HP 25C repair advice


Hello all,
I need some advice on repairing this calculator. I re-built the battery pack (pack works fine in my HP21) and installed it into my 25C and the display shows all zeros "across the board". It does not respond to any keyboard inputs. I swapped the 25C's display with a working HP25 and I get the same response. Any advice would be greatly appreciated. Thank you in advanced.


There is an over 95% chance that the ACT chip in the 25C is dead. It is the 22 pin 0.4 inch wide chip near the bottom of the calculator. The ACT chip from an HP21/25/67/97/29C will work in the 25C.



Maybe I'm wrong, but I remember reading something about these ACT chips having different characteristics. Is it correct? Does it apply only to the 67/97 series? Doc. Mike Meyers had a faulty HP67 that he could bring back to life after some transplantation.

I just would like to be sure about it.


Hello, I replaced the ACT chip and it started to work ( I was able to do some calculations). After a little while, the display started to act up- it showed all zeros again ( it acted like it was possesed and did not respond to keyboard inputs ). I have a working HP 25 and I will use it to try to determine what other componet is bad. I can take voltage readings from different spots, but I think that trouble shooting is going to be difficult without a scope. Does anyone have a rough schematic that I can work against?
Thanks for all of you help in advanced and have a happy holiday.



if you have both HP25C and HP25, they are differnet in some parts, as you have probably seen. ABotu the ACT chip, did you use the same type? I do not know how to identify.

Does anybody know?



This may help repairing your HP-25C :





Probably your ACT is not dead, read Katie Wasserman excellent article about Woodstocks (21-22-25-27-29 series) repairs in the Articles Forum, here at the MoHPC.


I think mayby the new ACT is also bad (probably was on the edge to begin with and heat/static did it in) or the machine has a power supply fault that is killing them.

The problem that Katie described does not match the 000000000 display, but her fix is very easy to try so it is definitely worth a go. My experience has shown that the 0000000000 display has always been ACT related... I have over 20 dead ACT chips that showed the same problem.

Although the various woodstock machines have different part numbers on their ACT chips, only the HP27 chip seems to be internally different. I have transplanted ACT chips among all the other machines without problems.



I have seen (and read) that the ACT chip is the Spice's (and some Classics) owners nightmare.

How is it inside? I have the HP21 block diagram and there are two chips dedicated to the LED assy: one is the 1820-1328J and the other is (what seems to be) a dedicated ROM with MK60025N (MosteK) 1818-0129 as ID. Which of them is the ACT? If it is something like a firmware with some resident logic, why not to emulate it with a uControler and a current driver? If it is simple logic, perhaps a single uControler would be enough.

Any suggestions? I think many spices and former Classics will be brought to life if we have a replacement solution for the ACT chip.



The ACT is a custom HP chip. It is the 22 pin chip (0.4 inch wide) at the bottom of the CPU board. The two display chips at the top are a ROM/Anode driver chip and the cathode driver shift register chip. Other chips on woodstock machines are one or more ROM chips and and one or more RAM chips. The RAM chips are particularly prone to failure also.

The spice machines have an integrated CPU/display driver chip (the big 40 pin package) and one or more RAM and ROM chips. All the normal ("E") spice machines use the same CPU chip and the CMOS "C" versions another CPU chip which is not compatible withe the "E" version.

There are no off-the-shelf replacements for any of these chips.

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