HP33C repairs

Hi Everyone,

I've come across a HP33C, which visually is in excellent condition (very little marking on it). However when I turn it on I get the low battery Led (in the top left hand corner of the display) and sometime when I jiggle the power I get random numbers appearing and sometimes all 0's appear or error 0. Can anybody help me out to get it repaired? it would be such a waste to throw it out. Look forward to any replies.

With Thanks



Hi, Marco;

there are mainly two types of Spices (the HP3x series) concerning their internal structure. One type is solderless, and all components connect with each other through a flex circuit that is actually 'pressed' against the IC's, LED display assembly and power board terminals. The behavior you describe is somehow common to this type of Spices, namely bad contact. The other one uses a rigid mainboard and all IC's, LED assembly and power board connectors are soldered.

Considering that the serial number of these calculators may be changed once they may have they back case replaced at any moment, I see no way to distinguish one type from the other by visual inspection. Two references may be added, though.

The solderless units are heavier because they have a fair piece of metal that ensures all contacts are made by offering a plane, uniform surface that regularly distributes the pressure to all terminals and contacts of the flex circuit. A solderless unit without batteries weights approximately the same as a soldered unit with their batteries inside. The other reference also needs comparison, though. The solderless units have a single flex circuit that also has molded keyboard contacts, while the units with soldered IC's have metal caps. As a consequence, when pressing the keys of the solderless unit it's needed a wider rotating angle to actually close their corresponding contact (they go deepre inside the calculator's case), while the keys of the soldered units need to move only about half the `path` to actually close their contacts as well.

The best way is having one of each in hands so you can feel for yourself.

Once you find out that your unit is one of the solderless type, DO NOT THROW IT AWAY! There are directions on how to open them at the MoHPC Articles Forum so you can open it and clean the contacts.

Good luck and success.

Luiz (Brazil)


Hello Luiz,

The behavior you describe is somehow common to this type of Spices, namely bad contact.

Or else this might be due to bad batteries, don't you think so? Perhaps just replacing the NiCd batteries would do.



Edited: 1 Oct 2006, 5:28 p.m.


Hello Luiz,

Thank you very much for your response. I have opened the HP33C since and have discovered that it is the soldered type (the LED display is soldered to the circuit as are the IC's). I have given it a very good clean, but the problem is still there. Important to note is that I am running the unit off the original power supply, there are no batteries in the unit. I have checked the power output of the power adapter and it is correct according to the output on the adapter (10VAC). I have cleaned it well, but still the same, when I turn it on, the LED dots appears, but if I jiggle the power switch, the LED shows random numbers but still won't respond to any key presses.

Any other suggestions?

I appreciate you getting back to me.



P.S. I won't REALLY throw it out!!!!


Hello Marco,

While Luiz doesn't reply to you himself, I'd like to remind you the 33C (and other 30-series calcs) won't work properly without batteries. You may confirm this here, anyway I observed this behavior on both my HP-33C and 34C. You should use flat-end NiCd cells otherwise the batteries contacts may break.



Edited: 1 Oct 2006, 8:00 p.m.


Hi Gerson,

Thanks for your reply. I had a little suspicion that this may be the case too and I haven't as yet tried to put batteries in plus power supply. I seem to remember somewhere that the HP97 had a similar problem too with not working properly without batteries inserted. Anyway, I'll grab a couple and give it a go. I'll be extremely happy if it works. I've given it a reaaly good clean so I doubt it's a contact problem. If anything one of the IC's has blown it's top, or as you say it could be something as simple as the batteries. My fingers are crossed and I'll let you know.

I appreciate your time.

With Thanks




Running the calculator off the power supply is critical and dangerous on the earlier 20 series. On your calculator this may not be so critical, it will only cause the calculator to work erratically and fail the self-test (at least according to my experience). Let's hope it will work fine when intalling new batteries. Again, be careful not to break the battery contacts, if they're the original ones (those on mine are not).




Hello Gerson,

You're a genius!!!!! Yep, it was the batteries. I replaced with some new ones and the unit fired up beautifully. Thank you once again I appreciate your time.

With many thanks


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