The Museum of HP Calculators

HP Articles Forum

[Return to the Index ]
[ Previous | Next ]

Increasing HP-42s Memory to 32K

Posted by Paul Brogger on 6 Sept 2001, 7:03 p.m.

Believe it or not, one may upgrade an HP-42s to 32K RAM.

Of course, I can make no guarantees about what happens if you try any of this. I have reported my experience as clearly as I can. I was able to upgrade two units a few days ago, and they seem to be working fine as I write.

I’ve asked for Tony Duell’s comments on my methods, and those are added below. Read everything before trying anything. If you’re hesitant about it, consider asking a more experienced person to do the touchy stuff.

To effect a 32K upgrade, you have to:

1. Open up the HP-42s
Pioneer calculators are held together by eight “heat stakes” – plastic poles that are molded into one half of the case, fit into holes in the other half, and are melted into mushroom, rivet-head shapes to effect a mechanical bond. The top four “rivet heads” are visible, recessed in holes beneath the battery compartment cover. These are accessible and easily undone. The lower four are arranged in a line, beneath and between the lowest two rows of keypads. The mushroomed heads are hidden behind the metal foil on the calculator face. These are inaccessible, but that turns out not to matter much.

Remove the battery compartment door and the batteries. Use a small (3/16” or so) sharp drill bit to cut out the melted tops from the four visible heat stakes that hold the top end of the calculator together. (I put drill bit in a pin vise or other holder and rotate it by hand.)
Caution: You only need to remove 1/16” of plastic or less, so don’t use a power drill, as it will rapidly cut right on through the calculator front!

Once the top end is freed, you should be able to easily pry the front and back apart at the top end, using your fingernails in the slot that runs down both sides of the case. (Don’t use a screwdriver or other prying implement – you’ll mar the finish on the sides of your calculator, and risk internal damage as well.)

As the case starts coming apart, I simply keep prying with my (fairly strong) fingers. It’s a bit of a struggle, but there are only the four hidden heat stakes holding the two halves together. With the right amount of force, carefully applied, they quickly pop one after the other in a zipper-like noise, and the calculator case comes apart.

Caution: Try to grip and pry the plastic case parts only. Don’t get your fingers into the electronics, and don’t bend the printed circuit board. If you apply pressure only to the case halves, the PCB is free and unstressed, and will not be damaged. Don’t use any prying device, as (again) you risk cosmetic and internal damage to the calculator.

2. Remove the 8K RAM chip (the only 28-pin chip on the board)
I use a small, decent-quality soldering iron with a finely-pointed tip. I also have a long, pointed steel “ice pick” prying tool, but have used a large sewing needle in the past. I wear magnifying lenses (cheap reading glasses bought at a discount pharmacy) to see better, and I work under good, strong light.

I work with the soldering iron in my right hand, the pry tool in my left. I work from right-to-left along a row of pins on one side of the surface mount chip. I put the pry tool in between two pins, and while prying against the pin on the left, try to lift the pin on right. I only apply prying force, however, after I’ve touched the soldering iron to the right pin, and the solder is melted. (You can see the solder subtly change appearance as it heats and melts.)

It’s touchy, delicate, detailed work, but easily accomplished. The final (left-most) pin on the side of the chip is lifted directly (there’s nothing to pry against), and the whole chip should be free enough to lift up a bit. Then I turn the calculator around, and start working again left-to-right. The chip should come free when the last pin is lifted.

3. Solder a 62256 32K (low power, 28-pin, surface-mount) RAM in place
The new chip should be the same size as the original, and should sit in the same place, and with the same orientation as the original. (There’s another set of surface-mount pads on the circuit board, but ignore them.)

I run the soldering iron over each newly-exposed pad, to level the solder there and remove any hardened “solder tails” or shorts with adjacent pins. A close, careful inspection of the pads should reveal no solder shorts (or undesired connections) to other pads and traces.

I place the new chip in position, with the correct orientation, and simply touch the soldering iron to each of the four corner pins to hold it in place. After waiting and blowing a little cool air on the chip, I start soldering the rest of the pins in place, maybe four at a time, and pausing to let things cool in between.

4. Cut one jumper pad near pin 28 of this chip
There are two “jumper pads” near the newly-installed chip. They look like a pair of upper-case letters “E”, arranged back-to-back. We’re going to disconnect one, and connect another.

