 25SX, which has no replacement planar upgrade) are the 7386 or the use of a Reply board. Reply board information is here, and the 7386 information is here.
A Word About Upgrade Planars
Content by William R. Walsh (original HERE). Modified by Major Tom.
Model 25 (8086) Upgrades
This model in the Model 25 line is the least upgradeable of all three models.
The only CPU upgrade you can get is to use an NEC V30 CPU in place of the stock 8086. This gives a slight speed boost, but will not enable your system to use software that requires the protected mode of the CPU as found in 286 and higher systems. Boards that upgraded XT systems to become 286 or 386 computers do exist, but I have only seen units made for use with the 8088 CPU, not the 8086 as used in the 25XT.
A Reply PowerBoard, PC Enterprises Genesis\PC or IBM EduQuest 7386 is really the only upgrade for this system that will bring you to a fairly modern CPU with more expansion options in terms of performance and software selection.
Most systems come with the maximum of 640 kilobytes of RAM. Although expansion of the RAM through use of an EMS RAM card is possible, most software that uses EMS RAM requires a 286 or higher processor.
The hard disk may also be upgraded with an aftermarket controller and drive. You will have to remove the hard disk already installed in the system if you have one. Such aftermarket controllers have their own ROM BIOS to boot from, as the system itself does not have support for standard drives. This is probably the best and easiest method with which to install a hard disk. The new drive will be faster, quieter, more efficient and certainly more reliable than the original.
Both IDE and SCSI controllers with their own BIOSes exist and can usually be found used at a very reasonable cost. You need a controller that works with an 8 bit ISA bus slot.
Model 25-286 Upgrades
The Model 25-286 can be upgraded to have a 486-class CPU at best.
The most common upgrade available is a Kingston SX Now! upgrade. These upgrades make your 25-286 into a 386SX system, at either 20, 25, or 33 MHz. You will need SX Now! upgrade model SX/xxLEM. Other models won't fit--this model is designed for the Model 25-286. The "xx" in the model number will be the upgrade's speed.
Other upgrades exist, including some that offer up to 486 processor levels. Speed on any of these upgrades is not going to be as good as having a "real" 386 or 486 based computer. Basically, these upgrades are "super 286" chips that run really fast, and they can run 386 or 486 instructions, thus allowing you to use software that would require a 386 or 486 processor. A lot of these upgrades also have a lot of cache on-board to speed operation even further. You will need a program to turn the cache on for most upgrades--or the upgrade will run at just about the same speed the original processor did. You can find the program for your particular CPU upgrade from whomever the manufacturer/vendor was.
You may also use a Reply board or the 7386 planar upgrade in this system. The Reply and IBM 7386 upgrades give you the most performance, with vastly improved RAM expansion options and a choice to use a larger/faster hard disk without the use of a separate controller card taking up one of the two valuable slots.
The memory in this computer can be upgraded to a maximum of 4MB on the planar board. Using an adapter card, you can achieve 16MB of RAM, but the adapter card must support changing its starting address so it won't conflict with the first 4MB of RAM. Unfortunately, RAM for this system doesn't come easily. IBM used chips with a different CAS/RAS refresh system and they also set up the parity differently. And while common 30 pin SIMMs can be hacked for use with an IBM SCSI adapter (which uses a similar system to the 25-286 for memory) they have proven to be less than reliable or totally unusable on the 25-286. However, some folks have reported that this may be a problem solved by using 9-chip 30-pin SIMMs instead of 3 chip ones.
Your options for a hard disk are the same as the 25-XT model above.
Model 25SX Upgrades
The Model 25SX can be upgraded to have a 486 class CPU at best. But not all systems can be upgraded. The very first Intel 386SX 16MHz processors don't have the ability to be disabled by grounding a certain pin the CPU itself. Fortunately, it seems that the vast majority of 25SX systems use a 20MHz 386SX processor--and all of those DO have the needed pin to be turned off when an upgrade is installed.
