The P70 came in two planar versions: the older 38F4688/65X1564, used primarily in the 20 MHz -061 machines (but also seen in some -121 20 MHz boxen), and the 38F6973/56F9085 used in 16 MHz and 20 MHz -121 models.  The 38F4688/65X1564 planars have the 386DX socket immediatly to the right of the 387DX coprocesssor socket, and two BIOS ROMs near the upper edge of the board.  The 38F6973/56F9085 has the 386DX socket above and to the left of the 387DX socket.  Another distinguishing feature of the newer models is the inclusion of video output filtering on the video card instead of on a 'daughter card' fitted in series with the video output cable.  The newer planar seems slightly  faster on some benchmarks, but the difference is hardly noteworthy.

For the H2 version, remove the plastic housing, starting at the left side.  Then remove the large screw that will be visible as you peek in below the bottom edge of the drive where the cable plugs in (use a flashlight).  The frame will then slide up and out. 

Blow the dust out of the drive and run a cleaning diskette through it before giving up on it completely; sometimes this is all it takes to get them operational.  Check Bob Eager's P70 page for replacement drive part numbers and sources. 

NOTE: if the machine is unconfigured (dead CMOS battery), it will ONLY   boot from a reference disk or a DOS boot disk that has been tricked up with Bob Eager's REFSTAMP utility.  Boot from the internal hard drive will not be possible until the machine has been configured.

Next, power the machine up and boot on the reference disk.  If the machine wants to run 'Automatic Configuration', say NO at this point.  Select 'Copy an Option Diskette' from the first menu, and follow the directions on the screen.  This will copy the option files to your reference disk, and the error message will not appear the next time you run the configuration.

It would seem that the MCMaster doesn't support clone-type dual-RAS (double-sided) 8 MB SIMMs in both sockets at the same time.  Other combinations should be possible with single-RAS chips; anyone who has information, please send it to me for inclusion here and I will give appropriate credit. 

It may be necessary, with faster than 33 MHz CPU's, to set the MCMaster to "Report MCMaster Only" memory, as the planar memory may not be able to keep up and lockups will result.  In fact, if you are running an advanced operating system like Linux, Window 9x/NT, or OS/2, you will want to set the MCMaster up to use only it's own onboard memory.  The reason for this is the MCMaster has to access the planar memory over the MCA bus; this is much slower than accessing the onboard memory, and will result in a signifigant performance penalty when using a "flat memory model" 32-bit operating system.  I found this out one time when I put an ISA RAM card in a fast 486 clone; OS/2 adjusted itself to run at the speed of the slowest memory in the system.  It wasn't a prettty sight.  More RAM is not necessarily better. 

The MCMaster utility diskette can be found on Louis Ohland's web site.  More information on the MCMaster can be found on Fred Spencer's 8580 Processor Upgrade page. 

Other memory cards should be able to break the 16 MB barrier with the "stock" 386 P70, as has been documented on Fred Spencer's pages for the PS/2 Model 70/80 (subject to the same limitations).  The main disadvantage to this approach is that you need to use a 32-bit memory card, which takes up your one-and-only 32-bit MCA slot.  This prohibits using an IBM SCSI adapter or long-card network adapter in the computer.  One potential RAM card is the Microram 386 (941366, Rexon/Tecmar Inc.).  Other possibilities may be the "enhanced" cards from Kingston, Acculogic, and IBM, among others.  If anyone has successfully done this, I would appreciate hearing from you (remove the spamblock from my email address). 

Various aftermarket plug-in 386-to-486 CPU-only upgrades should work.  I had a Cyrix 386/486 CPU installed for a while, that is pin-compatible with the 386DX.  It worked fine, although the P70 BIOS did not support the write-back L1 cache in the CPU.  DON'T enable write-back if you try this, or you WILL experience hard drive corruption!  The increase in performance is nothing to write home about, but if you just need to add a little speed, it's the most economical way to go.  You do need software from Cyrix to enable the internal L1 cache, though.  I will be publishing comparative benchmarks here when I get time to take some data. 

Kingston offered two types of 486 Now!  CPU upgrades (in SX and DX versions) for the P70: 

486/33PD3-P70 (DX) and 486/33PS3-P70 (SX) for the 38F4688/65X1564 planar used in the 20 MHz -061 model. 

486/33PD3-P70LP (DX) and 486/33PS3-P70LP (SX) for the 38F6973/56F9085 planar used in the 16 MHz -031 and 20 MHz -121 models. 

(Photos are ©Copyright 1993, Kingston Technology Corp.) 

Note that I have also seen the 20 MHz -061 planar used in the -121 model.  It is important that you get the right model for your planar, as the orientation of the CPU socket is different and the upgrade will not physically fit if it is not the correct one. 

