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lvt · 12-24-2022, 07:53 PM

Hi all,

I wonder why some HP calculators (landscape models) have small springs inside.

It may sound strange, but I have opened my HP 12C Platinum once, took a quick look to the PCB (I'm curious about what kind of chip my HP 12C has) then put it back together, and I didn't pay attention to those springs. In fact, I don't even know that the springs exist until I watches some videos on Youtube.

Are those springs functional? What happens if one is missing?

Thanks.

Steve Simpkin · (This post was last modified: 12-24-2022 08:14 PM by Steve Simpkin.)

They were used to electrically connect the front metal panel to the rear metal back plate. This provided enhanced ESD protection to reduce the chances that static electricity discharges would affect the calculator.

lvt · (This post was last modified: 12-25-2022 03:25 PM by lvt.)

(12-24-2022 08:07 PM)Steve Simpkin Wrote: They were used to electrically connect the front metal panel to the rear metal back plate. This provided enhanced ESD protection to reduce the chances that static electricity discharges would affect the calculator.

That makes sense, sort of Faraday cage?

But I remember a 12C that doesn't have a metal plate at the back, the instructions are printed on the plastic. I'm not aware that calculators with front metal plate need shielding. I used to have a couple of Casio calculators with front metal plates but I don't think they have springs like this.

So was it mandatory or it's just the tradition to make the 12C more authentic to the past models?

lvt · (This post was last modified: 12-25-2022 07:00 PM by lvt.)

So to clear my doubt, I decided to open my 12C again, and at my surprise, there is no spring at all!

Not even a single one.

That's why I didn't remember about the spring while opening the calculator for the first time.

Csaba Tizedes · 12-25-2022, 08:08 PM

(12-25-2022 06:38 PM)lvt Wrote: Not even a single one.

Those three grey circle on the left part seems connection points to the metal plate, maybe I'm wrong.

lvt · 12-25-2022, 11:49 PM

(12-25-2022 08:08 PM)Csaba Tizedes Wrote:
(12-25-2022 06:38 PM)lvt Wrote: Not even a single one.

Those three grey circle on the left part seems connection points to the metal plate, maybe I'm wrong.

Yes but it's plastic, I don't see how it helps with electromagnetism.

I bought my 12C brand-new in sealed blister. Model number is PHA127xxxx (27th week of 2021?).

**rprosperi** · 12-26-2022, 01:27 AM

It should be no surprise that a calculator made in 2021 had no metal plate on the inner rear cover to act as an EMI shield. In fact finding any metal at all in the case assemblies today is likely driven more by cosmetic choices than shielding strategies.

lvt · 12-26-2022, 04:26 AM

(12-26-2022 01:27 AM)rprosperi Wrote: It should be no surprise that a calculator made in 2021 had no metal plate on the inner rear cover to act as an EMI shield. In fact finding any metal at all in the case assemblies today is likely driven more by cosmetic choices than shielding strategies.

So if I understand well, because my 12C has a metal "cheat sheet" on the ouside of the back and with the front metal plate they already form a sort of shield, those springs aren't needed, right?

Steve Simpkin · 12-26-2022, 08:23 AM

Yes, the HP-12C is somewhat of a special case since it has been in production since it was introduced in 1982. There were at least eight hardware revisions of the HP-12C since 1981 (including one special issue). So the springs were probably not used on all versions.
https://en.m.wikipedia.org/wiki/HP-12C#Models

lvt · 12-29-2022, 05:55 AM

OK so my 12CP doesn't have springs but it has a quartz crystal, I wonder what the quartz crystal does in the calculator? There is neither digital clock nor chronometer function.

The only reason I would think is that the quartz crystal has something to do with the screen's adjustable contrast?

Steve Simpkin · 12-29-2022, 07:22 AM

(12-29-2022 05:55 AM)lvt Wrote: OK so my 12CP doesn't have springs but it has a quartz crystal, I wonder what the quartz crystal does in the calculator? There is neither digital clock nor chronometer function.

The only reason I would think is that the quartz crystal has something to do with the screen's adjustable contrast?

If it is similar to the HP 35s (which uses the same Sunplus CPU) the crystal is used as a RTC oscillator and is used for display refresh and keyboard scanning.

https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=130623