HP-25C It’s alive!

[Ethan Conner]

Back in 2016 a coworker found out I had a small collection of vintage HP calculators. He had been buying all kinds of vintage electronics at estate sales and brought me a HP 25C with a pouch. I asked him where the battery pack and AC adapter were. He proudly stated that he had seperated all of the batteries, chargers and devices into three giant cardboard boxes.

It had belonged to an engineer at the aircraft factory we work at and still had the materials pass sticker and name with employee number. It also had the name and address of the engineer which was three blocks away from where I grew up. Most likely it was used on the F16 and F111 programs which were both active in the mid 70’s.

The downside other than no battery pack was it had been used by a smoker and stored by a smoker. Last year I got busy and cleaned it up removing probably 90% of the cigarette smell. Today I received a battery pack and inserted it into the calculator and the display lit up! “0.00” So far everything seems to be functioning just fine.

In 1976/77 my father and I had an early morning routine of breakfast and reading the newspaper. He would give me the sections that had car advertisements and HP calculator ads. I begged him to buy me any HP calculator but it just wasn’t in the budget and my young brain couldn’t measure the price against our cost of living. He did buy me some no name teal led calculator that broke after three weeks.

When i see an led HP calculator it reminds me of those early mornings sharing the paper with my Father and dreaming of Hewlett Packard calculators.

[John Garza (3665)]

Great News! Another 25C lives. But remember...

NEVER
NEVER
NEVER

Plug in the charger to that calculator for any reason.

ALWAYS charge the batteries externally.

The moment you get a battery problem, or even a spot of oxidation preventing good contact, the charger will fry the calculator without good batteries in place making good electrical contact.

-J

[John Garza (3665)]

The 25C was my 'HP dream machine' too. While in high school, I had a summer job saving up for one. I kept some HP brochures around to keep me motivated. I was just captivated by that little gem.

-J

[Ethan Conner]

I have a modified battery pack running conventional “AA” batteries to avoid the whole charger fiasco.

[John Garza (3665)]

Perfect. That's what I do. Keep that little jewel running another 50 years.

[gregdressel]

Two years ago I put the Panamatik chip in my HP 25 with help from Bernhard Emese. After cleaning the keypad through the holes in the back of the plate, the calculator worked great.

The next project for the HP 25 were the batteries. Two Ni-MH 1300 mAh 1.2V 4/5 AA batteries were put into the original battery pack with a new spring. A second battery pack was constructed in a similar fashion using a newly printed battery holder.

The calculator is charged 100 % of the time with the batteries installed using the original charger hooked up to the calculator while charging. Never a problem. The calculator works every time. Based on my experience, the Panamatik chip is robust enough to allow charging the Ni-MH batteries while installed in the calculator.

[John Garza (3665)]

On the 'never a problem' post. The issue with the Woodstocks happens only under certain conditions when the battery pack goes bad, is not in place, or there is some slight corrosion that keeps the battery from making good contact. Zero corrosion batteries with shiny contacts in the calculator should work fine - that's what it's designed to do. The issue happens over time when the battery starts to fail, or makes poor contact. Never charging a battery in the machine is essentially a safety procedure. Like always stopping at a stop sign when you're driving. Corrosion and battery failure will eventually set-in if you use the machine regularly. So even if it works now, there is no guarantee it will work a few years down the road. And following the safety procedure could save your machine.

-J

[Maximilian Hohmann]

Hello!

(09-21-2023 04:04 PM)John Garza (3665) Wrote:  ... or there is some slight corrosion that keeps the battery from making good contact.

This corrosion of the battery connectors is inevitable in Woodstocks, because HP chose to use silver as a contact material instead of nickel or gold plating (like everybody else - and even HP in every other series of calculators). As we all know from our cutlery, silver oxidises in (moist) air no matter what one tries to prevent that.

Regards
Max

[Wilsom]

The corrosion of battery connectors in Woodstock calculators is an unavoidable issue due to HP's choice of using silver as the contact material, unlike the more common nickel or gold plating found in other calculators, including HP's own other series. As most of us have experienced with silver items, it tends to oxidize when exposed to moist air, despite efforts to prevent it.

[John Garza (3665)]

One solution I've found that works is to polish the metal disc contacts to remove the corrosion, then apply nickel bearing conductive paint. MG Chemicals #841AR

-J