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34C display problem - Printable Version

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34C display problem - Daniel CLEMENT - 07-11-2011


I'd like to fix a display problem on a (otherwise fine) HP-34C.

Here are the symptoms:
1) the rightmost digit doesn't light up;
2) on each other digit, the top segment and top left segment are weakly lit when they should be off on 2's, 3's and 4's.

Here is what I believe (I do my best to translate the technical terms from French...):
1) there is a transistor which toggles the right digit, and it is dead;
2) there are transistors which address the defective segments, and they leak. (But then, why would the 1's appear correctly?)

I'd appreciate someone confirms (or not!) these guesses. Can these faulty components be changed separately? A link to a diagram of the 34C electric circuit would also help much.

Thanks a lot in advance - regards, Daniel

Re: 34C display problem - Dieter - 07-11-2011

Daniel, you probably know that HP used a special manufacturing technique in their Spice series (34C, 32E etc.). In those days they used a solderless construction, i.e. the various chips and other components were fixed / clamped to the PCB by simple pressure.

This way, the contact may become insufficient over time. This also happened with my own 34C: Here and there one ore more segments or even complete digits may drop out. In these cases some careful (!) bending of the case makes the missing segments appear again.

In you happen to own one of the last 34Cs this might be one of the few that have been manufactured in the good old soldered way. If you are not sure, simply weigh it: with the batteries inside the solderless version is about 220-230 g, the soldered version close to 170-180 g.


Re: 34C display problem - Michael de Estrada - 07-11-2011

It doesn't sound like a contact pressure problem, since some of the LED segments exhibit ghosting. I've fixed the pressure problem on mine with a thin strip of electrical tape on the plastic back plate to act as a shim on the LED array fingers. In this particular case, the LED array probably needs to be replaced.

Re: 34C display problem - Mike Morrow - 07-11-2011

In you happen to own one of the last 34Cs this might be one of the few that have been manufactured in the good old soldered way.

I don't believe that "few" accurately characterizes the population of soldered vs. press-fit Spice models. All my Spice machines are of the soldered variety, made two or more years before these models were discontinued. I wouldn't be surprised if the press-fit models were the less common of the two, but that's just a guess.

Randy of FTC would be the best authority on the relative percentages.

Re: 34C display problem - Randy - 07-11-2011

I would agree many soldered models where manufactured and many solderless where created during repair but at this point I think it is more a case of how many have not made the one way trip to the bin.

My very un-scientifically estimate would at least ten to one, solderless to soldered, based on a sample of about 70 units that I have either purchased on ebay or repaired over about an eight year period. FWIW, the 34C and 38C lead the numbers of soldered machines, I don't remember ever seeing a single soldered 31/32E low end machine.

Re: 34C display problem - Michael de Estrada - 07-11-2011

I guess we all have different experiences in this regard. I currently have 12 Spices, all fully functional, and only 3 of them are of the soldered variety. However, one of the soldered units is an HP-32E and the others are an HP-33C and HP-37E. I have one HP-34C and two HP-38Cs, all of which are press-fit. Of course, many of the press-fit units were cobbled together from parts in several units, and I've had 4 or so other units that acted as donors to create the 9 good units. There was recently an HP-34C on TAS that I bid on but did not win, which I believe was most likely a soldered unit, based on the serial number.

Re: 34C display problem - Daniel CLEMENT - 07-12-2011

Thank you for your valuable input.

@Dieter: weight is 220+ g, hence not soldered (is it good news?) I did not know that the display itself was clamped. This sure can account for the last digit missing completely, but could it explain the ghosting on the others? I _carefully_ bent the case, but no luck.

@Michael: I'll try improving the contacts. The end of your answer is what I feared. I don't have a spare LED array --only on working calculators... Perhaps on a Woodstock, but I guess it's not the same.
However, a temporary exchange might help.

Yet, don't you like my transistor story? I thought it was pretty consistent with the technology... I have found the hpcc site which has some diagrams, but not the 34C, only the 33C: do you think I could use it instead? I'd like to find out if the display driving is performed by a single chip, or by separate components.


Re: 34C display problem - Tony Duell - 07-12-2011

Electrically there are only really 2 versions of the Spice series -- the -E models and the -C models. The various machines in each group differ in exact processor and memory chips fitted, but they are connected together in the same way. Keyboard, display, PSU module, etc are the same.

Therefore. the HPCC HP33C diagram _is_ of use in repairing a 34C. Just be warned that the IC part numbers will be different.

Re: 34C display problem - Daniel CLEMENT - 07-13-2011

Thank you, I'll get myself a copy of the hpcc diagrams CD.

Tomorrow I'll leave for 2 weeks so I won't be able to try anything on my 34C until August. Then I'll get back to the forum to report progresses (hopefully).