A coincidence, really, but I just finished restoring this HP 34C!


It started with another box with 4 spice calculators in it as well as 4 chargers, and 3 metal battery holders with one plastic separator. Also included was a 34C and 38E quick reference cards.

I have ignored the spice series for awhile and decided to tackle the 34C, as this unit really is impressive. Not only does it have the program continuous memory but also a 3 colour coded set of shift keys reminiscent of the HP 67.

This unit is coded 2045SXXXXX for 1980, Singapore.

Of course the first order of business was to inspect the case, top and bottom shell. You can see glue residue on the top LED display cover; figure 1. Also note the filthy keyboard and keys. The ON/OFF key was intermittant at best and would not stay on once selected. By pressing gently on the key I got the display to be stable, where upon I pressed "STO" followed by "ENTER". I waited a few seconds and up popped the -8,8,8,8,8,8,8,8,8,8,. Signifying a successful electronics test. The keyboard response on this calculator is nothing less then stunning. The positive click on each key is incredible.


I then turned the calculator upside down to document the bottom shell when I noticed a bunch of dirt fall out of the now inverted calculator. You can see, with reference to the Canadian penny for scale, the outline of the calculator (enhanced with the boxes); figure 2. This is the dirt that fell out of the spaces between the keys and the keyboard bezel. A close up of the dirt can be seen in figure 3 with R/S key for scale. I was amazed that the keyboard functioned at all as this calculator looks as though it was buried in the sand for a prolonged period of time! However this is a testament to the HP quality and the sealed keyboard.



Eventually I took a picture of the bottom shell with the glued on label; figure 4.


The battery compartment, battery holder and battery compartment contacts are all in excellent condition with no corrosion traces or cracks; figure 5, 6, and 7 respectively.




Of course the hardest part about a spice restoration is entering the case! After undoing the screws, I inserted a sharp long bladed knife between the upper shell and bottom shell at the foot of the calculator. This separated the bottom half of the calculator enough (a knifes blade in width) to allow me to simutaneously pull the upper half apart, and push the bottom shell down longitudinally to the calculator. You can see the L shaped clip that resides at the foot of the calculator that causes such a problem for shell separation; figure 8. This is the reason for separating the lower halves with the knife to allow the two halves to slide apart.


Once inside, luckely without breaking or cracking the case, I was able to expose the top shell back; figure 9. Also the inside portion of the top shell holding the framework and PCB; figure 10.

After removing the framework, holding the entire PCB in the case, by gently prying three clips out of the way on one side and lifting one edge of the PCB out, I was able to gain access to the keys and upper shell; figure 11.




The keyboard was now exposed indicating the problem with both switches. They were encrusted with a yellow stain and gritty dust; figure 12. A close up of the ON/OFF switch cleaned in contrast to the PRGM/RUN switch on the left can be seen in figure 13. Finally figure 14 shows both switch contact areas cleaned and greased ready for assembly.




This calculator has the floating (non-soldered) IC's and since the calculator passed the self test I did not think it wise to open up the next set of clips and expose the IC's. I did give this calculator a good dusting down with a blower and then tested it before reassembly. I must say that the dirt was confined to the exterior portion of the keypad and that the cello wrapped key pad was perfectly protected from dirt intrusion.

The case was gently cleaned in soapy warm water with natural soft bristled paint brush. After applying some citrus oil to the goop on the LED cover lense I managed to remove the detritus and expose the mildly scratched cover. A bit of plastic watch crystal polish cleared up the scratches. A touch paint around the ON/OFF switch bezel completed the top shell.

The bottom shell lable was also removed via the citrus oil and the case cleaned. Each key appeared to be covered in a dirt/oil/grime residue so the were individually scrubbed with a tooth brush.

Everything was dryed in my drying box and then the calculator was tested without the back in place with the self test; figure 15.

The end result as seen in figure 16 and a side by side comparison of before and after can be seen in figure 17.




Hope you enjoyed this little presentation!

Cheers, Geoff

Edited: 23 May 2009, 6:53 p.m.


Hi Geoff,

Again a beautifull restored machine.
Can you share with us how you restored the worn edges at the on-off switch, Did you use a filt pen, if yes what type?, Also it looks like the word "on" is now much scharper, as if it was painted again?

Kind regards


Edited: 24 May 2009, 5:09 a.m.


Absolutely correct, an airbrush would be better I think, have to wait until Christmas or Birthday for that though!

Extra fine sharpie black marker, indelible ink.

Cheers, Geoff


An amazing job - thank you for documenting it so thoroughly and for sharing it. It's rather inspiring to see examples of what is possible given skill, patience, and a lot of time!

