Tale about my dad's HP-35

[robjordan]

Hi all

Possibly this might be an enjoyable read for some of you. It's a bit out of the ordinary for posts here, but some of my friends enjoyed reading it. It's about how my dad came to own an HP-35 in 1973. I've recently been attempting to repair it, and it inspired me to write a piece of nostalgia. It's about a 10 minute read.

https://jordan-maynard.org/2022/12/a-sto...-robs-dad/

Happy Christmas!
Rob

[EdS2]

That's excellent - thanks!

[robjordan]

Thanks, glad you enjoyed it.

[agarza]

Thanks for sharing.

[lrdheat]

Great story. I have my dad’s HP 35 in pristine condition. He wisely packed it away with the battery pack removed.

How might I get a working battery pack that would fit the HP 35 case?

In 1973, the meteorology department at Florida State University had two HP 35’s in locked cradles on 2 study desks for students to utilize! At the time, it was easy to recognize science students as they were wearing their slide rules in holsters on their belts! I still have my best college rule as well as my dad’s, and also his shirt pocket sized K&E!

[robjordan]

(12-13-2022 12:49 AM)lrdheat Wrote:  Great story. I have my dad’s HP 35 in pristine condition. He wisely packed it away with the battery pack removed.
How might I get a working battery pack that would fit the HP 35 case?

I'm glad your dad took better care of his. Mine also left me two pristine HP-28S, and a 49G... yes, recently voted worst ever HP calculator

My battery solution, maybe not ideal for regular use but a very easy improvisation for testing. I used a slim LiPo battery of 350mAh, which should be good for 2 or 3 hours use. Mounted it on a stack of business cards, trimmed to fit the battery compartment, and glued together for stiffness. I added a TP4056 charging / battery protection module, which are 10 a penny from Aliexpress. Wrapped two copper strips around from the top (battery) side of the card, to the bottom (spring contact side). Two strips of stiff but compressible packing material to keep the copper tape in contact with the spring clips, but avoid squashing them. Another thin layer of foam above the card to apply a little pressure from the battery cover, down through the stack. Attached photos should make this a bit clearer.

I'm slightly wary of fully-charging the LiPo cell to 4.2V, since the original battery voltage was 3.75. On the other hand I think I've read that the original charger supplied more than 3.75V to the calculator when running on mains electricity. Can anyone confirm?

[Edit: I found this thread which confirms 4.2V is likely safe, but also has Tony's good idea to add a schottky diode to drop the voltage a little and protect against reverse insertion]

   
   
   
   

[lrdheat]

Thanks…I’m clumsy in these sort of enterprises, and at least for now, continue to enjoy to use it with his pristine power cord. Those bright red LED’s are wonderful to see. My dad was so thrilled when it arrived, and I still have the post marked box that it arrived in, as well as other items that were included.

Such a treat to have this memento of my dad, and such a wonderful memento at that!

[AndiGer]

(12-13-2022 12:49 AM)lrdheat Wrote:  How might I get a working battery pack that would fit the HP 35 case?

What about something like that?

https://www.ebay.com/itm/224002078421

Batteries from waterhosko are recommended as I read.
Andi

[vaklaff]

(12-13-2022 05:16 AM)AndiGer Wrote:  

(12-13-2022 12:49 AM)lrdheat Wrote:  How might I get a working battery pack that would fit the HP 35 case?

What about something like that?

https://www.ebay.com/itm/224002078421

Batteries from waterhosko are recommended as I read.
Andi

+1 from me, personal experience

[vaklaff]

(12-12-2022 05:42 PM)robjordan Wrote:  Hi all

Possibly this might be an enjoyable read for some of you. It's a bit out of the ordinary for posts here, but some of my friends enjoyed reading it. It's about how my dad came to own an HP-35 in 1973. I've recently been attempting to repair it, and it inspired me to write a piece of nostalgia. It's about a 10 minute read.

https://jordan-maynard.org/2022/12/a-sto...-robs-dad/

Happy Christmas!
Rob

I love to read these stories. Thank you very much for sharing.

[robjordan]

I'm pleased a few of you have enjoyed it!

[lrdheat]

Thanks AndiGer for the tip on the battery pack on eBay, and vaklaff for the report of a good experience with the same!