HP-28S Autopsy Photographs

[Paul Brogger]

Folks:

I've taken some amateur close-ups of a broken HP-28s, and thought some of you might like to see them.

1: The back case halves, with everything but the keyboard flex circuit removed.

2: The front case halves, with the PCB.

3: The closer view of the alpha keyboard's inside front.

4: The a pile of keys. (This should give you some idea of how the double-shot molded keys look and how the keys are hinged on the Clamshell models.)

5: The PCB front, with many LCD connection pads, above & below.

6: The PCB back, with RAM chip and keyboard connection pads.

7: A "personality shot" of the PCB, backlit by a slave flash unit. The idea is to emphasize the CPU chips suspended in space by a rectangular array of micro-fine wire connections.

These are all a first cut with various pieces of equipment gathered from the thrift store. I'll probably try again with better lighting, smoother background, etc.

The pictures look ENORMOUS when I link to them. If someone has a suggestion on how to reduce their initial presentation, I'll give it a try.

The HP-28s itself had been "flexed to death" in my son's over-full backpack. You'll notice that nearly ALL of the pins meant to connect the front and back case halves have broken. I may rebuild it using a drill press and a lot of tiny screws and acorn nuts. (It would be kind of neat to have an industrial-strength HP-28!) Meanwhile, it's a good subject for autopsy.

Enjoy!

[Daniel Kekez]

Hi Paul,

The photos look good. Thank you for making them available to us online.

Two rows and one column of keys on my 28S have stopped working. I suspect it's a false contact, but I haven't had the time to try and dismantle the unit. I'm glad that I can see what to expect when I try to take it apart.

As for your question about making them smaller -- perhaps use a program like Photoshop to resample them at 72 pixels/inch (screen resolution) and about 800 pixels wide. That would make them fit better in a web browser. But you might still want to provide a link to the high-resolution version for people that want to print out the photographs, or look at fine details.

Thanks.

[Paul Brogger]

The connector for the keyboard flex circuit lies atop a rubber strip that is intended to maintain contact between the flex circuit pads and the PCB. (Look for the blue strip beneath the black connector pads in the middle-right of the first photo.)

Pinching the calculator front-to-back just below the display should increase pressure on the connector, and may help solidify the diagnosis of the problem. In fact, by examining the photo, you may be able to tell which connector pads are involved. (If you're curious, let me know the rows & column, and I'll trace them for you.) Chances are, the connector pads are all together -- perhaps the three pads on one end of the connector or the other.

I've fixed Pioneer models with similar problems by "building up" the rubber pressure strip. Several layers of double-stick tape under the rubber strip will make it thicker, and hence exert more pressure on the connections.

But I've never dismantled a Clamshell before. I don't think the pins are mushroomed (melted over) -- at least not ALL of them. A few survivors still exist on my damaged unit, and they don't look as if their ends had been heated. I haven't taken the labels off of the front parts, however, and so I don't know what lies beneath.

Good luck! (If you do successfully dismantle yours, I'm sure there are folks here who would like to hear the details of your experience.)

[Vieira, Luiz C. (Brazil)]

Hi, Paul;

I must thank you for the opportunity you gave everybody. I had just one "exploded view" of the HP28S and it was a black-and-white scanned ficture from an original HP Journal issue. Yours are a lot better than the one I had to offer.

To those who asked me a copy of the exploded view, I can tell you these photographs are a lot more revealing.

Thanks.

Luiz C. Vieira - Brazil

[Hans Brueggemann]

paul,
if you want to improve the lighting, try to scan the objects with your scanner rather than photograph them.

cheers, hans

[Paul Brogger]

Thanks for the suggestion. I've scanned calculator exteriors before -- I don't know why it didn't occur to me to try scanning the innards as well . . . I'll give it a try.