The only advice I can offer on whether or not your Model 25 is one that can upgraded is to look inside in the system or check the CPU's stepping number. When looking inside the system at the CPU, you will see 1 of 3 possible things printed on the chip. The first will be a simple "80386SX-16". Most CPUs bearing this mark are not upgradeable. The second you could see is a chip marked as 16 or 20MHz with the additional printing "C Step". These processors support installation of an upgrade CPU. The third and final variation you may see is a processor that says "80386SX-20". All of these processors can handle installation of an upgrade CPU.
If you don't want to open your 25, software exists that can tell you your CPU's stepping number. This number is sort of like version numbers as used in software programs. Do a web search to find such a program and run it from pure DOS. It will put out one of two numbers. If you get a "2308h" stepping, your CPU is new enough to handle being upgraded. If you get "2305h", the CPU cannot be upgraded.
When you do get an upgrade, you will be looking for one that clips over the soldered 386SX chip. You might also find an upgrade that goes in the math-coprocessor socket. Evergreen Technologies marketed an upgrade that clipped over the soldered 386SX chip.
Other upgrades may also exist, but if you find one--make sure it is compatible with an IBM PS/2 computer. Also, when you install the upgrade, double check that the upgrade's pin 1 is oriented toward the installed CPU's pin 1. If you get the orientation wrong, not only will the system not work, but you may actually see sparks and smoke come from the upgrade, which will likely ruin it. This might also ruin your planar, so be careful.
Memory upgrades: The 25SX planar comes with anywhere from 1 to 4MB RAM soldered on the planar itself. You can add one SIMM, up to 8MB for a total of up to12MB RAM. You may use IBM Parity 70ns or 80ns SIMMs for PS/2s to upgrade the amount of installed RAM. These are fairly common and cheap in 2, 4, and 8MB sizes. You may also be able to convert existing memory.
Solder "pads" for another SIMM socket exist on the 25 SX planar, but based on the experiences of a comp.sys.ibm.ps2.hardware newsgroup member, installing a socket there resulted in strange behavior and failure of diagnostic tests. If you are really talented with a soldering iron or have professional soldering equipment and a 1MB-only planar, you could add in the missing chips to get 4MB. But this would take an extreme amount of soldering skill and is beyond the scope of this page.
The 25SX has standard IDE and a standard 4 pin power connector plug-in on the planar. You can use up to a 528MB hard disk, and beyond that with a disk manager such as Ontrack's Ez-Drive program.
An SCSI card with a bootable ROM BIOS is also an option, but with the performance of this system and the fact that it has IDE support already, it probably won't make much difference in performance.
The EduQuest models covered here include 30, 40 and 50. I don't have any other EduQuest at the time, so if yours isn't listed, have a look inside and compare to the notes here. Your system may only be slightly different and might be able to use the same memory that a model listed here uses.
Models 30 and 40 both use the same type of memory. They take 30 pin SIMMs with or without parity. There are four SIMM slots on-board. You can use up to a 4MB 30-pin SIMM. These models also have soldered memory chips (they're located underneath the floppy drive) and these can give you either 1MB or 4MB of RAM depending upon model. The easiest way to check and see how much soldered memory you have is to power the machine with all SIMMs removed or look carefully at the planar--sometimes there is a small table with two RAM configurations in it and it will have one of them marked with a dot.
The EduQuest Model 50 is a vastly different design internally and uses 72-pin SIMMs, parity or non-parity is your choice. I don't know at this point what the maximum size is, but I would think the machine could handle at least 2x16MB SIMMs in the sockets. There is no on-board memory in this system.
For the Model 30, no upgrade for the soldered IBM 486SLC CPU is known to exist. IBM doesn't seem to have implemented the disable pin as Intel did with the 386SX and clip over upgrades don't work.
Model 40: In addition to the soldered 486SX CPU, IBM provides what is called a 487 socket. This socket can be used with any 486 CPU--in fact, the 487 chip is just a 486DX with a different name and it turns the soldered CPU off. While some systems may require the use of a 487 or a CPU that is wired to ground the soldered the 486SX CPU's disable pin, IBM seems to have taken a more "intelligent" approach with this system and any 486 CPU that can be inserted into the socket should work just fine.
