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newbie needs help with HP-25 repair - Printable Version

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newbie needs help with HP-25 repair - Stan McFarland - 12-06-2008


I'm hoping someone can help me. I'm the original owner of an HP-25 which has seen only occasional use for the past few years. When I turned it on a few days ago, the display was dead, so I figured the batteries needed recharging (these are replacement batteries I put in the case myself several years ago). After charging with the calculator charger, still nothing, so I replaced the batteries. Now, the calculator is mostly dead - sometimes 0's will flash, sometimes I'll see 0.00, but in most cases, the display is blank. And even when I do see something on the display, pushing any button will cause the screen to go blank.

Does anyone have any suggestions? I'd hate to have to sell it for parts.


Stan McFarland

Re: newbie needs help with HP-25 repair - Randy - 12-06-2008

Unfortunately you fried the calculator by continuing to use the HP charger. At some point either the batteries failed or they lost contact with the calculator. Once that happened and the charger was connected, the charging circuit had no load. Since the battery charging circuit connects to the battery which is directly connected to the calculators electronics, they saw about 8 volts instead of the correct 2.5 volts when the battery provides a load.

It is without a doubt the most common way a Woodstock dies. The only solution is parts from another 20 series unit to repair the damage which is typically the largest chip on the board, the ACT IC. The flashing digits is a sure sign of ACT damage.

Most collectors use either an already dead Woodstock in which they charge their batteries or use modern ni-mh cells and charge them externally.

Re: newbie needs help with HP-25 repair - Katie Wasserman - 12-06-2008

It's likely that at least one of the problems is that the batteries are not making good contact with the calculator. Try scrapping and cleaning the small rivets used in the calculator to make contact with the batteries. Also, make sure that you don't run the calculator while charging especially if the batteries are dead or are not in contact with the calculator this can kill the electronics.

The simplest check for basic functionality: When you turn it on, if you see a 0.00 you're probably okay, if you seel a line of all zeros it's probably toast.

Edited: 6 Dec 2008, 10:58 a.m.

Re: newbie needs help with HP-25 repair - Stan McFarland - 12-06-2008

I was afraid that was the answer I was going to get. I'm gonna miss her. ;)



Have to agree with Katie on this... - Geoff Quickfall - 12-06-2008

first always try the little rivet contacts as these are under physical stress with the battery pack in. In some cases the rivets actually have a cold solder joint. Also on some occasions you may have deteriorating trace lines from the rivets to the circuits. This occurs with outgassing or leakage from the batteries. I have cleaned the circuit board and applied silver conductive trace paint to the original traces and also revived an otherwise dead HP 2X series

Clean the rivets.
Apply soldering iron and flux to the joint to reconnect solder.
Check traces for continuity and repair with silver conductive trace paint.

I had an HP 29C which was considered dead but all was due strictly to the joint at the rivet. The fact that you got a 0.00 gives me reason to believe it MIGHT be a solder joint. When the joint is not perfect it creates on some occasions an almost infinite resistance. This allows for some current to get through. This in turn causes display problems similar to a weak battery. Also, every time you press the keyboard you warp the case to battery interface and this may cause the display to go dead.

Cheers, Geoff

HP 29c restored from non-working condition due to weak rivet connections and cold solder joint.

Re: Have to agree with Katie on this... - Stan McFarland - 12-06-2008

Geoff, I'm hoping you're right. My first inclination was to believe there was a bad connection between the battery pack and the board, because I could wiggle the batteries a bit and cause a change in the display. I sanded the rivets clean of corrosion, but maybe a bit more cleaning is in order. I'll try it.



Re: Have to agree with Katie on this... - Stan McFarland - 12-06-2008

Geoff, I have you guys will probably consider a stupid question. I can only check for continuity on one side of the board, right? The underside of the board has what appears to be some sort of non-conductive film over the circuits. On the top side of the board, where I can test for continuity from the rivets, only one of the rivets has a line coming from it, so I'm not sure how to test for continuity from that one - any pointers?



Re: Have to agree with Katie on this... - Stan McFarland - 12-06-2008


I think I might have figured out the problem. When I put the battery pack in place and test the voltage on the side where the positive battery terminal touches the rivet, I get 1.25 volts, but on the other battery, I get 0.00 volts. If I take the battery pack out and just touch the battery to the rivet, I get 1.25 volts. So it appears to me that the rivet is not contacting the negative terminal of the battery when the pack is in place. I wonder if over time if the river hasn't worn away bit from use and corrosion. Anybody have any suggestions as to how to build the rivet back up a bit?



Re: Have to agree with Katie on this... - Geoff Quickfall - 12-06-2008

Hi Stan,

I think it sounds like the battery rivet. To answer your first post:

1.  remove the case back
2. remove the top circuit board (this will allow access to the
power tracings from the rivets.)
3. with one probe on the rivet work backwards from the rivet to
identify the problem
4. reverse the board and test the second rivet and trace.
5. IF the problem is found clean the board (vinegar rinse
followed by water rinse followed by alcohol and then a thorough
drying. (I use a shoe box with a hair dryer on COLD setting)
6. apply silver conducting paint.

On the second point,

1.  clean the rivet with a fiberglass brush; fine wire brush; or
sand paper.
2. If blue residue exist clean with vinegar.
3. rinse with 90% alcohol.
4. just stick a little solder flux on the rivet,
5. then apply the solder iron and heat until the existing solder
flows, add some if required.
6. clean rivet with alcohol to remove acidic flux.

There is enough spring action in the battery to force the contact between the rivet and battery terminal. I doubt the rivet is worn flush with the circuit board, if I interpret your question correctly. If it is then try filing some of the circuit board back to expose more of the rivet edge to the battery. OR, build up the rivet with solder.

Hope this helps, Geoff

Edited: 6 Dec 2008, 10:50 p.m.

Re: Have to agree with Katie on this... - Stan McFarland - 12-07-2008

All, thanks for everything, but it looks like it's indeed fried. There was a problem with the continuity on one of the rivets, but once that was fixed, it fairly consistently will show flashing zeros.

Thanks again,