Stupid HP-25 Questions



#9

I have just received a nice HP-25C. But there are a few confusing things that make me wonder if

1) there is something wrong with the 25C or
2) I don't know what I'm doing (most likely)

First, the basic operations seem fine. The storage registers seem to work ok. The values are saved between sessions. The batteries are fine.

But for some reason the programs act wierd. The don't seem to be saved between power on-off-on. I'm not sure if it has something to do with the way I write the programs.

Sometimes, after an off-on, the program is there, sometimes it's not, sometimes it gets corrupted. Wierd! Also, sometimes when the program gets trashed, the registers do also.

Everything seems to power up ok.

Anyone ever seen this or know what might be wrong. All I can think of is that maybe the power-up circuit is bad and it may corrupt data. Anyone?


#10

Tom,

I've seen the problem you describe, in an HP-25 in which I replaced the "old style" memory chips (two chips) with a "new style", single chip, I found that it was also necessary to add a 100K resistor, one that's present in newer model HP-25Cs but absent from the older ones.

Of course, unless someone tinkered with your machine already, this is unlikely to be of any assistance. My guess is that your machine may have a fried memory chip that works intermittently.


Viktor


#11

This has a year code of 1610A. It has 4 chips.

Two of these
S
7629
1820-1843

One of these
AMI
7607PE
1820-1741

One of these
Mostek
Mk60027 (or something like that; poor eyesight you know)

But I don't see any 100K resistor anywere. Which resistor was changed to 100k? There is a 10K at the end of the Mostek beside a diode and large cap.

Anyway, is there a web site with schematic of this model?

Thanks,


#12

Take a look in the Forum Archives back in August 2000 under the topic "Challenging Woodstock Repair". I came up with a fix for a problem that's not unlike your memory retention one. There are also suggestions from other collectors on that topic that might be of help.

Unfortunately, I don't know of a schematic for these machines. I was going to draw one up, but without having the IC pin descriptions, it's going to take a lot of work to infer their functions.

-Katie

#13

The 1820-1843 is the old memory chip (of which two are required), and in that configuration, I've not seen the 100K resistor I mentioned.

If you have access to a broken HP-29C or HP-19C (there are, sadly, quite a few of these around), you might be able to use one of its memory chips to replace the two chips in your calculator. In this case, however, you do need that resistor. If you're going to attempt this particular repair, write to me at vttoth @ vttoth.com and I'll give you the details.


Viktor


#14

After plowing through the archives, I have found that the problem is 100% consistent with the articles that posed the possibility that the memory chips need some sort of warmup (i.e. pre-charging or something) before they become stable.

Originally, when I first got this, I had the battery pack in and out many times for varying amounts of time. Whenever I found a problem, I might for instance, remove the battery to check (clean) the contacts. Of course, this would reset my problem.

After reading these articles, I let the calc sit for 15 or so minutes. NO PROBLEM ever again. I would then remove the battery pack for a period of time and THE PROBLEM RETURNED. Let it sit for 15 or so minutes and the problem went away again.

When I say it went away, I have eliminated the possibility of intermittent problem by performing numerous resets and running of programs and checking register data.

I can find no errors ever after the calculator has had the battery connected for 15 or so minutes.

I DO find this a bizarre behavior though.


#15

Sorry if I appear a bit thick as I don't know too much about the 25C calc but does the calculator have a battery back up capacitor so the memory is saved when you change batteries?

If this is the case then the circuit for charging the capacitor (including the capacitor itself) may have become highly resistive and hence takes 15 min to charge up enough to hold the memory.

This is just a pure guess but could be the reason why your calculator does what it is doing. High value capacitors do age badly and many battery holding circuits can be repaired by simply changing the capacitor. A copy of the calculator's circuit would be invaluable to solve this problem, am I right in thinking they are not available?

If I have grabbed the wrong end of the stick then please ignore this message.


#16

I only came to this conclusion by reading what others have found. Normally, I would expect that the charge-up path should be much quicker than the discharge path. It may well be that age is a factor.

All I know for sure is that this is the way it is working now AND others have seen this also. I guess I could just replace the cap and see if the behavior changes. But I suspect that others have the answer to that (been there, done that, you know).


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