Fellow PS/2'er Karsten Harder was kind enough to send me a 486/33PD3-P70LP fitted with an AMD 486DX-40 CPU from the factory (thanks, Karsten!).  Installation was simple and straightforward, and nicely documented in the Kingston manual.  If the planar is fitted with a 387DX math coprocessor, it must be removed (even if you only have the SX upgrade).  After properly setting the four DIP switches, the machine booted and seemed to run fine (although it seemed to take much longer to initialize the POST routines on power-up).  I see no reason why clock doubled or tripled CPU's could not be used with this upgrade, space inside the P70 permitting.  I did test the upgrade with an Intel 486DX4ODPR-100; it came up fine, and benchmarks reported a CPU speed of 120 MHz.  Be careful when removing the CPU from the circuit board; the Kingston board is quite thin, and is very susceptible to damage.  The DX4ODPR is tall enough that the heatsink will prohibit the use of a long card adapter in the 32-bit MCA slot, however.  The SX version should accept a DX CPU without trouble.  The downside: you are still using the slow (by comparison) planar memory.  The lack of L2 cache also hinders performance.  Still, it is a worthwhile upgrade if you can't find a MCMaster card. 

The best performing CPU upgrade for the P70, by far, is the Kingston/AOX MCMaster 486 cards.  My machine is currently running with a 25 MHz MCMaster that I managed to overclock to 32 MHz (yes, 32 MHz, not 33).  See Fred Spencer's page for the info on how I accomplished this.  Interestingly, the only CPU that would run reliably with a 32 MHz base clock on this board is an IBM (Cyrix) Blue Lightning DX4-100.  An Intel DX2-66ODPR and an AMD 5x86-P75ADZ (with interposer) would both lock up after a random period of time running Windows.  The problem did not appear related to cooling, so my conclusion is that some of the components on the card are slightly speed sensitive.  I would be interested in hearing from anyone who has successfully used clock-multiplier CPU's on a true 33 MHz MCMaster.  As mentioned above, I had to disable the planar memory in the MCMaster setup, as the machine would generally refuse to boot once the MCMaster took over.  Disadvantages of the MCMaster upgrade: actually finding one, although they do seem to show up on eBay occasionally (surplus auctions are also a good source, it's a good bet that any university or government agency that bought a lot of PS/2's will have a few floating around);  it takes the only 32-bit full-length MCA slot, which limits your options for adding other cards;  and you need a CPU heatsink without a fan, because there simply isn't enough room for a fan.  This shouldn't be a problem, as cooling seems to be adequate for a CPU with heatsink only. 

My little P70 (a.k.a. "Punkzilla , fastest P70 in the west") is currently running the MCMaster with the IBM DX4-100 CPU and 24 MB of RAM, and runs PC-DOS 2000 and Windows for Workgroups 3.11 very well.  I have a 3Com Etherlink III network card (with the blue handle removed) in the 16-bit slot, and it barely fits.  I have the computer set up to track Ham Radio satellites with Logsat Professional 5.1, and it has come in handy as a portable satellite tracking computer.  I've got an AUI transciever for the network card so I can connect to either coax or twisted-pair networks, and have used the machine with both NETBUI (Windows) and Novell networks.  The transciever and the reference disk fit neatly inside the I/O cover, and all I need is to run the reference disk to tell the network card which port I want to use.  These upgrade attempts have successfully turned what some would consider a slow, old dinosaur into a very usable machine for service work, portable word-processing or database functions, or just about any other use you can come up with.  Add a decent-sized SCSI hard disk with a Future Domain MCS-700 controller (at the expense of losing the network card), and it would probably even run Windows 95 comfortably (not that I would actually WANT  to...). 
 
 

The P70 uses a DBA (direct bus attachment) ESDI hard disk, that is not compatible with anything else except the PS/2 Models 55sx and 70 desktop computers.  Thus, a P70 with a 30 or 60 MB hard disk may be upgraded with a 120 MB disk from a Model 70.  Brad Parker and Thomas J. Watercott have sent me info about an 8570-161 that has a 160 MB ESDI drive in it.  I have verified this through the EPRM, and Thomas relates that he has successfully used this drive in a P70.  This is probably the best source of replacement/upgrade hard disks for the P70.  As an aside, these drives are referred to by some (and reported by a lot of diagnostic software) as "IDE".  In fact, they are and early form of Integrated (Intelligent, Imbedded) Drive (Device) Electronics (people can't even agree on what the acronym actually stands for!) drives, but don't bear much resemblance to IDE as we know it today.  Peter Wendt has this to say on the subject: 
 

