"Pioneer Internals"

Pioneer "Observational Internals"

I've found the Pioneers relatively easy to open, repair, and put back together. I thought I'd share my experiences with any who might be interested in trying themselves. This is a revison of an earlier HP Forum post, entitled "Re: HP42s, How to Open". I'd appreciate any comments on misused terminology, confusing verbage, or additional information!

(Thanks to Joseph P. III, Tom D., and Robert K. for supplying broken Pioneers at minimal cost, which GREATLY facilitated exploration!)

Opening the Pioneer Outer Case

After removing the battery cover and batteries, I use a 3/32" drill bit to cut off the tops of the four heat stakes. (I mount a sharp bit in a counter-sink tool, and rotate it slowly by hand -- one need only cut out the melted "rivet head", about 1/16" deep.) This frees the front and back case halves at the top (display) end of the calculator.

To fully separate the halves, I've then just carefully but forcibly pried the four lower heat stakes apart, working from one side to the other, with nothing other than my fingers. The keyboard is backed by a metal plate and remains relatively stiff, the PCB is free and unstressed, and the back attachment is plastic only.

At this point, you'll have the back cover and front (keyboard) separated. In the older models, the back is lined with an aluminum ground shield, presumably for the reduction of RF interference. In the models with beepers (e.g., 27S, & 42S), you'll also see a piezo disc attached to the plastic back.

PCB Removal

If you wish to remove the PCB (printed circuit board) from the front assembly, it comes off easily. Twist the six hold-downs in the obvious directions (back to their original form) and straighten them so they lose purchase and pass through the slots on the PCB. Work with them until the PCB is easily lifted from the LCD/keyboard assembly.

LCD Removal

If you want to go further, you'll have to contend with some nasty glue that attaches the LCD (liquid crystal display) along its edges to the steel frame which serves as the front assembly's "backbone". Work VERY slowly and gently here. I've started prying very gently on one end with a padded screwdriver (a Popsicle stick might be better), slowly but surely separating the LCD from the front assembly. Gentle pressure will free the LCD without breakage, as the rubbery glue gradually gives way from one side to the other -- you'll hear the glue separating as you go. The whole process should be dragged out over a couple of minutes -- don't rush it!

The LCD "zebra stripes" (soft, rubbery connection strips along the top and bottom of the LCD) should be kept clean, as should all of the interior contacts, or they won't work when you put everything back together.

Keyboard Disassembly

If you want to see the keyboard innards, you may be kissing your calculator goodbye, but not necessarily. There are over 50 plastic posts which have been "mushroomed" (melted flat, creating rivet-like heads) to hold together the keys, the springy contact sheet, the flexible (mylar?) keyboard circuit, and the metal "backbone". If you clip all these "rivet heads" off, it will be difficult (but maybe not impossible) to reassemble the keyboard.

I took one apart to see what's inside, but the only reasons I can think of for actual repair are either to clean out some sticky goo, or to replace worn plastic and mylar contact sheets.


With the LCD and zebra stripes in place, push the PCB slots over the six hold-downs, and use pliers to re-twist them. Some are used for ground connections too, so make sure adequate contact is made with the PCB pads where appropriate. And don't scratch other traces with your pliers!

I shave some of the excess "mushroomed" plastic from the four lower heat stakes to simplify re-insertion, and pop the cases back together. I can remove the back any time I want. I suppose that, if dropped, the back will come loose or completely off. However, the PCB / LCD / keyboard bond seems as strong as ever.

Miscellaneous Observations

Two Bezel Types (and incompatible parts)

HP has used two externally different (and readily identifiable) versions of the Pioneer over the years. The early calculators had a flat, two-step bezel around the LCD display. (See the MoHPC image of the HP-32S.) This featured an extra clear plastic (glare-reducing?) layer over the actual glass LCD. On well-used units, dust will often collect between the plastic layer and the LCD -- a very annoying trait, and hard to remedy.

On newer models, the border around the display is angled inward. (See the MoHPC picture of the HP-42S.) There is no plastic layer, and the LCD is itself directly exposed.

