HP25 scary moments



yesterday night I was testing a Thevenin (single, real components Eth and Rth equivalency) program in the HP55 and I saw it would work in the HP25, too. As I mentioned before, I cannot keep a calculator without finding something to do with it...

The program is 15 steps long, 55 version, and 13 steps long for the HP25 because it uses one register, and STO 0 / RCL 0 in the HP55 use two steps each.

I loaded it in the HP55 and run it: Rth and Eth returned fine. The problem was with the HP25: ON, PRGM mode and the display insisted on an annoying

 [nn       13 00]
(keycode for [GTO] 00) for each key pressed! "RAM is gone!" I thought. I tested the memories and they returned 0.00 for any value I stored on them. "RAM IS GONE!!!" I cried out loud inside my brain, reverbs and echoes granted.

Soldering iron plugged and lets try with a magnifying lens to find any "cold-soldering" (I learn fast...). None visible, let's go for plan B: heat-up all terminals in both 16- and 18-pin IC's, I would not touch the CPU. (I have one question in the end of this text). I also heat some passive components' terminals close to the same IC's.

Happy end: the HP25's RAM got back to life, I loaded the Thevenin program on it and results were fine. And I got to bed in the down (about 6:00 AM), but I was relieved. Detail: HP25 runs about two times faster than the HP55, at least in this program.

My question is: the HP 25 and the HP25C share the same AMI 1818-0154 chip. Is this the ACT chip? As both have exactly the same instruction and behavior set, I thought this could be ROM. The other two chips, CPU included, have different numbers: the 25 CPU is an MK6215N, ID 1820-1523 while the 25C's CPU is an MK6217N, ID 1820-1741 (as they deal with different RAM structure, they must handle data bus differently); the other chip, supposedly the RAM, is an MK6220N, ID 1820-1564 in the 25, while the HP25C has an HP5061-0469 2.

All of this is because the HP25C with me has an strange behavior: it lits on, accepts number keys and display control, stores and recalls numbers, stores any program keystroke sequence (key numbers are fine) but returns a [0.000000000] for each attempt of anything else, even stack manipulations, [CHS], [.], [EEX] or [CLx]. I think it's a ROM problem, and I would like to confirm if the common AMI 1818-0154 does it, or if this is the ACT chip and the ROM is inside the CPU.

Can anybody point the differences out?

Thank you in advance.

The THEVENIN listing and coments

The Thevenin prgm computes Rth and Vth equivalent for a single Vcc constant-current voltage source with R1 being the series resistance and R2 parallel to load resistance. The procedure is:

Input   Press   Display
Vcc [ENTER] Vcc
R2 [R/S] Rth
[x<>y] Vth
For testing purposes: suppose you have a 10 Vcc voltage source, R1=5 ohms (series with [+] pole) and R2=10 ohms (parallel with load). What are the values for the equivalent Thevenin Vth and Rth?

Input Press Display
10 [ENTER] 10.00
5 [ENTER] 5.00
10 [R/S] 3.33
[x<>y] 6.67

Solution: Rth=3.33 ohms and Vth=6.67 Vcc

The listing: (keystrokes are for both machines)

      HP25          HP55        Keystrokes
[01 21] [ 01 22] x<>y
[02 31] [ 02 41] ENTER^
[03 22] [ 03 23] Rv (roll down)
[04 15 22] [ 04 13] 1/x ([g][1/x] in the HP25)
[05 21] [ 05 22] x<>y
[06 15 22] [ 06 13] 1/x ([g][1/x] in the HP25)
[07 51] [ 07 61] +
[08 15 22] [ 08 13] 1/x ([g][1/x] in the HP25)
[09 23 00] [ 09 33] STO (STO 00 in the HP25)
[ 10 00] 00
[10 61] [ 11 71] ×
[11 21] [ 12 22] x<>y
[12 71] [ 13 81] ÷
[13 24 00] [ 14 24] RCL (RCL 00 in the HP25)
[ 15 00] 00

If you think it's worth, I'll put it in the Programs Library. It works fine in both 11C and 15C too, but their versions do not use registers, just the stack, because they use [R^], not available in the 25 and 55. And in the HP15C it computes complex results, if needed.

Let me know.


Hi, Luiz! I don't know the answer to your chip question, but I'm glad I'm not the only one..... when I finished "playing" at my workbench, it was 3 am this morning! I can't find it in the articles section, but Katie Wasserman once wrote about things to check when an HP-25 won't program. I'm glad yours is working again!

Remember ages ago I had a HP-67 that burned through two ACT chips? I thought I'd mention.... the third one's been a "charm". No problems since, and I use it daily, if nothing else, to show it off....(Not my only one, but the one I carry.) Even if you have more than one, it's always sad when these things die. Often, as you know and have shared parts, when they do die, their "organs" can bring life to another calculator.

Good luck with your 25C...



By the way, Luiz,

For the holidays and my birthday this year, I bought two glass display cases. They arrived just before a I had a few days off for Christmas and New Years. Now my collection is displayed, and I can "pluck" them off the shelves and play with different calcs. Much better than having them stacked in a drawer and on shelves! I like it. (My wife must be very patient, as they're in our family room...)



Hi, Doc;

good reading your posts.

Hey, is there a way to take some pictures for us to appreciate? I'd like very much to see hwo do your babes look like. I mean your calculators, for sure!

If you do not have an e-place to holst your pictures, send them to me, please. You know it's sort of relaxing activity to be prescribed...

Best regards and thank for sharing these moments with us.

(I did not answered "pronto" because I was trying to find the correct expressions)


The ACT (Arithmetic, Control and Timing) chip _is_ the CPU (it's the 22 pin chip).
1818-xxxx numbers are normally memory devices (often ROMs) -- ROM0 in ACT-based machines is an 18 pin chip, and IIRC the RAM is a 16 pin chip in most of these machines.
8 pin chips are ROMs, RAMs, or combination ROM/RAM chips (as in the HP67).


The -0154 chips is a ROM chip. The ACT chips is the arithmetic-control-timing chip. It is the big one next to the -0154. There is more ROM code stored in the display anode driver chip inder the LED module (the smaller of the two chips). The earliest HP25's had a bug in the code in this ROM chip.


Can you give any information about the bug and which date codes of calculator apply?

How can I chaeck to find out which ROM version I have?



The date code on the machine is usually earlier ones, although I have seen it in later ones.

Store a number like 1E-14 in a memory register (or do a trig function that returns a result in the range of 1E-10 to 1E-20), switch the machine into PROGRAM mode and back to RUN mode. You see a blank display in PROGRAM and ERROR in RUN.

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