HP-41CX Memory Registers



My HP-41CX shows "00 REG 155" instead of the usual "00 REG 219". Memory reset does not help. Can this be repaired? Thanks for your ideas.


What does SIZE? give you?


SIZE? gives me 36.
Any suggestions?


Do you have any module plugged in at the upper end? In that case, remove them. Is your HP-41 overclocked/speed up?

Starting with a <- ON sequence (MEMORY LOST). In that case the free program registers should be correct. I do not think the calculator actually test the integrity of the program registers. It checks the free program registers by counting zero registers until it sees something non-zero (or hit the limiting address 0C0).
Incorrect memory not reading 0 after memory reset could give this symptom of too few program registers.

It does however check the integrity of data registers, and if you have a faulty memory, SIZE? may come up short.



just to add another question: is it a coconut or a halfnut? The coconut type may be repaired, as the memory chips are 'discrete', 8-pin DIP IC´s, but you need a 'donator' coconut because AFAIK, those memory chips are not regularly sold. The halfnut units have a single hybrid chip under the LCD with all RAM/ROM and display driver circuitry (also called 'R2D2' in the Voyager structure) and are not serviceable.

To see the difference between both:


Luiz (Brazil)

Edited: 23 Feb 2010, 5:45 a.m.


Thanks Garth, Hakan, and Luiz,

The 41CX under discussion is a full nut. I had a look at the printed circuit board. It has a row of 8pin DIP ICs as mentioned by Luiz and they are soldered one above the other (piggy-bac arrangement). Do you think it is possible to get these ICs? Then the question comes on how to identify which of these are faulty? Any ideas?

As Håkan mentions this appears to the case of faulty memory.




the IC´s you want to search for are RAM IC´s, and these ones are in number of five (64 * 5 = 320). If you remove all of the 8-pin DIP IC´s you will see that five of them have the same prefix (usually ILE7xx) with the last digit being counted 1 to 5. They store 64 registers each, and the first one (lower numbered) relates to the HP41C basic memory. The other four relate to the Quad Memory Module, for instance.

If your HP41CX sees 192 registers (156 + 36, considering that the 3 bytes for the permanent .END. are counted as one less register), it means that the fourth RAM IC is faulty (ILE704). Interesting: I had two HP41CV´s in the same circumstance: the fourth RAM IC faulty.

If you want to confirm with a single test, provided that you have a single Memory Module, with the calculator OFF (could not miss mentioning that) insert it in port #3 and check if the memory count goes OK, then. If you do not have a single Memory Module, try this (I am not sure if this is the correct procedure, just have none of the faulty calculators at hand now):

- perform SIZE 191

- key in 256 (or any number from 256 to 319)

- try VIEW IND X

If I am not wrong, it will give no error message. But I cannot guaranty, OK? Consider that you have an HP41C with two modules, then you perform SIZE 191, then you insert a memory module in the fourth port. That´s what we achieve with the procedure above. So, the last 64 registers are available and there is a gap related to port #3.

Let us know how it goes.


Luiz (Brazil)

Edited: 23 Feb 2010, 10:51 a.m.


Hi Luiz,

I have a single memory module and tried that as suggested. No change in memory count.

I also tried the procedure you recommended without the memory module. The result is 'NONEXISTENT' display.

Any further suggestions?



What kind of memory module do you have installed? You should not use a 82106A or 82170A module in an HP-41CX. If that's what you have, remove it. A 82180A module is OK, but will not affect main memory.


Hi Michael,
I had first tried, before your message, memory module 82106A. After seeing your message I also tried 82180A memory module. Still the memory count does does not increase. It still shows "00 REG 155" instead of "00 REG 219". Any other suggestions?

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