HP Forums

Full Version: The CMOS memory has died in my HP-25C
You're currently viewing a stripped down version of our content. View the full version with proper formatting.

Yesterday I turned on my HP-25C and discovered that the contents of program memory and the storage registered had been lost. Thinking it was just a low battery, I replaced it with a fully charged NiCad battery pack, but still CMOS memory would not retain any entries. Even with the calculator powered on, I could not store data in any of the registers or a program. Also, statistical operations would not function properly, as they rely on the contents of the storage registers. This calculator has not been subjected to the killer AC adapter, since I charge the batteries externally, and only operate the calculator from batteries, so I don't think this failure is the result of any abuse or misuse. Has anyone else on this forum experienced this type of failure?

(added: I checked Eric´s HP Calculator Chips and I would like to call your attention to some data I posted. I seem to have seen some ACT chips elsewhere, but chances are that I mixed the pictures. Please, follow Eric´s link and confirm.)


I bought an HP22 some weeks ago and it has the same problem. I could also buy two HP25 and three HP21 that are not working, and along with some previous pictures I kept from other HP25, HP22 and HP21 I was able to repair, I consolidated some information.

Both HP22 and HP25 use the same PCB layout and differ only in their ROM chips. I found that three of the five IC´s in these calculators have the same codes. The ACT varies from some models, and I collected these codes:

1820-1596 (both seen in the HP21, too)
1820-1741 (seems to be used by the HP29C, too)
Display driver appears in all three of them with the code:
ROM chip varies:
1818-0163 + 1818-0164 for the HP22 (found in two different units)
1818-0154 + 1818-0168 for the HP25 (I seem to read 1818-0158 in one of them...)
1818-0129 for the HP21
I found the same RAM chips in both the HP22 and the HP25 (the HP21 seems to use available ACT registers only)
I also happen to have seen an HP25C with two RAM chips but their codes were not visible. In another HP25C there is a RAM chip written:
HP5061-0469 2
That is all information I have so far. Please, if there is something else to be added, I'd appreciate. If there is more information about this somewhere else, please, point the addresses out!

AND if I messed some information up, PLEASE, point it out!


Luiz (Brazil)

Edited: 6 June 2010, 12:19 a.m.