HP-34C display problems


I have a 34C in excellent physical condition. Judging by the weight and the very firm key action, I believe it is one of the models which has the chips held in place by a pressure plate or whatever, not soldered into place.

It works fine, exceot that occasionally one LED segment will go out. The same segment goes out on all digits simultaneously. The calculator still operates properly, and slightly squeezing or flexing the case just a tiny bit brings the segment back.

It seems likely that this could be due to a chip being slightly misaligned, or perhaps having a bit of oxidation or something on the leads.

1. Do the spice experts here think this would be a correct diagnosis?

2. How big a deal is it to diassemble and rectify such a problem?

Any advice is appreciated. Thanks, Don



I disassembled a few Spices with bad contact. All of them, no exceptions, where the sandwich, no-solder, pressure type. Simply cleaning and in one case using a metal polisher solved the problem.

The good news for me about your post is that the same segment in all digits appear loose. The process to "lit" them up is called "multiplex(ing)" and the same segment in all digits share one common connection each. This reduces the number of "wires"(connections) for the display segments (if you already know about this, just think it's for those who don't know, please...). This way, one single bad contact in one terminal of the display assy OR the control IC (CPU) is enough to cause the problem you describe.

If yours is a soldered-chips type (and I do not remember feeling differences from one type to the other... well, long time ago), no matter, too: resoldering terminals is not hard, too.

In both types, two major difficulties for those who have never done it before:

opening the calculators' case AND

disassembling the inside parts

If you already know how to do it, no matter. if you don't, it would be better to count on help from someone who have already done it. Just for making it easier.

Hope you have success.

Best regards.


Don --

Luiz is right about the older-design units with pressed-together (instead of soldered) component leads being problematic. I have two older-design HP-34C's and one newer-design (soldered) unit. The older ones exhibited problems; the newer one has no problems and has a better key feel due to improved snap rings.

Here is a post of mine that tells you how to separate the case halves of a Spice unit without breaking anything:

Spice disassembly


Before you open up the sandwich always record the position and orientation of the chips. Otherwise it may not work when you put it back together.


Thanks to everyone for the helpful advice. I'm going to try to take this apart and clean it a bit as soon as I have a little time when I won't be rushing through it.


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