Problem: amnesic HP-25



#2

Hello,
I recently bought a HP-25, my first vintage HP, but unfortunately it has some memory problems: I can't neither use the program mode nor the registers.
In fact, when I enter keystrokes in program mode, the step number normally increases, but the step I just entered is still GTO 00 (13 00), the HP does not remember what I typed. In the same way, no matter what I try to put in the registers, when I recall their content it's always 0.
On the other hand, the stack is functional, I can make any calculation I want provided I don't use the program mode or the registers.
Has someone here had tame problem? What can I do about this?


#3

Oops, I should have checked the archive first, the answer wasn't far away...dead RAM chip? No easy replacement? Cruel world :-(


#4

The RAM chip may not be dead. The symptoms you describe are common to this calculator and there's a fix that works sometimes. Read more about it here.


#5

Thanks, I'll try this fix...

#6

Quote:
The RAM chip may not be dead. The symptoms you describe are common to this calculator and there's a fix that works sometimes. Read more about it here.
Quote:
A seemingly common problem on some Woodstock machines is that they don't work until they are left on to "warm up" for some period of time. After leaving the on for several minutes (maybe up to 1/2 hour) then turning them off and back on again they start working normally.
Unfortunately, leaving my own 25 on for more than half an hour didn't make it work better :-( ...but I'll try the fix anyway...
#7

You'll also find stat functions missing as well.

Your 25 is in need of a working RAM chip, HP PN 1820-1564. The only source is from another HP22 25 or 91.


#8

What causes the ram to fail?


#9

Quote:
What causes the ram to fail?
The classic Woodstock failure mode. Just plug a charger into your 20 series unit that has a bad battery or dirty contacts, wait about 8 to 24 hour and *poof* your calculator has been fried. Crispy or Original, it's dead. If it isn't the ram, it's the act chip which exhibits itself as an flashing all zeros display. Sometimes it's both.

The battery is the load in a Woodstock charging circuit. If the battery fails to make contact or the battery is open (you're okay if it shorts), the voltage on the chips rises to about 6-7 volts of half-wave rectified DC instead of 2.5. UUUUggggllllyyyy.

So the next time you see a 25 on eBay and the seller says they plugged it in and it lights up, you better ask what was displayed. I almost never buy a Woodstock that has a charger with it. Cheap insurance. I would extend that rule on the 25C and 29C to NEVER unless the unit is known working and you can return it should there be a problem. Pressing the E+ key a few times to see it the display number increments is a easy way to test ram. Dead ram will stay at zero with each keypress.

Edited: 24 Aug 2006, 9:54 a.m.


#10

Quote:
Pressing the E+ key a few times to see it the display number increments is a easy way to test ram
Displays 1 all the time... the "mean x" function gives me an error too...

Sometimes also, I get a "negative decimal point": all decimal points are on, except where it should be...

Actually, the battery pack was leaked, I am currently cleaning the circuits, but I have very little hope...

#11

Quote:
Sometimes also, I get a "negative decimal point": all decimal points are on, except where it should be...

That's the normal low-battery warning. That part, at least, is not a problem!

- Thomas


#12

Oh, thanks for the info :-), I bought this machine for almost nothing, and it didn't come with its manual (nor its charger)...

I already know I can buy all the manuals on DVD, don't tell me... :-)

#13

Is it possible to modify the WOODSTOCK by adding a zener diode across the charger output to keep the output voltage within safe limits, or would that stop it charging...

As I now have a both an HP21 and HP25 (not HP25C unfortunatly) I'm beginning to take rather more of an interest in thes posts than before!

Mike T.


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