These are the IBM planars, NOT the Reply Corporation planars.
   On 25-MHz model 70 systems, the microprocessor is on a small card (processor board), that plugs into the system board.

   The 386 -A61 and -A21 can be upgraded by a 486/25 Power Platform. It uses the same planar, but the processor card has a 386 and a socket for a 387DX.  The only 486 system that supports memory expansion adapters is the model 70 A21
 

rf70486a.exe   PS/2 8570 486 reference diskette v1.06
Additional Standard Power Connector
Replacement CMOS Battery
Making a diskette drive adaptor

25MHz Planar
386 Processor Card
486 Processor Card

Axx BIOS  33F9987 Odd, 33F9988 Even
Bxx BIOS  64F3125 Odd, 64F3126 Even

Memory
Memory Min/Max on system board: 2/8MB (expandable to 16MB)
RAM: DRAM (PS/2 72-pin SIMM) 80ns parity checked (4 sockets) 
ROM: 128kb 
Cache: 8kb L1, 0kb L2 

8570-25MHz Models
A61 - 386-25 w. 64kb cache, 2MB, 60MB, 1.44
A21 - 386-25 w. 64kb cache, 2MB, 120MB, 1.44
B61 - 486-25, 2MB, 60MB, 1.44
B21 - 486-25, 2MB, 120MB, 1.44

GRAPHICS
The VGA port supports graphics and text modes including 640 x 480 graphics, 320 x 200 graphics in 256 colors, and 720 x 400 text, and maintains compatibility with Color Graphics Adapter (CGA) and Enhanced Graphics Adapter (EGA) modes.

The 2MB 80ns SIMM is for use on the 8570-A21 only. Three kits can be installed on the system board of the 8570-A21 to reach the maximum system board memory of 8MB.  The 1MB and 2MB 85ns SIMMs should not be used on the 8570-A21 system board.

ECA 021 - 8570 110 error 
  8570 mod A21 system board 110 memory error 
    8570 model A21 system boards P/N 15F7657 can be replaced. The ECA will be performed at customer request, while servicing the system for any system failure, or if a POST error 110 with five question marks occurs after a memory upgrade. 

The original pics for the Power Platforms came from Pete Rickard. Thanks! (Damn hard holding on to the bottom of the earth when typing!)

386 Power Platform FRU 15F7659
  Sorry, I know there never was an option called that...

J1, J2 Reverse of header
U13 387DX-25 Math co-pro
U14 386DX-25
U15-22 MT  5C6408-25
U24 50.0000 MHz
U28 82385-25 Cache controller
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

486/25 Power Platform FRU 64F0123 (?)

J1, J2 Reverse of header
U12 50.0000 MHz
U20 486DX-25
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

486/25 Power Platform
       The Model 70 386 (8570-A61 and 8570-A21) can be upgraded to the Model 70 486 by the installation of the 486/25 Power Platform. The 486/25 Power Platform was previously announced in Product Announcement 189-102, dated June 20, 1989.
   The 486/25 Power Platform is an optional upgrade kit for the Model 70 386 (8570-A21) that significantly increases performance by replacing the existing 25MHz 80386 microprocessor with
a new 25MHz i486 microprocessor.  In addition to the new 486/25 Power Platform, the kit includes two EPROM modules to upgrade the system board of the 8570-A21 to support the 486/25 Power Platform.

486 BIOS ver. is:  64F3125 / 64F3126
BIOS Ver.  Part No.  64F3126   Date  12/01/89

From Tam Thi Pham
 It *is* possible to beef up the Model 70, with just a tad bit of work. The 8570-B21 will run the  Kingston Turbochip (133MHz) nicely if you upgrade the crystal on the CPU card to 66MHz.  There's a slight issue with clearance for the CPU fan, and it requires that you shave away a bit of  plastic from the underneath side of the top-level drive mounts. Either that, or you could fabricate  your own low-profile heatsink device of some sort. I'm still figuring out all the logistics for an elegant solution.
   Now, the heatsink on the 486DX2-66 Overdrive already has a nice low-profile, and allows that  chip to mount in directly as is for an easy upgrade. I'm thinking you'd rather go all the way to  133MHz, though. Please keep an eye out on the newsgroup (and my site, probably) and I'll detail
 my Model 70 upgrade experience.