Can I take advantage of this HP-34C thread to ask a question about this machine's keyboard? My HP-34C is the only LED HP calculator that I have. The keyboard works well, as does the rest of the machine. However, the keys rattle! They are so noisy that I keep expecting them to fall out, although they seem secure. None of my other HPs have this "feature".

Is this normal for the machine, or is it a problem that I should be trying to fix?

Thanks for any advice that anyone can give me.




Is your HP-34C the soldered version or is it unsoldered? As mentioned in the previous thread, I have a soldered HP-34C with a deteriorated elastomeric keyboard overlay. The overlay is approximately 0.025" thick and is located between the plastic keys and the metal snap domes of the soldered HP-34C. The locations where the overlay is missing, result in loose keys since there is approximately 0.025" of extra space for key movement.

In comparison, I have an unsoldered HP-31e that does not use an elastomeric overlay. Instead it has plastic snap domes. The plastic snap domes are about 0.040" tall whereas the metal snap domes are about 0.016" tall. This difference in snap dome height corresponds to the thickness of the elastomeric overlay, which seems to imply that HP used the same case tooling and compensated for the snap dome height difference by inserting the elastomer.


I've haven't opened it yet (after all, it does work!) but the serial number starts "1850", which suggests that it was made in 1978. From what I've read, this isn't possible as this model wasn't released until 1979! Either I'm mistaken about this, or the calculator's back is from another 3x-series model.

Although I don't know which sort it is, your idea of a missing overlay sounds reasonable. The keys definitely have too much freedom in the vertical direction, which matches with what you suggest. I think that I shall open it up and have a look, especially since (with the false back) it must have been opened before. I would also quite like to find out whether it is soldered or solderless.

Thank you for the information.



It's likely that the production line was ramped up before the official introduction in order to have sufficient quantities available through the distribution channel.


It is very unlikely you'll find anything out of place with the unit, the version one units with the plastic dome switches did have very loose keys which rattled when shaken. The rubber dampener sheet made its first appearance on the 41 so the original Spices did not benefit from that improvement, only the second generation Spice keyboards had it.

My advise would be to let it be, there is a chance you'll chip the edge of the center case band when removing the back. Further, there is a risk of breaking one of the six tabs that hold the keyboard assembly in place.

I personally find the original Spice keyboard to have the worst key feel of any HP, the pressure required to collapse the dome is higher than any other keyboard design and the travel is too far for speed efficient entry.


I've slept on it, and I think that I'll take Randy's advice, at least for now. Cosmetically the calculator is in good condition, and apart from the rattle the keyboard works reliably with no missed or double presses. I haven't taken a Spice calculator apart before, and possibly a working HP-34C isn't the machine to practise on! I also don't have a clear idea of what material to put beneath the keyboard to stop the rattle, even if this is possible.

Thank you all once again for your comments. I'll attack a Commodore 5R39 instead - the penalty for failure there is much lower!




I agree, don't open the unit. My guess is it's a solderless 34C due to the early serial number and would not have the overlay.


It is very easy to determine whats inside without opening it, just weigh it without batteries.

~200 grams = solderless (V1)

~130 grams = soldered (V2)


The mass of my calculator - without battery pack, but with the pack cover - is 176g, so it's closer to 200g (solderless) than 130g (soldered). That seems to settle it - thanks!


Possibly Related Threads...
Thread Author Replies Views Last Post
  HP-34C Draining battery quickly Bill Crowell 2 905 09-08-2013, 08:30 AM
Last Post: aj04062
  HP-34C Curiosity Matt Agajanian 7 1,785 08-23-2013, 06:02 PM
Last Post: Stephan Matthys
  HP-34C Emulator BShoring 8 1,771 03-30-2013, 05:04 PM
Last Post: BShoring
  Matte-finished emblem of HP-15C M Habl 3 1,357 12-01-2012, 07:34 PM
Last Post: M Habl
  restoring HP29C - help needed Alberto Fenini 9 1,704 11-25-2012, 07:56 AM
Last Post: Alberto Fenini
  Looking for PRNG for HP-34C Dave Boyd 3 1,136 09-22-2012, 07:10 PM
Last Post: Mike T.
  Restoring an HP65 (trying to .... few pics) Alberto Fenini 31 6,110 06-29-2012, 10:55 AM
Last Post: Peter HOFMANN
  Restoring an HP-55, advice appreciated ! Alberto Fenini 4 1,124 05-15-2012, 03:43 AM
Last Post: Alberto Fenini
  HP-34C Storage Register Arithmetic & Programming Matt Agajanian 14 2,568 05-08-2012, 11:04 AM
Last Post: Matt Agajanian
  A longstanding HP-34C mystery Matt Agajanian 11 2,364 05-03-2012, 02:29 AM
Last Post: Alexander Oestert

Forum Jump: