HP-29C Low-battery warning, help needed please!

[jjohnson873]

Hi Andy...Jim Johnson here. I haven't done an exhaustive study on the low voltage warning, but will try to make some measurements this weekend using a bench power supply feeding the battery inputs and measure the low voltage threshold. My guess is pin 3 input to the ACT is the low battery warning signal. I have my doubts about the POR/Low Battery IC degrading. I looked again at one of my HP-29C's and found a 33ohm, 5% resistor (orange, orange, black, gold). However, I looked at my presentation from 2013 and saw that the HP-29C had a 3.9ohm resistor (orange, white, gold, gold) on the pc board. I'll have to do some more investigation of why HP used two different values for the POR circuit. If you're having problems with the low battery warning, you might look at the tantalum cap. While tantalums usually don't degrade too much, your calculator is probably 40 years old by now. The capacitor may have become leaky. Also, make certain you have good NiMH batteries. I've had issues with some that have failed. If in doubt, get a second battery pack. Remember to never charge your battery pack inside of your calculator. The battery charging circuit has no protection should the battery not be properly connected to the charger. The incoming charging voltage can destroy your ACT if the battery isn't properly connected in the circuit!
The Vgg voltage for the ACT chip shouldn't be too critical. This bias voltage for the ACT is an artifact of the process technology at the time. If you check your PHI1 or PHI2, you should see a good voltage swing, around ~19vpp on a scope.
I've yet to find anyone who has an HP-29C service manual. If we had one, we'd make an electronic pdf and include it in the HP Museum CD/Flash drive.
~Jim J.~