The Museum of HP Calculators

HP Forum Archive 08

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RAM chip on HP 25
Message #1 Posted by Laurent Damay on 4 May 2002, 8:56 a.m.

Hello and sorry for my poor english

I'm the owner of a later HP 25 (SN : 1805S24041). My calculator had two problems :

Bad keyboards reponses for somes numericals keys but it's not very important because all keys seems work.

More important : all statistical functions, memory operations and programmation sequences don't work.

For exemples : On program mode : the screen allway display 13 00 (GTO 00) Memory operations : 12 STO 0 CLX RCL 0 the HP 25 display 0

All mathematicals functions and stacks operations are working very good.

I think the RAM chip is corrumpted. Perhaps it's only a bad connection.

Can people help me ?


Laurent Damay

Re: RAM chip on HP 25
Message #2 Posted by Ellis Easley on 4 May 2002, 10:25 a.m.,
in response to message #1 by Laurent Damay

I have had the same problem with several HP-25's which I bought in unknown working condition. Some were like that from the start, another was like that after I got it to work at all. Finally I bought one from a collector that was known working. When I received it, I found its memory was a little flaky, I would enter a program and sometimes it would change a little. I discovered that the capacitor in the clock circuit had been changed, apparently to speed up the calculator. I can't remember whether I figured this out by comparing the component values on the several units I had by then, or if I measured the frequency and found it different from another unit or from something I read in the Museum, but I remember that when I found the difference, it was obvious that the capacitor had been changed. When I put the correct capacitor value in, the memory became stable. Also, along the way I figured out which chip was the RAM chip by brute force - from one of my bad-memory units, I removed chips one at a time till I found one that made no difference! Later I read the Museum Technology article for Woodstocks which explains how the functions are divided among the different chips. While I don't have my notes in hand, my memory (also somewhat flaky!) says that if you hold the circuit board component side up with the place where the keyboard attaches at the bottom (same orientation as if you were using the calculator), the RAM chip is the vertically oriented 18 pin chip (0.3 inch wide) to the left of the larger, horizontally oriented 22 pin chip (0.4 inch wide) which is the CPU (I think it is called the ACT). On my newest working unit, the part number of the RAM chip is 1818-0154. Also on this unit, the clock components, which are the capacitor and inductor connected in parallel and to pins 13 and 14 of the 22 pin chip, are a 330 pf capacitor (marked "331") and an inductor with the following bands (I've never learned how to read inductors properly): wide silver, brown, green, brown, narrow silver. (I guess that's actually a pallindrome.) My recollection is that the wide first band signifies an inductor (rather than a resistor) but I thought the first band was one of the significant digits, and silver can't be a significant digit, it can mean 10^-2 multiplier or 10% tolerance. The other bands, if it were a 5% or 10% resistor, would mean 150 ohms, so maybe this is a 150 microhenry inductor. 150 microhenries and 330 picofarads would have a resonant frequency of about 715 kHz, which sounds about right.

Because I found several units with this problem, I thought it might be a well-known problem and I have asked in this Forum if others have had the same problem, but I've never heard from anyone who has found this to be the case. There was a recent thread about another Woodstock that has a frequent failure and there is an article in the Museum Articles page about it, but I believe in that case it is a chip specific to that model that fails.

It is my understanding that several of the Woodstocks use the same RAM chip, I suppose any model with 8 data memories and 49 program steps would. The 25 is the only Woodstock I have, although I think I now have six of them (2 completely working).

The problem is, you have to find another calculator to take the chip out of, but unless that calculator is working substantially completely, how do you know if the memory chip is working? If you were lucky enough to find another 25 that worked except for a bad display digit or a bad keyboard, then you could replace your main circuit board with the one you found, this is very easy as the modules plug together.

One theory, at least, of why the memory chip goes out while the rest of the calculator is working, has to do with the weakness in the way HP designed the battery charging circuit. If the connections between the NiCad cells and the calculator develop a high resistance (such as when the cells leak and corrosion occurs), the voltage from the charging circuit at the calculator battery terminals, which is the power source for the calculator, rises to a higher voltage than normal. In other words, HP designed the battery circuit with the NiCad cells providing voltage regulation as well as power storage. The 2.5 volts from the battery is converted to +6 volts and -12 volts for the MOS chips (when the calculator is on), so when the battery voltage rises, the converted voltages rise proportionally. One thory is that the design of the RAM chip makes it more sensitive to a higher supply voltage, so it fails first.

Re: RAM chip on HP 25
Message #3 Posted by Laurent Damay on 4 May 2002, 11:28 a.m.,
in response to message #2 by Ellis Easley

Thanks for all yours explications


Laurent Damay

Re: RAM chip on HP 25
Message #4 Posted by Vieira, Luiz C. (Brazil) on 5 May 2002, 10:31 a.m.,
in response to message #2 by Ellis Easley


I have two questions:

- would it be good idea to solder a Zenner diode (say, 2.7 Volts), inside the calculator, in paralell with the batteries' contacts? This way, voltage would never go over 2.7 Volts

- Program an Registers' memory are fully shared in new calculators (10-series, 41 series, and so); it did not happen in the earlier ones (woodstock-series); do they have the same chip containing both program and register memory circuitry? If so, how can a program be recorded and a register cannot hold data? Is it an specific chip?