If you need more than 160 MB, you will have to go SCSI with an add-in card.  I will not go into detail on this, since it is described nicely on Bob Eager's P70 page, using an IBM SCSI adapter.  This is a 32-bit SCSI card, so if you use one of these, forget about putting in a RAM card or an MCMaster processor card.  If you need your 32-bit MCA slot for other goodies, another SCSI adapter ideally suited to the P70 is the Future Domain MCS-700 (short-card version).  The MCS-700 is a 16-bit SCSI-2 card capable of 10 MB/s transfers, has a standard SCSI-2 external connector, and also has a power connector on board to supply power to the hard drive.  It is not, however, a busmaster; it has no onboard processor and uses PIO mode for data transfers.  It is still a very nice controller; I used one in my Model 95 as a secondary controller, and it ran a CD recorder and two hard disks without a hiccup.  Louis Ohland has a nice page on the IBM OEM version of the MCS-700.  The IBM version (found mainly in the Model 76s/77s "Lacuna" machines) should be equally usable in the P70; however, it lacks the onboard power connector.  The solder points for the connector are still there, so it is easy to add in if you are comfortable with a soldering iron.  A ROM BIOS upgrade (version 3.61) was available from Adaptec at no charge if your MCS-700 exhibits the "hard disk LED not functioning" problem, or you can download the code from my IBM ROMS page and burn your own.

UPDATE, 1/21/2001; Casolai sent along this P75 SCSI info:
 

You can see Casolai's web site at http://www.angelfire.com/hi/casolai/index.html.  Beware of multimedia content!  :-)  6/13/2001 - link is broken. Cas, update me! 

Peter Wendt provides the following info on the video subsystem: 
 

System memory was 8 MB standard, upgradable to 16 MB on the planar.  In addition to the I/O connections found on the P70, the P75 I/O panel has an RS/6000 style external SCSI connector.  The machine also has one more of each (16- and 32-bit) MCA expansion slots, for a total of four MCA slots.  One 32-bit slot is an AVE slot.  The extra slots make the case slightly deeper than the P70.  I've been told that the P75 was marketed as a "portable server"; based on my experiences, it could certianly fulfill that function quite well.

The first, and most viable option for most people, would be to find one of the Kingston MCMaster 33 MHz cards.  There were versions of this card with clock-multiplied CPU's, but they seem to be extremely rare.  More common are the 486DX-33 and 486DX/SX-25 versions of the card, which should be able to support faster CPU's.  A 33 MHz base card with a DX2-66 or faster CPU fitted should perform nicely.  The advantage of this approach is that you can also easily upgrade memory beyond the 16 MB limit of the planar, with better performance than adding a memory card to the MCA bus.  The disadvantages are the loss of one 32-bit expansion slot to the upgrade card, and the height limitation on the CPU/heatsink combination imposed by the limited space inside the P75.  The hardest part would be locating one of these cards, but one place I have seen them for sale fairly often is eBay. 

The second approach is the more difficult, and should only be attempted by experienced hardware hackers.  I have personally upgraded four P75's by desoldering the CPU from the processor board and installing a 168-pin PGA socket.  This SHOULD NOT be attempted unless you have a lot of soldering experience AND access to professional desoldering equipment; it is very easy to damage the copper traces on the board.  That being said, the results were well worth it.  The two 'local' machines ran very well with an AMD 5x86-133 CPU chip (with 3.45-volt interposer).  Various other "turbochip" upgrades should work equally well; one of the two machines currently has a i486DX4-100 Overdrive CPU installed, running Windows NT 4.0 very nicely.  I don't think the P75 BIOS supports write-back cache, so any "turbochip" should have the capability of running in write-through mode for best results.  The disadvantage to this method, other than the risk of destroying your CPU board, is the height limitation imposed by the cramped quarters inside the P75.  Some of the CPU/interposer/heatsink combinations are quite tall, and will limit the use of the two adjacent MCA slots to short cards only; even at that, some 32-bit "short" cards may still be too long. 

UPDATE, 8/16/99:

Photos of one completed CPU upgrade can be found on Tam Pham's site here.  While you're there, be sure and check out the rest of Tam's fine collection of PS/2 information!

UPDATE, 1/21/2001:

On the subject of write-back cache, Brad Root has this to say:
 

The problem is, the CR2477 (with solder tabs) is not something you can just run down to your local battery shop and grab off the shelf.  They are somewhat more expensive than a 'standard' CMOS battery, and must be ordered from a parts distributor.  Assuming that there is nothing unusual about the CMOS memory/clock in the P75 (as compared to the Model 95, etc., that use the more standard and common CR2032 220 mAh cell for CMOS retention), there is no reason that I can see for using the CR2477 other than greatly extended CMOS battery life.  Two CR2032 coin cell holders liberated from 'retired' clone boards will fit nicely inside the battery pack shell.  They can be fixed in place with hot melt glue or epoxy and soldered with short jumper wires to the pads that the CR2477s connected to.  Two CR2032s cost me USD$1.36 each at the gorcery store today.  Much better than the $4-$9 each (plus shipping) that Bob found for the CR2477!  Top it off with a small chunk of non-conductive  foam or bubble wrap to keep things secure inside the battery case, and the result is an inexpensive replacement solution that looks stock from the outside.  The CR2032 only has about 22% of the capacity of the CR2477, so expect to replace them about every 2.5 years instead of every 10 years.