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isanchez · (This post was last modified: 05-20-2014 07:09 AM by isanchez.)

Dear friends,

I have an HP25 with an strange behaviour. I pass to describe it:

-Apparently turns on correctly, showing 0.00

-You can key numbers these are shown in the display, BUT when you try to press "ENTER" or make any kind of one key calculation, it shows 0.000000000

-This happens also when decimal point, exp, or chg sign is depressed...

- The SCI & FIX, functions work well.

-If you set it into PRGM mode you can enter any kind of programme. All the keys respond correctly, but you cannot run the programme. It shows 0.000000000 after trying to run it.

-Internally looks like new, no corrosion, nothing dirty or bad contacts.

By the way, it has a rare keyboard (at least I have never seen any like this one) the back side is all black, not white as usual. The S/n is 1706s23130

I would appreciatte very much your help to try to repair it.

Many thanks in advance.

Kind regards
Ignacio

ignazio · 05-20-2014, 06:33 AM

Hi Ignacio,
my name is... Ignazio, and I've one HP-25C with the same strange behaviour.
The calculator is arrived yesterday with a bad ACT that I've replaced with a good unit, the CMOS RAM work well, program and memory registers ok but when I push the ENTER key I see only '0,0000000000'.
This evening I'll swap the ROM 1818-0154 with a proved good unit, the only thing that I can try to change.
Let me know if you discover anything about your unit.

isanchez · 05-20-2014, 07:04 AM

Hi Ignazio,

same names...same problems! ;-D

I thought I was alone with this as I recieved yesterday many visits to my post...but not answers...

I have to confess that I am expecting for ideas. I firstly thought that it could be a bad contact, but no. The calculator is absolutely clean inside. After reading some old posts, I wanted to have confirmation about replacing the ACT (but from your experience it seems this is not the problem)... I am really blocked.

We keep in touch and thanks for your answer.

KInd regards from Valencia, Spain
Ignacio

isanchez · (This post was last modified: 05-20-2014 09:41 AM by isanchez.)

Dear Ignazio,

after spending some time in the forum, I have two possible answers to check:

1.- I have found the following Article by KATIE WASSERMAN. It states that the ACT is not fried, the problem is the noise that must be removed.

http://www.hpmuseum.org/cgi-sys/cgiwrap/...i?read=267

It recommends to install a 10 K resistor in the ACT chip...connecting pins 11 and 12.

2.- The problem is in a bad ACT module. Maybe your replacement ACT was not good. Many people associates 90% of 000000000 display problems to a bad ACT chip. The only difference is that, in our case, the calculator reponds to the keyboard input of numbers and programming mode...what makes me doubt...

Kind regards,

Ignacio

ignazio · 05-20-2014, 10:12 AM

Hi Ignacio,
It's not our situation because we are able to write in the program memory and register data in the memory. The CMOS static RAM is deputed to the store of program and data, even the LastX is registered here. The X registry and the other stack registries (Y,Z and T) are resident on the ACT CPU. So the culprit can be only or a brocken ACT or the ROM 1818-0154, I fear this last most probably.

Let me work on the problem this evening.

Take care and hope.

walter b · 05-20-2014, 10:48 AM

(05-20-2014 08:49 AM)isanchez Wrote: It recommends to install a resistor in the ACT chip...

Don't be afraid - it's not in the chip but from one pin of the chip to ground.

d:-)

isanchez · 05-20-2014, 10:51 AM

Hi Ignazio,

you are ABSOLUTELY right in your suspects.

Look at this post:

http://www.hpmuseum.org/cgi-sys/cgiwrap/...read=34519

It reflects exactly the same problem in a 25c and the problem was in the ROM...this is a bad new for me, as I don't have this spare parts.

Kind regards

Ignacio

ignazio · 05-20-2014, 01:40 PM

Ignacio,
me too, I've not this spare parts, so far. I'll search another 25/25C not working but with good ROM, I hope.

Good luck even to you.

ignazio · 05-20-2014, 06:39 PM

Ignacio,
another element that confirm that the ROM is dead: with or without this component the behaviour of the calc is the same. We just have to search for a working ROM...

isanchez · 05-21-2014, 08:38 AM

Hi Ignazio,

yes, it is pretty clear now. Let's see if we can find the part....difficult, but.

Again, thanks for your time and communication.

Kind regards

Ignacio

PANAMATIK · (This post was last modified: 02-11-2015 07:33 PM by PANAMATIK.)

(05-21-2014 08:38 AM)isanchez Wrote: Hi Ignazio,

yes, it is pretty clear now. Let's see if we can find the part....difficult, but.

Again, thanks for your time and communication.

Kind regards

Ignacio

Hello Ignacio,

It is a long time since your last posting in this thread and obviously there was no easy repair possible. Could you repair your HP-25 calculator meanwhile?

As a side effect of my work of an ACT replacement, I have some spare original ACTs and some spare original ROMs. But the HP-25 has two ROM chips and one is integrated in the display driver. Unfortunately I cannot spare this chip, because I still need the display driver function. So I could help you only with an original ROM if you know that you need the second ROM.

But anyway the new ACT will likely solve your problem, because while using internal ROM code it replaces automatically the potentially defective external ROMs and RAMs.

Bernhard