HP-25 calculator shows all zeros, or blank display

B

Bill Bowden

Guest
Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and wouldn't power up correctly. Sometimes the display
indicated 12 zeros and other times the display was blank and a few
keys would indicate a single "0" in various places.

I took the thing apart. cleaned the board and keyboard with alcohol
and toothbrush and contact cleaner and managed to get it working for a
couple hours using a couple alkaline AA batteries.

Put it all back together and everything seemed to work right but the
next day it went back to the original erratic operation. I read an
article indicating possible noise problems on the data line that was
fixed with a 20k resistor termination on some particular line. I tried
that idea on a few pins without sucess and cannot get the thing to
power up with further cleaning. Haven't looked at the clock signals on
a scope yet. Hard to find a schematic for old HP calculators.

Any ideas?

-Bill
 
Something that old, certainly by now has some cold solder connections. I
would reflow the whole board, especially if you say alkaline leaked out of
the batteries onto the board and the cleaning had some positive effect. If
there are any electrolytic capacitors on the board, change them too.



"Bill Bowden" <wrongaddress@att.net> wrote in message
news:ffadb930-98e4-4718-bc4e-1f19d1fb6636@s31g2000vbp.googlegroups.com...
Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and wouldn't power up correctly. Sometimes the display
indicated 12 zeros and other times the display was blank and a few
keys would indicate a single "0" in various places.

I took the thing apart. cleaned the board and keyboard with alcohol
and toothbrush and contact cleaner and managed to get it working for a
couple hours using a couple alkaline AA batteries.

Put it all back together and everything seemed to work right but the
next day it went back to the original erratic operation. I read an
article indicating possible noise problems on the data line that was
fixed with a 20k resistor termination on some particular line. I tried
that idea on a few pins without sucess and cannot get the thing to
power up with further cleaning. Haven't looked at the clock signals on
a scope yet. Hard to find a schematic for old HP calculators.

Any ideas?

-Bill
Click to expand...
 
On Apr 23, 3:57 pm, Bill Bowden <wrongaddr...@att.net> wrote:
Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and  wouldn't power up correctly. Sometimes the display
indicated 12 zeros and other times the display was blank and a few
keys would indicate a single "0" in various places.

I took the thing apart. cleaned the board and keyboard with alcohol
and toothbrush and contact cleaner and managed to get it working for a
couple hours using a couple alkaline AA batteries.

Put it all back together and everything seemed to work right but the
next day it went back to the original erratic operation. I read an
article indicating possible noise problems on the data line that was
fixed with a 20k resistor termination on some particular line. I tried
that idea on a few pins without sucess and cannot get the thing to
power up with further cleaning. Haven't looked at the clock signals on
a scope yet. Hard to find a schematic for old HP calculators.
Click to expand...
Have you tried the fansite, hpmuseum.org?
 
On Thu, 23 Apr 2009 15:57:04 -0700 (PDT), Bill Bowden
<wrongaddress@att.net> wrote:

Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and wouldn't power up correctly. Sometimes the display
indicated 12 zeros and other times the display was blank and a few
keys would indicate a single "0" in various places.
Click to expand...
The battery compartment isn't sealed and the batteries tend to leak
corrosive goo into the guts.

<http://www.hpmuseum.org/hp25.htm>
<http://www.hpmuseum.org/hp25c.htm>

I took the thing apart. cleaned the board and keyboard with alcohol
and toothbrush and contact cleaner and managed to get it working for a
couple hours using a couple alkaline AA batteries.
Click to expand...
Argh. Use NiCad batteries instead. Alkalines have a higher terminal
voltage (1.5V) than NiCads (1.35V). I don't know if you can damage
anything with the higher voltage, but it doesn't hurt to be safe. If
you don't have NiCads, use a bench power supply at 2.70VDC.

Contact cleaner leaves an oily residue. That's fine for the slide
switches, but not a great idea for the rest of the circuitry. Remove
with alcohol.

Did you inspect it for any corroded traces, especially near the
battery compartment? The various IC's are quite close to the battery
section:
<http://www.hpmuseum.org/25cinter.jpg>
Cleaning UNDER the IC's is tricky. If there's any corrosion under
there, it can be scraped out with a stiff piece of wire shoved under
the rows of IC leads. Also, brush between the leads.

Put it all back together and everything seemed to work right but the
next day it went back to the original erratic operation.
Click to expand...
Something might be loose or dirty around the clock oscillator area.
However, my intuition suggests that it might have something to do with
the over-voltage.

I read an
article indicating possible noise problems on the data line that was
fixed with a 20k resistor termination on some particular line. I tried
that idea on a few pins without sucess and cannot get the thing to
power up with further cleaning. Haven't looked at the clock signals on
a scope yet. Hard to find a schematic for old HP calculators.

Any ideas?
Click to expand...
Ask on the HP Museum forums:
<http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/forum.cgi>
but first check the archives:
<http://www.hpmuseum.org/archives.htm>
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
On Apr 24, 9:33 am, Jeff Liebermann <je...@cruzio.com> wrote:
On Thu, 23 Apr 2009 15:57:04 -0700 (PDT), Bill Bowden

wrongaddr...@att.net> wrote:
Recently aquired a HP-25 programmable calculator (1975) that had dead
batteries and wouldn't power up correctly. Sometimes the display
indicated 12 zeros and other times the display was blank and a few
keys would indicate a single "0" in various places.

The battery compartment isn't sealed and the batteries tend to leak
corrosive goo into the guts.

http://www.hpmuseum.org/hp25.htm
http://www.hpmuseum.org/hp25c.htm

I took the thing apart. cleaned the board and keyboard with alcohol
and toothbrush and contact cleaner and managed to get it working for a
couple hours using a couple alkaline AA batteries.

Argh. Use NiCad batteries instead. Alkalines have a higher terminal
voltage (1.5V) than NiCads (1.35V). I don't know if you can damage
anything with the higher voltage, but it doesn't hurt to be safe. If
you don't have NiCads, use a bench power supply at 2.70VDC.
Click to expand...
The old alkaline batteries I used measured 2.5 volts DC.

Contact cleaner leaves an oily residue. That's fine for the slide
switches, but not a great idea for the rest of the circuitry. Remove
with alcohol.

Did you inspect it for any corroded traces, especially near the
battery compartment? The various IC's are quite close to the battery
Click to expand...
Yes, I inspected every solder joint and trace for problems with a high
power magnifying glas. Been doing it for years.

section:
http://www.hpmuseum.org/25cinter.jpg
Cleaning UNDER the IC's is tricky. If there's any corrosion under
there, it can be scraped out with a stiff piece of wire shoved under
the rows of IC leads. Also, brush between the leads.

Put it all back together and everything seemed to work right but the
next day it went back to the original erratic operation.

Something might be loose or dirty around the clock oscillator area.
However, my intuition suggests that it might have something to do with
the over-voltage.


I read an
article indicating possible noise problems on the data line that was
fixed with a 20k resistor termination on some particular line. I tried
that idea on a few pins without sucess and cannot get the thing to
power up with further cleaning. Haven't looked at the clock signals on
a scope yet. Hard to find a schematic for old HP calculators.

Any ideas?

Ask on the HP Museum forums:
http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/forum.cgi
but first check the archives:
http://www.hpmuseum.org/archives.htm
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Click to expand...
I'll do that.

Thanks,

-Bill
 
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