It is my experience that later-model displays won't work directly in early-model ("stepped bezel") calculator frames. The space between the bezel front and the metal "seats" for the PCB seems more generous in the older model, and the six metal hold-downs don't seem to be able to create a firm enough bond to make adequate contact between the PCB and the newer display. (I have made one work, by padding the display in front to make it "thicker" and therefore providing the contact needed.) This indicates that there are different versions of the displays, and that one is thicker than the other. (Though I couldn't discern any such difference upon examination -- maybe I'm missing something!)

Different LCD's

There are at least four different LCD units employed in the various Pioneer models, and (as noted above) perhaps a couple variations of some of those.

The high-end models (17B, 27S and 42S) apparently use the same 16x131 dot-matrix display. I'm guessing that the HP-17BII does as well, but I haven't had any of the 17B's apart, and don't know for sure. (If this is so, it should be possible to replace a broken HP-42S display by buying a current-model HP-17BII.) This display enables the two-line display, labeled menus and limited graphing capabilities.

The mid-range models (14B, 22S, 32S, etc.) seem to use the same single-line, large-numeral, alphanumeric LCD. This display utilizes 5x7 LCD dot matrices for more readable characters and numerals, but it is not a general-purpose graphic display.

The HP-32SII display is a variation on the 32S, with even larger, more readable characters and two shift annunciators.

The low-end Pioneer display is a 7-segment display used in the 20S and 10B calculators.

Keys and Keyboards

The Pioneer calculator keys and keyboard are molded in one piece, as part of the front of the calculator. Plastic of contrasting color is injected (or whatever) from the back to produce essentially indelible legends on the keycaps. Each key attaches to the plastic surrounding it via two thin hinges (again, all part of the same plastic casting). The metal-foil faceplate covers the hinges and the voids which surround the keys and which allow for the keys' movement.

All Pioneer models seem to have the same circuitry implemented within their keyboards. That is, despite their differing keyboard layouts, it seems all Pioneer keyboards are functionally identical. It is each different model's particular CPU and ROM logic which implements the varous functions embossed on the keys. The keyboard is connected to the PCB via a 15-conductor "contact connection", made when the PCB is fastened to the calculator frame.

Circuit Boards

The older Pioneer models utilize a (to me) novel form of surface-mount technology, with rigid plastic frames holding the spidery CPU chip in place via its many leads. Indeed, the CPU is set in a hole punched in the circuit board, and is entirely suspended by its many (VERY many!) hair-fine electrical lines. At first glance, the silvery sheen of these leads looks opaque, but by bringing the PCB close to the eye (or by back-lighting it), one can appreciate that the CPU is "riding on a hammock" in thin air.

The three older USA-built models I've dismantled all had handsome white, translucent circuit boards with anti-corrosion gold plating on the electrical traces. (See the MoHPC internal picture of the HP-32S.) The models from overseas all had green circuit boards, also with gold plating.

The high-end models (at least the 27S and 42S) have longer circuit boards, with spring contacts for the piezo buzzer. They also have had two chips -- the CPU and another 28-pin surface-mount chip, that appears to be an 8K (or so) ROM. There are two rows of pads for another 28-pin chip on each board, but in the several units I've examined, those pads have been empty. There are also several "jumpers" -- pads of interleaved but separated traces, where a small blob of solder will complete an electrical connection. These appear to be associated with write enable and chip select lines. (On one 42S, I tried mounting a static RAM chip, and switched a few jumpers, but never got my "RAM upgrade" to work.)

All the low-end models have had short boards with one chip and a couple of other surface-mount devices. All of the circuit boards appear to be two-sided, with no internal layering.

The one 27S and several 42S boards I've seen differ visually only in that the 27S has a transistor near where the 42S boards have had a diode. The 27S has a clock, so maybe that's what accounts for the difference. Presumably the 27S and 42S ROMs are very different.

