Classic (HP-67) charger / battery problem

[Johnboy]

Hi All,

I am trying out a few battery packs, all rebuilt, but with varying ages of rebuild. Some are NiCd and some are NiMh. They all seem to charge to around 3.6V ok, and when charging, the calculator turns on and works fine.

If I remove the charger, the calculator doesn't come on. I have check the continuity between the two outer charge socket pins, and it's all good. When I measure the voltage between the charge socket middle pin and outer pins, it's only 3.0V with the calculator turned off, and less that 1.0v when I turn it on.

I get similar measurements for all (5) battery packs. Does this indicate the battery packs are all bad despite charging to 3.6 (even 3.8 for one) ?

Any suggestions welcome.

Thanks
John

[teenix]

It seems like the batteries are not charging.

The charger may not be able to charge the batteries or has limited current ability due to a fault.

The charger has two sections, one for powering the calculator while on charge, and the other section charges the batteries. Maybe the charger section is faulty, broken wire, loose plug.

Other problems may be internal calculator wiring and /or the charger socket, corrosion etc.

cheers

Tony

[AndiGer]

Check the batteries under load. With a small bulb e.g. for bicycles (6V). If it does not light the batteries power breaks and voltage drops. I.e. batteries dead or not charged.

[Johnboy]

Thanks for the responses Tony and Andi. I rebuilt a battery pack with fresh 1000mAH NiCd cells, and I left it charging over night. This new battery has charged enough to run the calculator on the battery alone, but the low voltage led is on.

Sounds like it's not or not just the battery, so I'll try another charger.

Tony do you know the pinouts for the supply vs charge on the charger plug ? I'm trying to get a good 65 and 67 to test the teenix boards you sent me :-)

Cheers,
John

[teenix]

If you look under the calculator, the 3 charger terminals should be obvious and arranged in a triangular shape.

With the calculator upside down and looking top to bottom, the left pin is direct to battery +ve and is where the charger connects to do its work, the middle (closest to top) pin is battery -ve and the right pin is for calculator power via the power switch.

You should also see a shorting link between the lower left and right terminals, this connects the battery +ve to the power switch when the charger is not connected.

This connection is broken when the charger is plugged in. In this state, the battery is disconnected from the calculator and is charged via the left pin. The charger also powers the calculator via the right pin when it is switched on and the middle pin connects the charger -ve to the battery/calculator -ve.

The schematic (from Tony Duell) can be downloaded in PDF format from here

cheers

Tony

[Johnboy]

Thanks for the info Tony.

I also have a few other classic chargers I can use for comparison.

Cheers,
John

[Johnboy]

(04-20-2023 01:44 AM)teenix Wrote:  If you look under the calculator, the 3 charger terminals should be obvious and arranged in a triangular shape.

With the calculator upside down and looking top to bottom, the left pin is direct to battery +ve and is where the charger connects to do its work, the middle (closest to top) pin is battery -ve and the right pin is for calculator power via the power switch.

You should also see a shorting link between the lower left and right terminals, this connects the battery +ve to the power switch when the charger is not connected.

This connection is broken when the charger is plugged in. In this state, the battery is disconnected from the calculator and is charged via the left pin. The charger also powers the calculator via the right pin when it is switched on and the middle pin connects the charger -ve to the battery/calculator -ve.

The schematic (from Tony Duell) can be downloaded in PDF format from here

cheers

Tony

Yep the charger I was using has 0 volts on the charging pin, so found a couple of other chargers and all good now.

Cheers,
John