HP41 battery pack circuitry

[Jaime]

Hello.

Some weeks ago I replaced the batteries of the battery pack of my old HP41 with Ni-Cd 1/3 AAA and it worked quite fine.

Suddenly the batteries didn´t recharge anymore. I had a look at the circuitry and discovered that the small regulator (I think the small black three-terminals component is a regulator) had broken one of its pins. I´m afraid I was a bit clumsy when I rebuilt the pack. Now it´s irreparable. So...

Does anybody know a possible replacement for this regulator? Does anyone know its characteristics?

Thank you in advance.

[Ellis Easley]

If it is like mine, it is marked with the HP number 26-0275. I think it is a 78L12, 12V 100 mA regulator. Tony Duell might have verified this here in the Forum (he has a list of some equivalants for HP part numbers).

The part on my unit is like this (I traced out the circuit some time back): looking at the flat side of the regulator, with the terminals going down, the center terminal is connected to the -ve side of the unregulated supply; the right terminal to the +ve side. The left terminal is connected to a small capacitor to the center pin, and to the anode of a rectifier diode whose cathode is connected to a 365 ohm resistor. The other end of the resistor is connected to the +ve terminal of the battery. The -ve terminal of the battery is connected to the -ve side of the unregulated supply. A zener diode is connected across the battery, cathode to the +ve side and anode to the -ve side. With no battery present, and relative to the -ve side of the unregulated supply (and the battery), I measure +15-18V at the right terminal of the regulator, +12V at the left terminal of the regulator, and +6.7V at the cathode of the zener diode (and the +ve battery terminal).

Unrelated note: I recently cleaned up a BBC model B computer I bought on Ebay from a guy in Australia, and have found quite a bit of literature and software for it on the internet. The Acorn manuals use the conventions "+ve" and "-ve" for positive and negative, as Tony does. I've started doing the same, it's a great shortcut! (Now I'll have to adopt some emoticons.)

[Tony Duell]

My list shows that 1826-0275 is an MC78L12ACP. Any other 78L12 would be fine too, I think. It's a standard 12V 100mA regulator. However, in an old PPC journal I found a reverse-engineered schematic of the pack which shows the part as a 78L06. I suspect the latter is incorrect, but....
The convention of +ve and -ve is _very_ common over here. I can't remember where I learnt it from, but I think it's been around for well over 50 years. I am suprised it's not universal. Another convention which is common in the UK and Europe, but which seems to confuse Americans is to use the prefix character in place of the decimal point in component values. Things like 4k7 for 4.7k (4700 Ohms). Or 5n6 for 5.6nF (5600pF) and so on. Very common here, and the letter is less easy to lose than a period.

[Vieira, Luiz C. (Brazil)]

Hi;

I also have seen the "+ve" and "-ve" in some deutsch technical literature (very, very old ESK documentation; ESK is a Telephone Central Switching Network, from Siemens). Would it come from "Volt Eingang", that stands for "Volt Entrance", or Volt Input?

Any comments?

[Ellis Easley]

You might have something there. I have been thinking of the terms "+ve" and "-ve" as sort of hieroglyphic abbreviations for "positiVE" and "negatiVE" and also means to give more weight to the symbols "+" and "-" alone. In my German dictionary, I see that the words are spelled the same as English except without the final "e". But it could be that Germans adopted the English abbreviations. "+v" and "-v" alone imply something too specific to replace the general meaning of the original words.

[Jaime]

It´s marked 1826-0275. So your information is just what I needed. Thank you all.

[Ellis Easley]

I think I've seen actual resistors marked "4k7" for 4700 ohms and I'm quite sure I've seen low value resistors marked "4R7" for 4.7 ohms. And I've seen this kind of notation used on schematics and parts lists. It makes perfect sense and can avoid a lot of trouble in the case of a fax or Xerox copy or a scanned image!

Regarding the PPC Journal schematic with the 78L06 - did it also have the resistor and 6V Zener diode?

I have some boards from old oil well logging equipment that use multiple Zeners in decreasing voltages, cascaded, to improve the regulation at the end of the chain by each Zener pre-regulating the next.

[David Smith]

There are two versions of the battery pack. One has three pins in the charger plug (the center one is actually a switch contact). The other has two pins. I think one type uses a 6V regulator. The other is 12V.

BTW, the three pin version is not as good a design. Besides the charger plug switch contact, it has a weird tape-flex circuit that forms one of the battery contacts. If the cells go, it can overvolt the machine. When I rebuild them I put a 6.8V zener diode on the ourput side just in case. I also put a couple of layers of kapton tape under the tape contact so that if the battery spring wears through the tape contact it will not make contact to the battery cell underneath it.