With the HP-32SII and newer low-end models, HP has greatly simplified the PCBs' layouts. The new PCBs have traces on only the front side, with a ground shield mesh on the back. (This has eliminated the need for metal shielding in the back of the calculator.) Both the front and back are etched copper, with no gold plating. (The new models may be more susceptible to corrosive moisture, though with the older units' delicate CPU leads so exposed, that may be a moot point.) The CPU chip mount has been simplified: it is still set in a rectangular hole in the PCB, but the chips themselves have more conventional plastic bodies and a surface-mount form.

The new chips' leads have been assigned with simple construction as the goal. As a result, the display and keyboard connections require no crossovers, the circuit board has no feed-through holes drilled, and no jumpers are installed. The new-model calculators could hardly be further simplified, and so are masterpieces of economical design. They lack, though, the nerd-aesthetic appeal of the shiny, more complex older units. (I presume it's the MBA aesthetic that dominates, these days . . . )

I'm curious to see how HP proceeded from the 17B to the 17BII, and what those PCB's look like, but I've not had the opportunity to open either yet.

DO Try this at Home!

If you want some practice before digging into your favorite, or you just want to take a look, you're encouraged to buy one or two HP-14B calculators from InterFab at only $9.95 each. (No, I don't work for InterFab!) They're surplus service-replacement stock, obtained with the buyout of EduCalc. The ones I bought are all older, stepped-bezel, made-in-America units, mint in plastic.

Now to me, an algebraic business calculator is just about as useless an item as has ever been created. But at the very least, their LCD's (if not the keyboard components) should be useful for repair of a much-loved 32S. And some come with the nicer, stitched vinyl slipcovers, themselves unused and in mint condition. Finally, the sight of that little CPU chip suspended in space by its external connections is just downright poetic.

Right now, I've got a 42S circuit board and LCD (from a VERY beat-up unit) in a brand-new 14B "body". I've spray-painted the keyboard black (the keycaps were almost all wrong, anyway) and I've memorized the layout. (Besides, you can hold down most 42S keys & see what the functions are . . . ) I call it my HP-42BS. A real conversation-starter, and very designer-ish-looking!



Remember, the quicker way to open the cases - the method I used to obtain inexpensive calcs - is to allow your kids to carry the calculator in their bookbags. Shortly thereafter, open Pioneers!

I have two HP32SII "opened" in a similar manner but I just can't part with them. At our offices we provide training for individuals, so from time to time someone brings their 32SII. During their training, I pick up their calculator to "double check some figures", turn away for a moment and get the broken 32 with a faint screen. Next comes the set-up. "Look, your batteries just went bad. I'll get some for you." I then go to my desk and return with the other boken 32 with the peeled-back face plate. In my most sincere tone I ask "Where do the batteries go?" Paul, you should be there to see the looks on peoples faces.

Thanks for the serious info, Joe


Highly informative, thank you! (I own a 20S and a 32SII). Do you happen to know why the rubber feet on Pioneer models are different? What would be a reason to have different feet on different models?

Very nice you made yourself a 42BS (bull shit? ;-)). Did you ever think of the fact that nobody will be able to use it?


I hadn't noticed any difference in the rubber feet among Pioneer models -- I'll take a look.

And yes, there are probably few on this earth who would make sense of my 42BS -- but then that's one of the benefits of RPN calcs, right? (The uninitiated leave 'em alone.)


Having just obtained my first HP-17BII, I've found that its circuit board and display seem identical to those of the HP-42S. I swapped a -42S circuit board into a -17BII and had a -17BII case that behaved like a -42S. I suspect that the ROMs and the externals (keycap legends, color) are the only differences between the two models. (Which makes the continued availability of the HP-17BII and UNavailability of the -42S even MORE tragic!)

My next experiment (when I get enough nerve) will be to swap an HP-42S ROM onto the HP-17BII circuit board. (Hey, if someone could make copies of -42S ROMs and use them to upgrade -17Bs, would anyone be interested?)

For the time being, -42S fanatics should recognize that the best way to insure the longevity of their calculator may be to buy a HP-17BII and tuck it away somewhere for future spare parts (esp. display). (Do keep in mind that, for the time being, HP will still service/replace the HP-42S -- $69.00 at the time of writing.)


I'm interested in swapping the roms!! I miss my 42s.

See my message of today.


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