I just found it interesting, mostly cause I have an HP25C (the 25C has an extra fourth IC) that has the following problem: cannot execute any functions EXCEPT memory manipulation, say, stack and STO/RCL. Any attempt to perform other operation (a single [+], or [sin], or else) results in a 'zero's full-filled display. In other hand, it saves program steps and register data, no trouble. I understand it lost its ROM, and I am also looking for a solution. If someone can help...

Cheers and thanks.

Re: RAM chip on HP 25
Message #5 Posted by David Smith on 5 May 2002, 4:04 p.m.,
in response to message #4 by Vieira, Luiz C. (Brazil)

Yes, it is probably a bad ROM chip, although it could be the ACT chip. If you have a donor HP25, you can use chips from it. They are identical (or compatible) with the 25C. Only the RAM chips are different.

The HP25 has ROm code in two chips, the vertical one on the left side of the circuit board next to the ACT, and the ROM0 display anode driver chip under the LED module. Most likely one to fritz is the ROM0 chip.

Re: RAM chip on HP 25
Message #6 Posted by Ellis Easley on 7 May 2002, 11:54 a.m.,
in response to message #5 by David Smith

Then is the RAM in the HP25 the 16 pin chip sitting parallel to the ACT? If so I gave Laurent bad info!

Re: RAM chip on HP 25
Message #7 Posted by David Smith on 7 May 2002, 5:24 p.m.,
in response to message #6 by Ellis Easley

Depending upon the revision the RAM can be either parallel (on top) of the ACT, vertically to its right, or (for the small capacity RAM chips) both positions. There is also a 100K resistor that is different between the single and dual RAM chip versions.

Re: RAM chip on HP 25
Message #8 Posted by Ellis Easley on 10 May 2002, 6:52 p.m.,
in response to message #7 by David Smith

I examined all my HP25's and realize I gave bad information regarding which chip is the RAM, sorry Laurent!

Regarding the display anode driver ROM0 chip - is this the one on the left or the right under the display? On my units the one on the right is always an HP-made chip, so I'm guessing that if HP went to Mostek and AMI for the ACT, the RAM and the other ROM, then the ROM0 is the one on the left. Here's an interesting relationship: of my 6 HP25's, the two with working RAM chips (which I now understand is the 16 pin chip above the ACT on all my units, P/N 1820-1564) both have Mostek ROM0 chips. The four units with bad RAM chips have AMI ROM0 chips. Of the working RAM chips themselves, one is Mostek and the other is AMI. By any chance, is this the failure mode of the ROM0 chip - does it cause the RAM to appear to be defective, possibly by responding when the RAM is addressed?

And how about this piece of irony: my original good RAM unit now won't go into program mode! The switch is fine as are the connections to the ACT. In the RUN setting the switch connects ACT pin 4 to ACT pin 1, which is the +6V connection. Then in the PRGM setting, the switch is open and pin 4 is driven to 0V (equal to the negative battery terminal or ACT pin 12) internal to the ACT on a good unit, but on my formerly all-good unit, pin 4 only drops to +4V. Both supplies are working, I measure +6.37 on ACT pin 1 and -12.47 on ACT pin 2, relative to ACT pin 12. If I tie ACT pin 4 to ACT pin 12, the unit goes into program mode but doesn't work correctly (in this case pin 4 sources from 3 to 11 mA, changing all the time - probably shorted to an active signal). I guess I better look for an external short before I jump to the conclusion that the ACT is bad, but if it is bad, at least I have several to replace it with!

Regarding the other chip under the display, which on my units is always black with HP molding, P/N 1820-1382 (is this possibly the display cathode driver?) - the second line of text on these parts, which I would think would be datecodes, vary quite widely. One is illegible, one is "7539-C" which seems in line with the other chips on the board, one is "1535K" which looks like a reasonable HP-style (1960+) datecode, but the others are "8768-D", "8861K" and "C-8074-K". Is it possible these units were reworked by HP into the late 80's?

One final thing: all my ACTs are P/N 1820-1523 except one, which is 1820-1741. This is in my oldest S/N unit (1601S) although the chip datecodes are the newest of all my units, 7608 to 7635. I guess this unit had its board swapped. Similarly, all my ROM0 chips (if it is the one on the left under the display) are P/N 1818-0168 except one, which is 1818-0153. Is anything in particular known about these different chip P/Ns? My RAM chips are all 1820-1564 and my other ROM chips are all 1818-0154 (the 18 pin chip I had said was the RAM).

Re: RAM chip on HP 25
Message #9 Posted by Laurent Damay on 11 May 2002, 3:02 a.m.,
in response to message #8 by Ellis Easley

Thanks for yours greats documentations



Re: RAM chip on HP 25
Message #10 Posted by David Smith on 11 May 2002, 3:19 p.m.,
in response to message #8 by Ellis Easley

The ROM0 chip is the smaller one on the left.

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