HP-35S disassembly photos?


Has anyone opened up a 35S yet?


Yes. There are six screws: four in the battery compartment covered by round rubber plugs, and two under the wide rubber foot. After the screws have been removed, there are still four plastic catches holding the case together.

My good camera is dead, so I was only able to take photos using the terrible camera in my Treo 650 phone. I suppose that's slightly better than nothing. There are three photos here.

The GeneralPlus microcontroller and the static RAM are both mounted chip-on-board under blobs of epoxy. I'd hoped that the SRAM might be a normal surface mount part, so that it might be possible to replace it with a daughterboard with the SRAM and a CPLD, to add some I/O capabilities. Oh well.


HP35s is beautiful outside and inside...

Here is an CT topogram image of the HP35s I took this morning



FYI only, your tomogram doesn't show up here.


Yes, I did FTP the .jpg to hpmuseum and emailed the curator as documented. I guess it will show up soon. I can email it to anyone directly if they are interested.



Ok here is an image of the HP-35S I took with a CT scanner.

I do see alot of screws or other mechanical fastners etc.



It seems fitting that the 35s is so full of mechanical fasteners (screws). After all, it commemorates the 35 which if I'm not mistaken was designed to be repairable.



25 screws to hold the main PCB to the case...


Back when I was repairing Televisions, I composed a "rule of thumb"

"The cheaper a television, the more screws hold the back panel in place."

RCA's and Sony's used 4 screws, off-brands had as many as 14.


dona nobis pacem


That remindes me of 'ARDs Law of DECSA Construction' (The DECSA -- Digital Equipment Communication Server Aparatus, or something like that -- was an early ethernet-X25/etc bridge, with a PDP11/24 CPU in the middle of it).

Anyway, the law states 'The lower the importance of a part of a DECSA, the more screws hold it in place'. It is illustrated by the fact that :

All the main PCBs plug in to the backplane without any fixing screws

The PSU(s) are held in by 2 screws each

The fan tray is held in by 4 screws

But the grille over the fan tray is held in by an amazing 28 screws (yes, I counted them all out and back again when I had to dismantle a DECSA for repair)


The use of 25 screws is almost certainly because it significantly stiffens the case. If you remove most of the screws the case can flex much more. From the Woodstock series through the Pioneer series, the same thing was accomplished through the use of heat stakes. Apparently Kinpo must find it more cost-effective to use screws, though that seems counterintuitive, or they might want to be able to send an assembled calculator that fails final test back for rework, though that seems even less likely.

Given how many people in this forum have complained about heat stakes in the past, I'm surprised to find people now complaining about screws.


Eric, you're right: from our point of view as (more or less) educated users, screws are optimum. Nondestructive service becoming possible again after many years :)


I wonder, would the choice of screws vs. heat stakes say something about the projected size of the 35s' production run? (Would the setup have been more expensive for heat stakes than it was for screws?)

Or how about the early units being put together with screws, with the option of converting to heat stakes at some point in the future?

With the chips encased in blobs of epoxy (or whatever), I doubt that much in the way of service is anticipated.



Given how many people in this forum have complained about heat stakes in the past, I'm surprised to find people now complaining about screws.

I'm not REALLY complaining,
it was more of a smart-aleck observation.


dona nobis pacem

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