HP-34C missing led


I have a HP-34C with a missing led all along the display. It´s not a problem of bad contacts. I am afraid is the cathode chip with one circuit failing. Does anybody knows if this chip can be repaired with a transistor?. Can I use any chip from other spice calculator? I have spare parts from a 33E.




Hi, Jon;

it's a common symptom and it is usually observed in the "solderless" spices, the ones I name "sandwich" assy.

I have an HP38C that sometimes comes to "life" with a missing segment in all digits. I currently twist it slightly and gently and it comes back to normal operation. When it first occurred I disassembled it and cleaned all of the surface contacts. It worked fine for more than a month. Then it came with the same missing segment and I instinctively twisted it (gently). It worked fine again.

I'm almost sure it's not an internal chip problem, maybe it's a cracked trail or a bad contact inside the LED module.

If you want to try a gentle twist before opening, go ahead. Otherwise, you should open and check for the type 34C you have. If it's a soldered type, you may also have a cold solder, what is somehow easy to repair.

Hope you have success. Let us know if you need extra advice.

Luiz C. Vieira - Brazil


Jon -- dont give up on that 34C yet.

Luiz is right ....

Rather than presume the semiconductor IC is damaged,
it may be that your unit is solderless, AND,
one of the solderless connections has become open.

Is the 34C fairly HEAVY? That's one way to conclude
it is solderless.

It would be good news for you, because if the chip is blown no, you cannot replace it with one from a 33E . But the bad news is, you need to know EXACTLY what you are doing to get inside of a 34C and succeed in making restorations.

If you are technically inclined then proceed. However you seriously consider boxing it up and sending it to Luiz . He is the best for working with the solderless SPICE calculators and I bet he would like some well-paying evening work. He'd probably quote you a price that would work.


Thanks everybody. I am quite busy these days. As soon as I have time I will "play " with it. If I have success I will post it in the forum.

Thanks again



Well, good luck with the 34C Jon.

There are many pitfalls, starting from when you try to open the plastic case halves (it will seem they cannot be separated, and it is very easy to damage the plastic).

After that comes risk of ESD damage to these chips, which are very ancient and not design to be rugged at all. In a nutshell, if you have the wrong pair of socks on, you may fry the chips, and then you are truly stuck.

There are some articles here in HP Museum that will help you with knowledge about servicing the unit correctly. To take a line from the antique radio crowd "any service work should be done carefully and correctly, for the item is an irreplaceable heirloom, and you are not likely to be its final owner."

- Norm


Hi, Jon;

I read your post again and I see something I did not notice in the first time (I think I read it in a hurry, sorry). It seems you're sure about something when you mention "It´s not a problem of bad contacts.". Is there any additiional information about it that you believe would help diagnosing the problem?

Please, let us know. Thanks.

Luiz C. Vieir - Brazil



I have opened it. I have the experience with other spices that a missing led is just a problem of bad connection of the display contacts. If you press it the problem comes and goes and is easy to fix. But not this calculator. I though it could be a chip problem because it is described something similar for the HP-67 and I have read the HP-97 service manual where I can see the circuit (spice design is not so different). If somebody is interested I can post the information related to the HP-67 that I found somewhere in internet

With the information you gave me I will investigate further.




If your machine is a "compression" machine where the chips, etc are not soldered then you can just pull out the LED module. Test it by connecting a 400-1000 ohm resistor to a 3 volat battery pack. Fumble around a while with the leads on various pins and in various polarities. You will soon see a pattern of where the LEDs segments start to light up. The LED segments and digits are arranged in matrix (segments on the rows of the matrix, digits in the columns). Typical failute modes are a missing digit (bad column connection), a missing segment in all digits (bad row connection), or a single segment bad (bad LED segment).

If you can get the offending segment(s) to light, the problem is in the big chip or (more likely) the connection from the big chip to the LED module. Connections between the big chip and the keyboard etches can be hard to get right.

The LED module can be replaced from any other spice machine. The big CPU chip will have to come from another machine of the same type. You may also have to replace the ROM chips from the other machine. Some CPU chips are not compatible with all ROMs.

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