HP 35 - Working with AC Charger but not Battery Pack

[Michael Lopez]

Hi all,

I recently purchased a HP 35 which seems to be in pretty good condition. Whilst the calculator came with 3 very old NiCAD battery packs, I also decided to purchase a new battery holder for the Classic series that uses 3 AAA Alkaline batteries.

Haven't tried running with the old battery packs installed as these cannot be in suitable condition. However, when I tried running the calculator with the new battery holder it will not turn-on.

With the original AC charger (820002A) plugged-in & without batteries, the calculator turns-on & works fine.

Since I am new to the Classic series, I was wondering if you have had any luck running these calculators on 3 AAA Alkaline batteries in an aftermarket holder? The seller of the new battery pack assured me that it should work fine.

Are there other replacement battery packs you would recommend? And finally, is there something likely to be wrong with my calculator that may be preventing it from working with the new battery holder that I may be able to fix?

Many thanks,

Michael

[mfleming]

Sounds like a messed up charger port. A 35 and most Classics will run from the charger without a battery pack installed. I use the 3 AAA replacement pack in a number of calcs as well.

Turn the 35 around and check the port where the charger plugs in. You should see three pins in a triangular arrangement pointing up, with a brass spring clip below shorting the two lower pins together. If that clip is missing or not aligned then the battery is not connected. If present you may need to realign the clip or even open the calc to reposition it. If missing, use a bit of foil or brass strip to short the lower two pins together. You'll need to remove that connection each time you want to connect the charger, but it's easier to charge batteries externally. Goes without saying, don't plug in the charger if using alkaline cells in the replacement pack

~Mark

[Steve Simpkin]

To add to the above description. It’s a bit hard to see but when you unplug the charger cable from the calculator two tabs on the metal bar shown in the photo below short the outer two pins together which connects the battery to the calculator electronics. Any corrosion at the point where the tabs contact the outer pins will also prevent it from working with the batteries.

[AndiGer]

A third option: if you have the battery installed and the charger plugged to the calculator but not to the mains the bridge of the outer pins is missing as well (pushed open by the charger) and the machine won‘t work.

[teenix]

...and a fourth, depending on how much the internals have been fiddled with over the years and battery corrosion, the wiring can sometimes become frayed at the ends where they are soldered. This could result in a break in the wiring or poor joints at the mentioned charger terminals or on the battery connectors themselves. If this is the case, then repair is easy.

cheers

Tony

[John Garza (3665)]

Corrosion in the charger port is more likely than a missing/damaged clip. Try scrubbing with deoxit in situ with a trimmed down qtip to fit. The metal flaps must have good contact with the end posts (left and right - not center). If this doesn't work, try a short piece of wire in the holes on the top of the two end posts. If it works then corrosion/poor connection with the flap/posts is the problem. You can either keep the wire in place, if you like the 'Macgyverd look', or disassemble, sand the corroded flaps, and cover with conductive paint. Deoxit is usually sufficient for the posts. For some reason, the flaps on the clip seem to go first.
-J

[Michael Lopez]

Hi all,

Thanks very much for the useful information & suggestions.

As I previously indicated, I am new to the Classics range (had experience from HP41 series onwards) & wondered what that Brass clip on the HP35 was for. When I first saw it, I thought it might have been some kind of earth for the AC charger but it didn't mate with anything on the plug. However, I would never have guessed that it was like a mechanical "switch" to go from AC power to batteries :-( .

There was light corrosion on the pins & clip so I cleaned both in-situ but it still didn't work. Seems that the clip is quite loose & not exerting enough contact pressure against the pins. But the aluminium foil trick Mark suggested works fine. Just pushed some carefully folded aluminium kitchen foil between the clip & pins & all working now. So fortunately it didn't turn-out an internal corrosion problem (there is no evidence of leaks in the battery compartment either).

I presume the said clip isn't connected to anything internally? Am tempted to pull it off & then re-bend it so it isn't loose where it slips over the plastic ridge in the body.

For now though, very happy with my first Classics calculator.

Cheers,

Michael

[Steve Simpkin]

(04-09-2023 08:40 PM)Michael Lopez Wrote:  ...
There was light corrosion on the pins & clip so I cleaned both in-situ but it still didn't work. Seems that the clip is quite loose & not exerting enough contact pressure against the pins. But the aluminium foil trick Mark suggested works fine. Just pushed some carefully folded aluminium kitchen foil between the clip & pins & all working now. So fortunately it didn't turn-out an internal corrosion problem (there is no evidence of leaks in the battery compartment either).

I presume the said clip isn't connected to anything internally? Am tempted to pull it off & then re-bend it so it isn't loose where it slips over the plastic ridge in the body.
...

Yes, it is just a floating piece of metal. It is probably still not making good contact.

https://www.keesvandersanden.nl/calculat...inside.php

[teenix]

Definitely bend the two fingers outward a bit to increase the contact pressure. I have had to do this in the past. It is possible that the charger was connected for a long period of time or had many charges in the 50 or so years of its life and compressed these inward.

If it goes loose again after periods of charging, then the metal may be fatigued, and a replacement might be in order.

cheers

Tony