Rebuilt HP 67 battery pack problems

[OldHPCollector]

I have a question for you electronic wizards out there. I had a 67 battery pack recently rebuilt by a reputable company that's rebuilt my 41 and 97 battery packs in the past. With the 67 in the "off" position I tried installing the pack. As soon as the leads touch the calculator contacts the card reader motor comes on. I tried it with another 67 and the same thing happens. This doesn't happen by using the adapter only. Has anyone experienced this? I would greatly appreciate any suggestions to remedy this problem. Thanks

Regards,

Ed

[BobVA]

The card reader motor has a connection to the battery pack via an IC that does not go through the power switch, so there is a path for current to flow to it with the switch off, as seems to be happening for you.

The only possibility I could imagine is that the battery pack has been wired incorrectly, with the polarity reversed, and this is somehow allowing the current make it to the motor through the IC? (I've got no idea what's going on inside that IC, so this is pure speculation.)

I'd definitely check both the voltage and the polarity of that pack before using it again.

[teenix]

I hope you are lucky if the battery was reversed and didn't turn the calculator on.

I'm not sure how the 67 CPU board will handle a reversed battery, although I think the switch mode power supply design will block any reverse voltages. The 47uF power supply bypass capacitor will have reverse battery directly across it and being a tantalum type will not like it - smoke being a telltale sign.

As Bob mentioned the sense chip seems to pass the reverse battery voltage though the Vbb pin to the motor output pin Vm (motor running backwards). Also not sure why, but hope it wasn't damaged. There is a tantalum capacitor on the motor connections, this might grumble at the reverse voltage as well.

In the battery compartment, the brass spring terminal closest to the top of the calculator is the negative terminal.

cheers

Tony

[OldHPCollector]

Hi Bob and Tony. You guys are sharp. He reversed the polarity. Luckily I didn't turn on the calculator as it trips the card reader motor as soon as you make contact with the leads. I appreciate your help

Ed