Warning: To avoid damage to your CPU, make sure that if it runs on 3 volts that you have an appropriate voltage regulator installed. Otherwise the CPU may be destroyed over a period of time or immediately when the power is turned on.
The Model 50 is by far the most upgradeable EduQuest. It features a Zero Insertion Force (ZIF) CPU socket and it has the needed extra pin row to accommodate the Intel Pentium OverDrive 66 and 83MHz processors. This socket can also handle 3 volt 486 CPUs with no additional voltage regulator as one is built on the board.
NOTE: While it is believed (at this time) that the EduQuest Model 50's 3 volt regulator for the CPU is automatically enabled when needed, this has not been proven. Proceed at your own risk.
For the Models 30 and 40, video is provided with an on-board ATI video chip. For the Model 40, this is an ATI Mach32 Graphics Accelerator chip. 512K of VRAM is provided with both systems, and an open socket exists where you can install an upgrade memory chip to presumably reach 1MB VRAM...which will give you more colors on screen at once for all supported resolutions. However, it is questionable as to whether or not this upgrade VRAM socket is usable or not. I have installed countless VRAMs into it with no change in the detected amount of video memory by ATI diagnostics. These same VRAMs have worked in many other video cards with no problems, so it is possible that the feature was provided but never implemented in these systems.
The Model 50 uses a Cirrus Logic 5434 video chip. This comes with 1MB VRAM and you can upgrade to 2MB. I have attempted this upgrade using VRAMs that didn't seem to work in the 30 and 40...and the new VRAM was detected and set to work immediately.
An external monitor can be plugged into all models for dual display of the same screen or the internal display can be disabled in setup if desired. This is handy if you want to run a display mode beyond the abilities of the built-in screen.
With the exception of the Model 40 having on-board SCSI and IDE, all EduQuests can use any standard IDE hard disk you can find. For sizes larger than 528MB, you will need to use a disk manager program to help the system use your hard disk to its fullest capacity.
Note: For some reason the EduQuest 30 and 40 are unable to boot the DR-DOS operating system as used by many disk manager programs. This issue can be resolved by hacking the boot disk on which the disk manager is stored and replacing the system files with those from DOS or Windows 9x. I had originally intended to place a disk image up here, but it would only work for a limited brand of hard drives and might be subject to copyright protection or some other condition which would not allow redistribution. So you'll just have to make your own if the disk manager available to you doesn't boot on the EduQuest like it should.
The EduQuest Model 40 has on-board Future Domain TMC-950 SCSI. This SCSI system is bootable, but maximum size limits for the hard disk are not known at this time and it may be better suited to devices like CD-ROM drives as opposed to actually being used as a bootable device.
A Word About Model 25 Upgrade Planars
Locating and installing a CPU upgrade may be your best choice, but...
UPDATE! Three upgrade boards exist for certain:
The Reply Board upgrade planar is the be-all, end-all of Model 25 upgrades. It features a true 486 CPU that can be upgraded to a 486DX4-100MHz CPU at best as of this writing. Standard 72-pin parity SIMMs are used and Cirrus Logic SVGA video is provided.
Trouble is, the Reply planar is rare at best, so you may have to do quite a little bit of looking to find one.
Even rarer than the Reply board is IBM's EduQuest 7386 upgrade. This upgrade features a 386SLC processor operating at 20MHz. IBM's manual for the 7386 hints at versions with other more powerful processors being available, but I've never seen anything except the 4 386SLC-20 powered ones I have.
Rarer yet is the PC Enterprises Genesis\PC board for the Model 25. This is another replacement planar. I have recently acquired one of these from a kind soul on the IBM PS/2 newsgroup, but I have yet to dive in and really work at documenting it. PC Enterprises may still be in business, but as of this writing their website appears to be nonfunctional.