HP 67: charging of battery pack

[PANAMATIK]

(04-25-2017 05:47 PM)Den Belillo (Martinez Ca.) Wrote:  RE: "The HP-67 is save to charge and work without batteries like the HP-35,45,55,65,70,80."

Bernhard;
You may well be correct. If so that's good news, with a clear explanation.
However; I've read the opposite here for probably 15 years from other knowledgeable people. So until I'm sure, I'll err on the safe side and urge "new" members with that question to err on the conservative side too. IF there is any greater chance of frying the ACT in powering it that way; I'd rather avoid the penance of buying another of your chips, the couple of hours changing it out, and the peeling and re-sticking of the backplate. I always hate that part.
I admit that charging the battery outside is an extra step. Probably everyone, except one of us, who has a working 67 also has a classic charging cradle or a relatively cheap 35, 45, or 80 though. So the hardware part of this is already taken care of. Even another classic with keyboard, display or other problems should work.

Hi Dennis,
I met with this problem, when I once got an HP-67 which was running only with charger and not with batteries. I found out that the metal strip at the charger slot was missing. This metal strip connects Pin 1 and 3 when no charger is plugged in.

The original HP-67 manual lists the HP 82002A as charger, which is the same as for the classics (HP 82010A for EU version). Here is a description with schematic.

Charger HP 82002A

You can see the three pins of the charger slot in the charger circuit with 4,7V zener diode at Q1, which delivers about 4,5V to the calculator circuit while the battery plus is connected to the other part of the charger circuit, which delivers a constant current and potentially higher voltage by Q2 Q3. If the two outer pins are connected by the metal strip when the charger is not plugged in, the battery delivers its voltage to the circuit.

But you are right, you may be safer than safe when you always charge the batteries externally, because the charger circuit could be defective and could deliver a higher voltage than normal. Or the charger could be set to 110Volt accidentally which could potentially destroy a HP-67 in europe. On the other side, when charging externally, you could accidentally reverse the battery polarity, when reinserting them into the battery case. There is always space for making a mistake somehow.

My conclusion: When you state that the "Classics" are safe to operate without batteries inserted then also the HP-67 is safe.

Bernhard