HP-34C Intermittent Display


I have an HP-34C with an intermittent display. My 34C accepts keyboard inputs and passes the self test. However, frequently when I first turn on the calculator and start inputting data, various LED segments will not light correctly. I have noticed that a sharp tap to the back side of the calculator corrects the problem and data as previously entered is displayed correctly. Since this is a repetitive problem it would be nice to repair the calculator.

I opened the HP-34C and determined that it is the earlier solder-less design. Since I don't have prior experience with the Spice series, I haven't disassembled it further. I have tested the display PCB by moving it slightly from side to side and the intermittent problem is easily replicated. Observing the backside of the LED PCB it is obvious that the through hole pins have no solder. However, before I proceed to solder the display PCB pins or disassemble the unit further, I am curious if there are any recommendations or suggestions. For example is it likely that soldering the display will correct the problem? Is there a down-side to soldering the display?




I'd strongly suggest NOT to solder the display. I'd rather go for a cleaning procedure. I have two HP38 with solderless assembly, and they work pretty fine.

I used liquid metal polisher and let the PCB and the display contacts shinning (must be careful using it without 'contaminating' unnecessary areas). After that, I allowed a thin coat of micro-oil to remain in their surface, so it would reduce oxidation occurrences. It does not interfere in the normal operation because the pressure of the assembling components is enough making sure the necessary electrical contact to happen.

In any way, I'd avoid, by all means, to solder it. The flex PCB would not resist higher temperatures, and even if you have the necessary equipment, you should align it correctly to fit in the molded chassis. And, as a final remark... if it would work better as soldered assembly, chances are it would come this way, though.


Time's up... running back to normal life, though. Cannot check for spelling, sorry.

Luiz (Brazil)

Edited: 4 June 2007, 7:20 a.m.


Luiz, just curious about some details. The 34C LED display PCB appears to be typical FR-4 material so it shouldn't be damaged by soldering. From the 34C battery compartment view, the through hole pins are not soldered. Did HP solder these pins on the opposite side of the PCB (the LED side) or did they rely on some type of pressure contact?

The row of LED display PCB pins fold 90 degrees and then are fed into the hard plastic backbone. I assume this is where the display pins contact the flex circuit and where you are recommending against soldering?


Hi, Robert;

Did HP solder these pins on the opposite side of the PCB (the LED side) or did they rely on some type of pressure contact?
AFAIK, I have never seen explicitly soldered terminals in these Spice LED assy. In one particular case, I remember having a damaged unit and I tried to remove one of these terminals. I got it loose after some effort, but it seemed not to be soldered.
The row of LED display PCB pins fold 90 degrees and then are fed into the hard plastic backbone. I assume this is where the display pins contact the flex circuit and where you are recommending against soldering?
The whole flex PCB, IC´s and PSU flex contacts are hold together by both metal clamps on each side of the set. The display may be removed and placed back without the need of disassembling it all. After removing the LED assy, you'll see that the only contacts are the ones in the position you are mentioning, opposed to the plastic backbone. You see, the Spice soldered version has another kind of plastic backbone, and there is room for the remaining solder mass on each terminal for each IC. The area under the LED assy contacts has a different cut, though.

My primary concerns about soldering refer to the fact that the soldered LED assy will add stress to the flex circuit surface, and you may have broken copper tracks in the future. Depending on what is used to solder the LED display, the copper tracks may be detached from the flex surface, enhancing the problem.

Sorry to be such a Devil´s advocate... d:8^(


Luiz (Brazil)



Thank you for the advice. All of your comments were correct.

As you suggested, I removed the 34C sandwich assembly from main body, but did not need to disassemble the metal sandwich clips. At this point the LED display PCB was easily removed from the sandwich connector. The LED PCB through hole pins are not soldered. They rely on a very tight compression fit and were rigidly located. However, the LED PCB to sandwich connector had a very loose fit. Possibly due to aging of the sandwich foam which has decreased the compression force. Although the LED assembly pins and the sandwich receptacle appeared corrosion free, I lightly cleaned the areas. I then reassembled the LED PCB slightly offsetting the LED pins to take advantage of the sandwich locations that have thicker plastic. I expect this was not HP's original assembly intent, but it seems reasonable now.

I reassembled the 34C and it is working perfectly. However, there is a certain irony. I own two Spice models. The 34C and a 31E. Both models are of the non-soldered style. While searching for the 34C charger, I decided to use the 31E. Whoops, the 31E died while in storage. No life signs whatsoever. Presumably similar non-soldered issues, and another project.



Hi, Robert;

good news, indeed! At least, mostly.

Hope you succeed fixing your HP31E. I had two working units, but I decided sending one as a gift to one of our contributors, in fact, an e-friend. And if you check for its value, the HP31E is somehow rare, it´s worth bringing it back to life.]

One small advice: IIRC, there are two different Spice chargers, and I remember reading that it is not a good idea to use one of them with the 'other kind'. I am not entirely sure about this, but it is something related to clamping diodes used to protect their internal circuits. This subject has been discussed here last year, but I did not check the messages. Chances are that some Spices had their internals put in risk with different chargers...

Best regards and success!

Luiz (Brazil)

Edited: 6 June 2007, 7:34 a.m.



Thanks for the advice regarding the charger. Fortunately, I have the 82087B which is the recommended 34C charger. The prior Spice discussion centered around some extra Zener clamping diodes present in the 82087B but missing in the 82087A. It would be interesting to have the charger development history from an HP engineer. And interesting to disassemble both models.

I will let you know how the 31E progresses.

thanks, Rob

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