|HP35s Internal Investigations re: Creation of the PH35sx|
Message #1 Posted by Jeff O. on 10 Jan 2008, 8:37 a.m.
There was much discussion late last year regarding the desire for and creation of a new calculator, either by HP or by a dedicated independent group. Many people spent a lot of time and effort defining what such a calculator might look like, the functions it should have, how it might be produced, likely hurdles to an independent production, etc. At some point, the possibility of modifying the 35s instead of building something from the ground up was raised. DaveJ conducted a poll on the idea. In that discussion, a link to the Newhaven Displays web site was provided as a possible source for an LCD display to use in a home-brew or retro-fit project. Just for fun, I poked around that web site and found what appeared to be a number of candidates that could work. I was surprised that they weren’t all that expensive, so, in a fit of irrational exuberance, I ordered six displays of various types and sizes just to see them in person and hold up next to my 35s to assess how they might work. Being primarily interested in a multi-line graphics-capable display, I ordered the following:
NHD-10032AZ-FSPG-YBW, 100 x 32 pixels
NHD-GDSC-12864WM-09-RN-GBW, 128 x 64 pixels
NHD-12032BZ-FSW-GBW, 120 x 32 pixels
NHD-12232DZ-FSPG-GBW, 122 x 32 pixels
NHD-LGC12865AZ-RN-FBW-3V, 128 x 65 pixels
NHD-LGC-12864GG-RN-GBW, 128 x 64 pixels.
This is what they look like next to my 35s:
I really liked the large 128 x 64 pixel display. It compares very favorably in size with the 48G display. Here's what it looks like on top of a 35s:
It looked like there might just be enough room for this display, with the goal of achieving a transformation like this:
I shared the above information with DaveJ and we exchanged some e-mails on the project. It became obvious that to go any further in figuring out which display might work as well as many other things, it would be necessary to fully disassemble a 35s. So, in another fit of irrational exuberance, I ordered a 35s to be used as a dissection subject. (I already had two 35s’s, but didn’t want to tear into either one of those for “sentimental” reasons, I guess.) I ordered a new one from newegg.com and received it shortly before Christmas. The disassembly process went as follows. Opening the case is easy. Remove the rubber plugs in the four corners of the battery compartment, then peel off the rubber strip on the bottom of the calculator, and remove the six screws. The front and back halves of the case are also held together by five plastic catches, one in the middle on the top edge, one in the middle on each side, and one near the bottom on each side. These catches are not exceptionally tenacious; some gentle tugging at the case halves and careful prying between them will fairly easily separate the front and back, yielding this:
(credit and thanks to Lyuka for the above picture.)
It certainly appeared that the PCB was held in by those small screws (25 of them), so I removed them all. Gentle prying on the edges of the PCB did not yield any movement. Close inspection revealed that the posts into which the screws were screwed were mushroomed over at the top, just like the heat stakes in other HP calculators. Taking a cue from the tips for disassembling those calculators, I attempted to trim off just the mushroomed-over portions of the posts, i.e. leave the center part of the post sticking through. Due to lack of skill, lack of close-focus ability (damn presbyopia!), lack of sharp knife, type of plastic, or some combination thereof, I was unable to trim them cleanly enough to free up the PCB. I then decided to just trim the posts off flush with PCB, and with a little gently prying, was finally able to free up the PCB. (I then realized that this was likely to be necessary anyway to enable the screws to hold the PCB back in when it is re-installed. Otherwise, the screws would not be able to tighten down against the board to hold it firmly in place.) Once the PCB comes out, next is a thin rubber membrane, then the keys themselves. The keys are in two groups with a number of keys connected to a plastic frame, with the four cursor keys adhered to their own separate rubber membrane. Here are some pictures of the disassembled calculator:
Main PCB, back side:
Main PCB, front side. The white covering is a thin sheet of plastic of some sort that is adhered to the board. The little bumps, (not all are visible) appear to be metal dome key switches:
Key assemblies, front:
Key assemblies, back. The small post in the center of each key is what actually presses the key contact through the black rubber membrane:
Rubber membrane. Sorry about the poor quality of the photo. This is the side that faces the keys. It is flat except for small circular raised areas below each key, some of which are visible. The back of the membrane (the side toward the PCB) is flat:
Front case half, outside:
Front case half, inside:
There does not appear to be any sort of contact strip or similar which interfaces the key switches on the front of the PCB with the electronics on the back side. My uneducated guess is that the traces that interconnect the key contacts come through the board in various locations, which would make interfacing a new board with the factory key switches challenging. At first I found this disheartening, but then I wondered if it might be possible to basically strip all components and traces off of the back side of the PCB, find the spots where the connections are made to the key contacts on the front side, and solder to those points. (By strip off the components, I mean taking a soldering iron and removing everything that can be unsoldered, scraping the epoxy (or whatever they are) blobs off of the integrated circuits, desoldering or prying or cutting the chips off, then holding the board against a belt sander to take all the traces off.) I shared my above findings and ideas with DaveJ. Suffice it to say that he was not enamored with my idea for stripping the back of the PCB to use the existing key switches. Dave feels it would be entirely feasible to create a new PCB for the new electronics on one side and with new metal-dome key switches on the other. I’ll leave it to Dave to explain this more fully.
