While I cannot sleep well for it's too warm and humid tonight here in Yokohama JAPAN, I disassembled an HP 35s just for fun and took some photographs.

http://www.finetune.jp/~lyuka/interests/calc/hp35s/hp35s-disassembled.html

Good night.

Edited: 14 Sept 2007, 4:16 p.m.

Fun stuff!

As Eric told us earlier, there is no prospect of switching out the CPU on that guy!

Regards,

Howard

what about switching out the crystal? maybe it has a lot of inherent performance headroom, at the expense of battery.

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Fun stuff!

As Eric told us earlier, there is no prospect of switching out the CPU on that guy!

Regards,

Howard

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It wouldn't be too hard to design a complete new board for it... if you were that keen. Hardest part would be re-attaching the LCD flex cable.

I can only see a (presumably) 32.768KHz watch crystal. Maybe a PLL is used to generate a higher frequency clock?

BTW, what's with the *24* screws holding that board in?? Seems a tad excessive.

Dave.

I'm no industrial design expert, but the reason for using so many screws is probably to stiffen the case.

If you like the tactile response of the 35s' keys then you like those screws. Imagine if you only had screws on the perimeter of the board; every time you tapped a key in the center the board would flex.

Edited: 14 Sept 2007, 10:10 p.m.

I'm not a design expert either, but I was impressed that screws were used at all considering heat staking is the less expensive approach you'd expect for calculators fabricated in China.

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If you like the tactile response of the 35s' keys then you like those screws. Imagine if you only had screws on the perimeter of the board; every time you tapped a key in the center the board would flex.

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Not if you have a sufficiently thick PCB. It's all a trade off between where you want your cost to be, room in the case, construction options etc.
Can anyone confirm what thickness PCB is used in the 35S? (just curious)

In the 35S it looks like the PCB designer laid out a full complement of screws on a fixed grid and simply removed those where space was required for the parts and tracking. So, PCB first, and then the case moulding was designed to match the remaining screws holes. Interesting.

Dave.

I think they should have put even more screws, that would make the calculator heavier :)

reth

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I think they should have put even more screws, that would make the calculator heavier :)

reth

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A thin lead plate glued on the inside back cover is much more effective for that.

Doesn't quite make the device ROHS compliant though :->

Dave.

I used to think that the screws in calculators fastening the PCB to the case top were to improve the rigidity of the PCB. However, my own experiments seem to indicate that the PCB tends to be more rigid than the case top, and that the screws serve more to increase the rigidity of the case top. Either way, though, it's fairly important.

Yes, I was surprised by that as well. With automated manufacture, perhaps there is less difference in cost between screws and heat stakes than we thought, but it's hard to believe that screws could be less expensive than heat stakes.

Note that many inexpensive calculators from Casio, Sharp, etc. also use screws.

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... the screws serve more to increase the rigidity of the case top.

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I suspect that a stiff case prevents it from acting as a sounding board when keys are pressed. The sound of the key press is a large part of the HP keyboard "feel".

-- Richard