HP-25 Calculators



#5

I have two HP-25 Calculators that have corroded battery contacts. Has anyone developed a procedure to repair these or am I out of luck. They appear to be silver plated button contacts that are riveted or soldered on the PCB. They extend on there side past the PCB tabs and make contact with the battery terminals on the battery pack. Mine have corroded so that the edge making contact with the battery is impossible. Is there a way to remove and replace the old ones, replate the old ones, or piggyback new ones on top of the old? Thanks.


#6

Hi Jim,

normally it's sufficient to take a sharp knife or some sandpaper to establish good contact again. If the riveted contacts are really badly corroded I recommend to remove the corroded material until the metallic surface shines up again and place a big ball of solder on each contact. There may be more elegant methods but I think that's the easiest.

Best regards, Walter


#7

Quote:
place a big ball of solder on each contact

You're right, that's not very elegant - and IMO, downright ugly. You'll also find it doesn't work - solder will not stick to the oxidized areas nor will it adhere to the chrome plate, should the rivet have any left. Even if you could get the solder to stick, need I mention it makes a very poor contact? Can you say "Fried Woodstock"? Do you charge your batteries in the unit? You'd better not with solder balls for contacts.

The only way I've repaired really, really trashed contacts is to remove all the solder, drill them out and replace them with a small piece of brass U channel cut down to an L shape. 1/8" works fine, it will be almost exactly the same length extending past the PC board.

I've never been able to find the same style rivet. Perhaps Keystone or some other electronic hardware manufacturer might have something but it is such a rare need, I just go with the brass.


#8

Thanks for the info. I to hunted for a replacement contact and I can not find any. The brass seems like the best bet. If I drill out the old contact, how do I establish a good connection between the brass and the PCB land? As an alternative could I solder the brass to the top rivets they are still in pretty good shape. I was thinking if I drilled a hole in the brass the same size as the rivet head, I could then solder the rivet head to the brass. Then I would not have to worry about the PCB land connection. The only thing is the brass would overhang on the opposite side of the PCB. I could put a right angle in the brass so that it wraps around the PCB and old contact.

The damage is only on the edge of the contact that interfaces with the battery. The back of the contact is still good. Do you think it is possible to turn the contact so that a fresh edge is facing the battery. Will this destroy the contact with the PCB land.

I plan on selling these calculators. Do you think it is worth my effort to try an repair these or just sell them as is for parts. Will my efforts increase the price. Thanks


#9

Hello!

Quote:
Will my efforts increase the price.

It almost certainly will! An alternative to the replcement of the rivet (or replacing it a blob of solder, as I have already done, I confess :-)) could be to place a small bolt or screw with cylindrical head in place of the rivet. If you find an appropriate size brass bolt, you can cut the shaft very short and solder it to the pcb.

Greetings, Max


#10

Max

Thanks for your response. Brass bolt with right head size and shaft size may work. Still concerned about getting good contact between the new contact and the PCB land. Do you know how I would make the contact? Do I expose the land at the contact and solder a small copper wire between the land and new contact? Do I try and solder the new contact directly to the land? I want to do a professional job so it is durable and the look does not harm the value. Thanks.

Jim


#11

Hello!

Quote:
Do you know how I would make the contact?

First, I would very carefully drill out the original rivet from the back side of the PCB. The calculator is easy to disassemble, but upon reassembly, you must take care to insert all the contacts, that link the two PCBs, in their correct holes.



If the gold plating underneath the rivet is still intact and free of corrosion, it should be sufficient to either tigten the brass bolt firmly with a little nut or to fix it with a drop of solder from the back side.



If the PCB has corrosion, then a little bit of conducting silver underneath the bolt head should help to make good contact. You could also "paint" the head of the bolt with it, so that it resembles the original rivet. Some people on this forum do not like conducting silver, but I only have good experiences with it!

Good luck, Max


#12

Max

Thanks for your help. It sounds like there is conductive eyelet around the rivet hole that should give me good contact. I need to be careful when I drill out the old rivet not to damage the eyelet.

I was wondering about conductive silver paint. Where there is only small damage to the contact I may try that first. I also found a product called Quickshine Pure Silver Plating Spray and several others on Ebay. They are supposed to replate the base metal surface. The base metal appears to be brass so they should bond properly if I can clean the contact well enough. Not sure how well they work but it is worth a try. Thanks again.

Jim


#13

Hello!

Quote:
1. I need to be careful when I drill out the old rivet not to damage the eyelet.

2. ... Pure Silver Plating Spray

3. The base metal appears to be brass so they should bond properly if I can clean the contact well enough.


1. Do it from the back side, even if that means that you have to disassemble the calculator, and there is no risk of damaging the eyelet.

2. Stay away from spray, but use a small brush instead. With spray, there is a big risk of creating short circuits everywhere!

3. Conductin silver should stick niceley on most metals you will find inside the calculator (copper, gold, brass). Just use fine-grain sanding paper or even the finest nail file of your wife (but make sure she dosen't find out what you've been using for :-) ) to clean the metal surface of oxidation and apply the silver paint immediately. Use no chemicals when preparing the metal surface for coating with conductive silver.

Greetings, Max


#14

Just a general comment. I once repaired our Amiga home computer with silver conductive paint; there was a tiny break in the circuit near one of the heavily used keys. The repair was fine and lasted the rest of the life of the machine. I don't know what is sold on eBay, but the stuff I used came from one of the car repair parts place (I live in the UK) and was meant for repairing heated rear windows. It was applied with a tiny brush.


#15

Thanks. I will check the automotive stores for a defroster repair kit.

Jim

#16

I clean mine with vinegar, then distilled water. I use a dremel tool with a wire brush. FInally I treat them with Caig DeOxit. If the contatcs are worn too far down, wrap them in thick aluminium foil or copper tape..


#17

Dave

I did use vinegar to clean most of the crud off. These contacts appear to be plated. Looking through a loop it appears the corrosion has eaten through the plating and has pitted the base metal. This is just on the side where it mates with the battery. Have you had any success at polishing the base metal so it makes good contact with the battery? I will have to look into the caig deoxit product and see what that does. Thanks for the help.

Jim


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