I then turned my attention to how or if any of the displays might fit. First I placed the large 128 x 64 pixel display in the display area of the inside of the front half of the case. There are four screw posts in this area. The large display will not fit between these posts side to side or top to bottom; at least two of the posts would have to be removed. I think it would probably be OK to remove the two posts at the very top of the case. The big display might then (barely) fit between the two remaining posts and the top of the case. It might be necessary to remove some of the inside edge of the top portion of the case, which would also necessitate removal of the plastic catch in this location. I don’t know if the remaining fours screws and the plastic catches on the sides would be enough to securely hold the front and back case halves together. Here are a couple of pictures:
Large display with LCD facing front:
Large display with LCD flipped over:
I think that the large display could be made to fit inside the case and would provide a very nice display. With 128 x 64 pixels, I figure a display that would show a row of soft-key labels, the X, Y, Z, T and Last X registers, plus have room on top for some annunciators and a clock display could be created. (That’s based on using nine pixels for the soft-key label and register rows, leaving ten pixels at the top for the annunciators.)
However, if the large 128 x 64 pixel display just won’t fit, the 128 x 65 pixel display would almost certainly fit. It is narrow enough to fit between the two screw posts at the bottom of the display area, but the top two posts would still have to go. However, this would allow the display to move down enough that I don’t think it would be necessary to remove the top plastic catch. This display looks like this:
For completeness, here is the small 128 x 64 display. There would not appear to be any reason to consider this one as the top two screw posts would still have to be removed:
Finally, if none of the above displays would work, the largest of the smaller displays (the 120 x 32 display) should not require the screw posts to be removed.
Other issues and observations – Even if it is possible to make a new PCB with a new processor and install a new display in the case, the issue of re-labelling the keys and the keyboard will remain. It seems like getting new legends on the keyboard itself should be doable via several methods. One way might be to merely paint over the existing keyboard, then apply new legends via silkscreen (never done it, don’t know how hard or costly) or rub-on lettering or other. The whole face of the calculator appears to be a thin aluminum sheet similar to the back labels on the classic calculators, so perhaps it could be removed and a new one fabricated. (Such fabrication should be possible, see the thread of some years back regarding fabrication of replica or “fake” rear labels for classics.) I’m sure there are other possibilities. Re-labelling the keys themselves looks like it will be a tough problem to me. If we could live with the factory legends on the key faces, that would help, but almost any good new layout will require some changes I think. At a minimum, I think all agree that we need “STO” to be an unshifted function, and I’d like Pi to be primary. For example, my idea of a good layout would look something like this:
The above layout would require 21 of the keys to get new primary legends (only 19 if the existing shift keys could be tolerated.) While not everyone would necessarily agree with my layout, I’m guessing most alternatives would require a fair number of keys to get new primary function labels on top. Then of course there are the legends on the sloped front part of the keys. Getting rid of those would be doable with sandpaper or paint, but placing new ones on the keys would appear to me to be very challenging.
About the only thing left that I can try would be to take a dremel and start carving out the case in the display area and see which of the displays will fit. I'm willing to do that, as well as anything else that would require the services of my donor unit, if doing so will actually help this project along. For the time being, I think my unit could be re-assembled, so I don't necessarily want to do anything irreversible yet. In fact, after I took the above pictures, I put the PCB back in and reattached it with the screws. The screws seemed to thread back in OK and hold the PCB securely, and the key-press feel seems to be as good as new. Unfortunately, I broke off one of the battery connection wires (the yellow one) and have not had time to re-solder it. The calculator does turn on this way and seems to function, but it shuts itself off after a few seconds. If I hold the wire in place, it stays on and seems to work fine, but it's a little awkward to give it thorough test. However, I'm confident that it does still work.
At this point, I've probably rambled enough, so I'll invite Dave to comment on my findings, and the next steps in the project.
Edited: 10 Jan 2008, 9:10 a.m.