Classic Series Enter Key


Hello everyone,

When I got my first HP-35, I disassembled it to clean the switch contacts. I noticed that the "enter" key has only one pressing surface on the back, yet there are two contacts behind the key. It's as if the double sized enter key should have two pressing surfaces. Does anyone know why this is so? If I had to guess, I would say it was done in the interest of saving money and time

When I cleaned the switch, I decided to post a facebook note about it, with pictures, so everyone could see how much of a calculator nerd I was (am)!

In the photos section of the note, you can clearly see that the enter key has only one surface, just like all the other keys.

HP-35 facebook note



Hi Dan,

Does the Carriage Return key on your computer keyboard have two pressing surfaces?



Hi Dan,

Does the Carriage Return key on your computer keyboard have two pressing surfaces?


I don't know if it, or the space bar for that matter, does *<:-) I only mentioned that site because it was the only place I knew I could get the picture, taken by me in my own house, from the Internet. I don't think you do need an account to see it though, I can view it just fine without being logged in. If someone can suggest an alternate way of putting the photos on here, I'd be happy to do it.


Here's something called a 'guest file directory'. Look under 'Accounts'. Works very well. That book of faces doesn't care for data privacy and is thus not liked here.


Edited: 18 Dec 2012, 11:39 a.m.


Here's something called a 'guest file directory'. Look under 'Accounts'. Works very well. That book of faces doesn't care for data privacy and is thus not liked here.


Thank you Walter, I have requested a guest directory. Thank you also for bringing that to my attention, I was not aware of that fact. I couldn't find anything in the forum rules that expressly prohibits talk about the book of faces, but as another user posted, another post was edited on the grounds of off topic conversation, which could be claimed in my case also.

Anyway, I got the answer I needed, and I was sure that HP made their choice in an effort to save money and time.




Only the first picture is linked, but you get the idea.

You might want to take a look at the following, in case you haven't done it yet:



P.S.: I only was able to see your pictures because my wife has a FB account.

Pictures and captions below by Dan Lewis:


The HP-35 (One of the first versions)
The legends on the keys are formed using an expensive, but very effective, double-shot mold process. The legends are cast all the way through the key, ensuring they can never wear off!

The workstation, showing the all important anti static mat and hairdryer (Thanks Mom!) The black coil in the lower left corner is the static suppression cord that connects the mat to the bracelet I'm wearing on my right wrist.
The back label of the calculator (before I got my hands on it), It even shows you how to "work" RPN!
Working very slowly with the hair dryer and razor blade, I pulled the corners up just enough to get to the screws. The calculator was TOASTY!

Here's goes nothing (everything)! I lifted the back off to peek at the inside. Now I was wondering If the static kit would work. I don't know what that white chip is, but apparently, it's OK!

A closer view of the big chip. Sorry, my hands were a tad shaky. The number 300 can be seen in the lower right corner. Your guess is as good as mine.

October 1972, a good year! See that black IC there, can't get that at RadioShack!

Time to dig myself deeper into the hole I'm in! The screws are backed out, releasing the circuit from the calculator's kung-fu grip. Those three silver canisters are the calculator's ROMs. It uses these, like an instruction manual, referring to them for every calculation.

Here can be seen the front half of the calculator separated from its "guts". the backsides of the keys can be seen on the left. More on these later.
This was really the only part I was concerned with, the on/off switch.

Flipping the calculator's innards over, we can see the surfaces that the keys press upon. These are Beryllium Copper strips...plated in gold! The key design has not been changed to this day. Start at the lower left hand corner and go right 4, up one. See it? One looks like it hasn't been pressed right? That's because the ENTER key is double the size of the other keys, but only has one pressing surface. The plastic sheet is for spill proofing. I wouldn't try a "whole" cup of coffee though.

Here's a close up of the ENTER key. See? I told you. Only one does anything, the other is just for show.
Here can be seen a close-up of the LEDs that make up the display. Each one of the 15 LEDs has eight individual segments. HP was the first company to produce LEDs such as these. LEDs of the time used too much power, so HP made their own, better ones. The two chips below the LED matrix also have the hp logo on them. Won't find these at "The Shack". Or nearly anywhere for that matter. You can imagine why I was so scared of static discharge.

Another close-up of the LEDs. The grime on the switch contacts can be seen on the right, between the chips and the key surfaces.
A close up of the four little black lines that had been causing all of my problems.

A little abrasion took care of the problem. Notice that most of the parts on this calculator are either gold or gold plated. Copper would've worn away long ago.
Here's the back of the enter key. I still don't know why they decided to make it that way.

Altough you may not be able to see it, before leaving, so to speak, I penciled in my initials and the date.
A closer look at those contacts on the bottom of the circuit reveals that they are actually snap-like in form. That way, the sides can be assembled separately and then snapped together.

Imagine there are two buttons, push one, you die, the other, you live. I'm exaggerating of course, but that's how I felt flipping that switch. I saw the 0, so I tried the famous 8888888888 CHS EEX CHS 88 that lights all segments of the display. To my delight, it works!





It would be nice to see your work without requiring a facebook account!



Perhaps it was found that if both contacts were mechanically 'struck', the tactile feedback to the user was noticably greater than the feedback from the other keys.

It would be interesting to know if the PC foil patterns below the contacts are both there.


Edited: 17 Dec 2012, 11:37 p.m.


The TI-66 'equals' key and the DM-15CC enter key, among others, both have two contacts. This works since those keys aren't hinged - on the TI-66, either of the contacts are pressed, rarely both. On the 15CC, it's nearly impossible to activate both contacts with one finger.

OTOH, the 35 keys *are* hinged so the user would always (have to) press two contacts, which would be quite irritating I guess.


It was done to decrease manufacturing costs. It allows for the same formed switch strip tooling and installation methods to be used along with keeping the layout of the PC board symmetrical.

FWIW, the Voyager design had the same "feature" on the enter key. The second dome was there, just no mechanical actuation by the keytop.


Aaaaargh, don't mention that book face site!!!

The moderator kills threads about it:


And then Bart (UK) asked me to delete the Bart (UK) account to protest it. (Which I obviously didn't.)

Hopefully everyone is calm, now.


And thus I now post here having been bullied into not mentioning Facebook here, even if it is HP calculator related.


Thank you. However, just to be clear, none of your comments in that thread were actually about HP calculators. The thread had many uses of the word "troll" and a claim that certain people do not "have a life", but contained no statements about HP, calculators, math, science...

The thread was reported to the moderators 6 times. I could find no real on-topic content and trimming out certain portions could imply blaming certain people for "starting it" so I just trimmed the whole thread.

Facebook is obviously popular. On the other hand, in a recent poll, social media edged out nuclear bombs as the technology people would most like to un-invent. ;-)

The only other website that I can recall causing threads like this is ebay, so let's try to leave the ebay debates on ebay and the facebook debates on facebook as much as possible.


The link is obviously still there. I remember that thread has caused a lot of anger among us, which left Dave no other choice than to remove the link and subsequent posts.

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