HP 34C display problem


Good day all

Seeing Matt's topical post prompted the following.

My beloved 34C has lost the leftmost unit of the LED display so negative numbers look the same as positive numbers (it's the display rather than the CHS key as simple arithmetic confirms). From what I've read here it seems that it is normally the right digit rather than the left that goes as a result of battery leakage.

I can't be certain but think it went just after my bag in which it was suffered a knock when I was jolted walking along a train corridor. However, judging by the weight (6.5 oz) I think mine is a soldered unit so unlikely to be a loose connection. Bad solder?

I havn't plucked up the courage to open up the unit yet although having removed the two screws in the battery compartment I can see that the threaded part that one screw goes into has been built up with resin so someone has been there before.

Any thoughts?

I'm guessing the only option is to try and locate a donor unit for the display as I assume that the chances of finding a replacement display are somewhat less than getting Captain Zener (ah... the good old days!) to say a kind word about the 33S (I see from an earlier post that even Randy has no Spice spares).

At least it is livable with as all the rest of the functions are ok - it's just a pain having to do an extra step to check if the result is negative or positive.



Edited: 5 Apr 2012, 3:32 a.m.



Would you check if your HP34C is the solderless or soldered type? In both cases there is a chance of bad contact, but in each case procedures are different. I prefer dealing with the solderless type because I think repair is easier and if you do not heat IC's terminals neither touch them with the soldering iron tip, chances are you'll have less problems with ESD.


Luiz (Brazil)


Hi Luiz

I suspect from the weight (or lack thereof) that mine is the soldered type but guess I will have to open it up to be sure, hopefully will get some time over the holiday weekend.

Kind regards



HI, James.

I am not completely sure about this, but chances are that the leftmost digit has one common terminal and another one shared with the other digits. Based on what I read about the low-battery indicator, I cannot tell you it is also shared with some other common segment. Anyway, if it is a problem of cold soldering, briefly touching the tip of a soldering iron (the finest tip possible) might melt the solder and reconnect the missing segment. If you can, just give it a try.

On the other hand, there is always the other end of the coper trail. If the connections for the display are OK, once the calculator is opened then you can also check if the IC's ends are also OK.


Luiz (Brazil)


Thanks Luiz.

Yes, I did have thoughts of running down the battery to see how the low battery indicator responded.




I fear the worst .... I haven't opened it up yet as I'm trying to run the battery down but whilst waiting I had as close a look at the display as I can manage by shining a bright light through the red plastic shield. The leftmost digit (minus sign and low battery display) chip which lies under the magnifying lens looks displaced, it is lying at an angle so that the bar which makes the minus sign, rather than being 90 degrees to the vertical is more like 110 degrees.
Having had a look at the 32E repair pictures on Kees' site I fear the chip may have come unstuck from the substrate and damaged the bonding wires.

Time will tell!


ps The battery has finally run down, no sign of the low battery indicator. I can see one bonding wire coming from the minus sign and one from the point for the low battery indicator. I assume there must also be a third common wire somewhere?


Well, opening it was a lot easier than I expected thanks to following Geoff Quickfall's guide - thanks Geoff!

The previous repair by persons unknown to the scew-post is shown circled in red above.

At least I know for sure that mine is a soldered unit. All soldered connections look fine with the exception of the circled one on the 8 pin chip nearest the display.

I understand from this post that this chip is part of the constant memory so I think that the connection is OK despite the appearance so will leave it alone for now.

The general appearance inside is quite clean with only a little bit of green corrosion on the ribbon cable to the battery contacts which I am trying to remove with a dilute vinegar bath - the shot below shows the main board resting on a cup with the battery board soaking in the vinegar/water mixture,

Finally, a shot of the display unit. Unfortunately it is not too clear but the red lines show the rough alignment of the minus bar which is presumably a result of the jolt.

So I guess I'm now on the search for a donor machine with a good display.


Edited: 6 Apr 2012, 11:27 a.m.


HI, James.

First, congrats on opening the calculator. Good to know!

Second, thanks for the pictures. They help a lot to understand what is going on.

I had two spare LED modules for the Spice series, but I am not sure if I have already shared the last one of them. One has surely gone some years ago when someone asked for a replacement - his HP34C had the rightmost digit fully missing. At that time I had a nonworking HP38E so I traded it for the bad module and I remember recovering it with a touch of conductive ink in the base of the microscopic digit. It took me concentration, dried/firm hands, plenty of light and a magnifying glass (this I could not do with the naked eyes...).

I'll let you know if I find them.


Luiz (Brazil)


Hi Luiz

Thanks very much, that would be great if you find one.

I remember recovering it with a touch of conductive ink in the base of the microscopic digit. It took me concentration, dried/firm hands, plenty of light and a magnifying glass

Respect, Luiz, that was some job given the small size - I'm not surprised you need a magnifying glass, and very steady hands.

It's funny, I had been very wary of opening up the 34C because of the risk of breaking plastic clips and hearing stories about the difficulty people had opening them. But Geoff's method worked great. It is just as well that the 34C is the only Spice I have as otherwise I would have carried on and opened any others that were around just because I could!

I was also a bit wary removing the main board as I was again worried about breaking the plastic retaining clips but at least with the board you can see what you are doing.

Thanks for all your advice, it is very much appreciated.



Edited: 6 Apr 2012, 3:35 p.m.


Hi, James.

I agree with you, these little fellas are somehow tricky because of their form factor. They have only two screws, all of the rest is kept together with two metal clamps and six plastic clips, plus the big plastic interlocker. I have seen many Spices with more than one of the six plastic clips already broken, and it was hard to keep the keyboard assembly in place.

One of the most difficult tasks for me is not braking the four little plastic guides used for aligning the keyboard assembly with the uppercase. I have not found yet a way to preserve them when removing OR replacing the keyboard assembly, I have broken so too many of them that I gave up trying to keep them in place.

Believe me, you are not the only one to be wary when dealing with them.


Luiz (Brazil)


Hello James.

I am very glad my previous post has inspired you to investigate your 34C glitch. I hope things turn out well.

Edited: 5 Apr 2012, 1:58 p.m.


Thanks Matt.

I just hope I can get it sorted.



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