HP41 rechargeable packs - another query



#2

.....or three!

Having rebuilt my first pack, I managed to get hold of another (much earlier pack - serial number 21xxS) and was rather disconcerted to see it had a three pin contact for the power supply cable rather than the two pin contact I had expected. There is also a copper blade that appears to make contact with the middle pin closing a circuit when the power cable is disconnected.

There was further consternation when on opening the pack I discovered that the metal spring strip contact on one side that conencts with the internal contacts in the calculator does not make contact with the top of the rechargeable battery (as I had expected having rebuilt a 'two pin' pack) but was insulated from it and instead was connected via a flexible ribbon to the circuit board in the pack.

Can anyone enlighten me as to the logic behind the 'three pin' pack?

Now to the rebuild:

One of the metal spring strips which form the connection between the pack and the internal contacts in the '41 was badly corroded and broke in two on removal - any ideas where I can get a replacement? (worst comes to the worst I should be able to use a coil spring)

The printed circuit track on the flexible ribbon mentioned above has corroded away in a number of places and will need replacement - any ideas on a suitable source? (worst comes to the worst I can run a cable from the metal spring strip connector back to the circuit board)


#3

The three pin pack disconnects the battery from the calculator when the wall wart connector is inserted.

The small TO-92 three terminal regulator in the three pin version is a 78L05 or 06, I forget which. It provides regulated DC to the calculator while the battery charges, disconnected from the calculator. In the two pin version, the TO-92 regulator is a 78L12 and only provides the charging current to the battery. The calculator is powered from the batteries. In these versions, there is a 6.2V zener across the battery to prevent overvoltage from reaching the calculator. It becomes a fail safe should the battery ever open as the thermal rating is exceeded and it fails shorted.

With a little work, you can convert a three pin to a two pin version and completely eliminate the flex circuit that always gets eaten by the battery leakage. It involves replacing the regulator, the series charge limiting resistor and adding the zener. Yes, a fair bit of work but then you'll have a safer battery with fewer parts in the end.

BTW - tinned beryllium copper is the material of choice for the contacts. I don't know a source in the UK (I doubt RS has it) but here in the US - http://www.smallparts.com/products/descriptions/bcs.cfm is one source of supply.


#4


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