First, find out what you have.  Under "Settings", "System", you can see how much RAM your computer has.  If it's about 6000 k, your computer is full and can't have more added at this point.  If it's about 4000 k, then you can add 2M to the RAM/ROM card (if you have an A11 or C140) or to the computer's motherboard (if you have an A10 or C120).  If it's about 2000 k, then you can add 2M to the RAM/ROM card and 2M to the motherboard.

You will need a number of items:  Your computer; a Windows95 or NT computer to back up its contents and restore them; an antistatic clean work area; soldering equipment for 0.8mm surface mount package soldering; and, for each 2M of RAM to be added, a 0.1 uF surface mount capacitor and a 1Mx16 DRAM.  The DRAM should be either a Hitachi HM51W1616xyLTT-6, where the "x" is either a 0 or a 5, and "y" is not anything, or "A" or "B".  The NEC uPD42S16160LG5 and uPD42S16165LG5 should work as well.  I ended up finding the HM51W16160LTT-6 at Reptron Electronics for about US$21 each.

Other needed tools would be small (jewler's) Phillips screw drivers, flat blade screw drivers, magnifier to check work, good eyes, and steady hands.  I recommend getting the service manual from Casio as described on my Windows CE Hardware Page.

Once you have everything, back up your CE machine.  Turn it off.  Remove any modem or other PCMCIA card and stylus.  Turn it over and remove the backup battery cover, the backup battery, and main battery.  Be sure to put parts you remove where they won't roll away, get mixed up, etc.  Be sure to take all antistatic precautions from here on out!

Remove the RAM/ROM card cover and card (next to the backup battery cover).  If you have an A11 or C140, you already have 4M in your computer and can only add 2M to the RAM/ROM card.  Turn it over - if there's no IC on the back (U305), that's where the extra 2M can go.  If you're only adding memory to the RAM/ROM card, solder the IC to the U305 pads with pin 1 by the white "L" shaped mark.  Solder the 0.1 uF capacitor to the other side at C305.  Reinstall the card, reversing the steps listed above.  Test as described below.  If you're adding memory to the RAM/ROM card and the motherboard, add it to the card for now and test your work, then disassemble to this point for the motherboard modifications.  The RAM/ROM installation will help prepare you for the more involved steps ahead.

If continuing to the motherboard, remove the 5 Philips self-tapping case screws visible in the case back, the 3 in the battery backup compartment, and the 2 that go through the PC board visible in the RAM/ROM socket area (once the RAM/ROM card is removed).  Don't lose or mix up the screws!

You can now lift out the serial port cover and auxilliary port rubber plug.  In the battery compartment, the case halves latch together near the auxilliary port plug which can be unlatched by carefully pulling the upper case tab out slightly.  By carefully pushing the lower case toward the back of the computer, another catch near the opposite back foot can be released.  The third and final catch is behind the IR cover just next to the serial port.  Some careful manovering will free that.  (Photographs are coming).  At this point you can carefully lift and remove the entire bottom cover.  Don't lose the battery catch/switch, case latch, and such when doing so!

Carefully remove the PC Card on/off switch.  You will see a small silver Philips screw in the PC Card socket near the battery compartment - remove it.  You can now carefully fold the main PC board away from the keyboard, revealing the main circuitry of the computer.

You can now add U8 - the missing DRAM - to the set of pads next to the chip that's already there in the corner of the computer.  Orient it so pin 1 is facing the same direction as the DRAM that's already there.  Add the 0.1 uF capacitor to one of the empty set of pads just above the Pin1 end of the DRAM (the empty pads closer to the other large ICs are the pads used by the A11 / C140 but they're equivalent).  Note that unlike the RAM/ROM card the pads already have solder on them so reflow soldering is somewhat more challenging if done a pin at a time.  Be sure to doublecheck your work!  When I soldered my RAMs, I used a professional temperature controlled antistatic grounded SMD soldering station with extremely fine point and 28 AWG Ersyn 62/38 solder.  Be alert for solder bridges, contamination (including finger prints), etc.

Time to reassemble your HPC by following the preceeding steps in reverse order.  Holding the battery latch / switch, the IR port cover, the PC Card on/off switch, and the rest of the back cover all in place while snapping the cover on is tricky but not as bad as it may appear.  When rescrewing the 6 small Philips screws, remember that they are self-tapping and will cut new threads if given a chance - so after a few uses they will cut so many threads that there will be nothing left for them to hold onto, given a chance.  So turn them backwards at first until you feel them "click" into their old threads, then turn them forward to run them into the thread that are allready there.

When ready to put the batteries back, don't put the backup battery in.  Only do so once operation is verified to keep the HPC from draining the backup while the main battery isn't in.  Do be sure to set the battery switch properly as the HPC won't run unless both switch halves are set properly.

You can invoke the built-in diagnostics by holding the Notification button (on the edge next to the case latch release) and pressing Reset, then releasing the Notification button, then pressing Ctrl, CapsShift, and On while tapping Reset, the releasing the three keys.  You should get a "Main Menu" of 8 choices (if not, check display contrast and your work).  If so, press 3 (MEMORY) then 1 (RAM TEST) and make sure that the extra memory is shown (4M or 6M total).  If so, press Enter then 2 to WRITE to all RAM.  When it gets done (several seconds), press 3 to READ and verify that the RAM worked OK.  If so, you can turn the computer off, put the backup battery back in with its cover, and turn your computer back on with another Reset.  Go through the setup again, then restore its contents from your Windows 95 or NT machine.

For a final check, check the "Settings" / "System" memory display - and select "Memory" while there and revel in the vast expanses of free memory awaiting your use.  Do enjoy it!