A tiny ball of solder (maybe 1mm in diameter) should indicate the jumper that is connected. (It’s between the corner of the chip and the edge of the PCB, right near pin 28.) I use solder wick (copper braid) to take this away. Alternatively, you can wipe your solder iron clean several times, and then each time touch it to the jumper blob to take some away. It’ll take a few hits, but eventually it’ll be removed. (Proceed slowly, and pause between each, blowing gently to cool things off.)

Interestingly, even though the solder pads seem to be designed to be connected with a tiny ball of solder, there also seems to be a connection already etched in place underneath the solder ball. This should be cut away cleanly with a razor knife.

5. Solder across the other jumper nearby
Right nearby, opposite pin 26, is another jumper pad that should be connected. I build up a blob of solder on the soldering iron, and touch it to the pads. The idea is to get the little ball adhering to both pads, to complete an electrical connection that did not previously exist.

6 Reassemble and test
Before reassembling, I shave off the tops of the four lower heat stakes, attached to the back half of the case. Then the case may be snapped back together (be sure to align the battery contact with its place in the battery well) and slip in the batteries.

Mine came up with a “Memory Clear” message. Then 2nd/CATALOG and MEM should display something like “Available Memory: 31,553 Bytes”. Pretty cool!

Additional Notes:

The jumper changes alter the function of pin 26 of the chip. Originally Chip Select 2 (which is tied to Vcc), it becomes a new address line.

You want to use a low power CMOS RAM chip. Some 62256s take rather a lot of standby current and they will drain the batteries quickly. Pick one that has a low standby current.

Be sure to orient the new chip correctly. The “pin 1 end” of the chip is marked with a notch or dot, and that should be pointed to the nearby edge of the circuit board.

Of course, increasing the memory doesn't get around the problem of the lack of any form of input port . . .


Tony Duell ( suggested the upgrade in a letter to the MoHPC Forum, and he supplied all of the important technical details.

The methods described for opening the calculator, and for removing and soldering the chips, are those that I’ve employed in upgrading my own two HP-42S devices.

Tony Duell’s Commentary:

I offered the first draft of this document for Tony’s comments. Here’s what he had to say:


1. Open up the HP-42S
I [Tony] always remove the metal overlay and drill out those posts as well. If you are careful it’s not hard to remove the overlay without damage. Just make sure you don’t crease it when pulling it off.

2. Remove the 8K RAM chip (the only 28-pin chip on the board)
Don’t even attempt it without a temperature-controlled soldering iron with a fine tip. You will damage the HP42…

Get some fine wire (5A fuse wire is ideal if you can get it). Feed it under 1 row of pins. Solder one end of it to a convenient large pad on the PCB (ground, for example).

Heat the pin nearest the other end of the wire. When the solder has melted (and not before!), pull up on the wire to lift the pin off the PCB. Then free the wire from under that pin and repeat for the other pins in the row.

Now one side of the chip is free. It’s easy to bend the chip back and forth to break off the other row of pins (you don’t care about the 8K RAM chip, right). Remove the body of the chip.

Now desolder the remains of the pins from the PCB one at a time.

3. Solder a 62256 32K (low power, 28-pin, surface-mount) RAM in place
I used desolder braid (solder wick, soldamop, whatever you call it) to remove the old solder. Then place the new chip in position, and solder down 2 diagonally opposite corner pins. Use the temperature-controlled iron and fine solder. I use 30swg silver-loaded solder (which is sold for surface-mount rework).

Check the chip is correctly positioned and the right way round. Check it again. Then solder the other pins down.

Don’t worry too much about solder bridges when you are fitting the chip. After you’ve fitted it, use desolder braid to clean up any bridges.

[This, BTW, is the only way to do fine SMD rework by hand. Clean up the bridges at the end. It’s how I replace chips in Voyagers, etc]

4. Cut one jumper pad near pin 28 of this chip
“Interestingly, even though the solder pads seem to be designed to be connected with a tiny ball of solder, there also seems to be a connection already etched in place underneath the solder ball.”
Yes, I found this, too . . .

5. Solder across the other jumper nearby
It’s generally not hard to get solder to bridge this jumper pad.

6. Reassemble and test
Put the batteries in and test it. It'll work without the case (OK, the beeper doesn't :-)). If you don't get the right messages or the machine won't turn on, or something, take out the batteries and check your work.

I [Tony Duell] call the result a “42SV”. ;-)


So, there you have it. The distilled HP-42s upgrade experience of two calculator fixers – the rookie (me) and the expert (Tony Duell).

Good luck!

-- Paul Brogger (


[ Return to the Message Index ]

Go back to the main exhibit hall