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HP Forum Archive 13

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Display on HP-25C shows 000000┤s
Message #1 Posted by Matti Ívermark on 13 Oct 2003, 4:34 a.m.

Hi gurus,

I have recently obtained an HP 25 C. However I cannot get it to work. I inserted new batteries, rechargable, in the battery compartment, but all it shows is a full display of 0┤s (zeros). In one instant the last zero became a 3. Very wicked, I┤d say. The original batteries were dead. Any clues?

TIA Matti, Sweden

      
Re: Display on HP-25C shows 000000┤s
Message #2 Posted by Victor Koechli on 13 Oct 2003, 5:22 a.m.,
in response to message #1 by Matti Ívermark

I'd suggest taking the calculator apart and carefully cleaning the interior contacts that connect the keyboard and the display to the main PCB. While doing so, you can also check for broken traces or loose solderings on the main PCB.

Another oddity of these calcs is that some of them only work after a warm-up period of up to an hour, so if cleaning does not help, try leaving it turned on for some time. I have a HP-25 with that behaviour. It's supposed to be a problem of the ACT chip, and there are a number of fixes you can try (search this forum's archives for more information).

One thing, though: Never, under any circumstance, connect your HP-25C to the AC adaptor when you have no battery installed or if there is even the slightest chance that the battery contacts could be bad! You could kill the calculator's RAM chip just by plugging in the adaptor! In the C-version, the RAM is connected to the power supply even with the calculator off.

Cheers, Victor

            
Re: Display on HP-25C shows 000000┤s
Message #3 Posted by Albert on 13 Oct 2003, 1:13 p.m.,
in response to message #2 by Victor Koechli

Hello,

Don't touch PCBs with your fingers! Static voltage could destroy ICs.

Check up space / gaps between integrated circuit (ICs) and display pins. Using a sharp / fine plastic, you can remove any dust or suspicious tiny objects. In addition, with a magnifying glass check both printed circuit board faces, ground metal paths mainly. Clean the keyboard printed circuit also. If battery pack was placed for a long time inside calculator, corrosive damage could "blur" device metal contacts.

Good luck,

Albert

      
Re: Display on HP-25C shows 000000┤s
Message #4 Posted by David Smith on 13 Oct 2003, 3:10 p.m.,
in response to message #1 by Matti Ívermark

This problem is almost always caused by a dead ACT chip (the wide 22 pin chip near the bottom of the circuit card). The only source is another Woodstock series machine (except the HP27). The chips have different part numbers, but they are all compaitible.

            
Re: Display on HP-25C shows 000000┤s
Message #5 Posted by Matti Ívermark (Sweden) on 14 Oct 2003, 5:39 a.m.,
in response to message #4 by David Smith

Hi,

Thanks for all hints. I let it (the HP 25C) stay on for a while, a quarter of an hour or so, then I occasionally by turning it on/off reapeatidly got it to show 0.00 in the left corner. But then (sigh!) it failed to show anything.

I think the HP-25C is opened by removing the rubber feet and then there ought to be some screws to work with.

Anyone suggesting what I should do? There seems to be no surplus stock of these calcs around here...

Best regds Matti

                  
Re: Display on HP-25C shows 000000┤s
Message #6 Posted by David Smith on 14 Oct 2003, 5:54 p.m.,
in response to message #5 by Matti Ívermark (Sweden)

By bet is still a failing ACT chip, but you might try opening up the machine and cleaning the circuit board with 91%+ isopropyl alcohol. Sometimes corrosion (often invisible) or solder flux residue will stop the machine from working.

If there is a lot of green or white corrosion on the board you may need to use vinegar. Basically stand the board up in a glass of vinegar up to the top of the battery contacts for 30 minutes or so. Scrub it down with a stiff toothbrush. Rinse VERY well. Shake all the water off... better yet blow it off with canned "air"... do not use an air compressor (they blow oil and air). Let dry for at least 24 hours.


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