Help on HP28S

[Cancuino]

Hi, I was going to buy a second hand HP28 that was supposed to work perfectly. The previous owner bought two new batteries to show me that the calculator was ok. The fact is that nothing showed on the screeen and the batteries were hot just 5 seconds after pressing on.

He gave me the calculator and I’m trying to find the problem.

The batteries he used are Panasonic LRV08 that is 12V each. 3 batteries equal 36V!.
The batteries that the calculator needs are type N, that is 1.5V each, 4.5V total.

Is it possible that something has been burned inside the calculator?.
Can I fix it somehow?.

Thank you.

[John Keith]

It is very likely that something has burned out in the calculator. If so, it is probably irreparable. Only way to test is with three known good N cells.

Don't feel too bad, you're not the first person to make this mistake.

[Cancuino]

I have N batteries that I use on my HP41CX but the HP28 doesn’t work with them.

[grsbanks]

(01-24-2019 09:19 PM)Cancuino Wrote:  I have N batteries that I use on my HP41CX but the HP28 doesn’t work with them.

It's doubtful it ever will. It is most likely fried after the 12V cells used in it.

[Geoff Quickfall]

Sorry, but a big oops on the 12 volt battery insertion. Some 41c’s have suffered the same fate.

Having said that, the damage may have been limited to the power side of the circuit. Usually an electrolytic cap fries first. Now the problem is “is it worth it to disassemble a calculator not made for easy disassembly. Then track down the circuit in the hope the 36 volts did not make it to the ACT or any other IC’s.

Find a working one on ebay with the knowledge that the battery cover and case design are problematic.

Search clamshell battery cover issues and heat stake issues:

Opening an HP28S

[mfleming]

(01-24-2019 09:26 PM)grsbanks Wrote:  

(01-24-2019 09:19 PM)Cancuino Wrote:  I have N batteries that I use on my HP41CX but the HP28 doesn’t work with them.

It's doubtful it ever will. It is most likely fried after the 12V cells used in it.

I bought an HP28 on TAS for a very low price. The seller said "The display fades away a few seconds after turning it on." He was right. Opening the battery compartment to see if they were dead, instead I found the same three 12V batteries you did. Popped in the right ones and all was well.

HP made them tough at one time, so give yours a try...
~Mark

[jonese]

Is the 12V the "A23" battery? Slightly smaller in dimensions compared to the proper 1.5V "N".

12V A23 28.5 X 10.3 mm
1.5V N 30.2 X 12.0 mm

The very first photo from https://en.wikipedia.org/wiki/N_battery shows how similar they are.

Disappointing...

[Jlouis]

Sorry to say that, but this 28s is fried. No help.

You can find others in TAS. It is cheap enough, and a pleasure to use. I always have one at hand.

Good luck.

Cheers

[Geoff Quickfall]

Many an hp n cell calc has been fried because of the similarity in size. If Duracell or Eveready even the cell looks the same except for the very tiny print.

[isanchez]

Hi,

I fully agree with Geoff. I have seen this mistake twice, both in HP41 calculators. I also share the same opinion about if it does worth to open a HP28...I have bad experiences with this dismantling process...hard breaking posts work...not very "friendly" design...

Regards

Ignacio

[everettr]

Don't feel bad. I purchased an HP 28S on TAS a few years ago. The seller included batteries, and I did exactly the same thing as you. Ouch!