HP-97 troubleshooting

[teenix]

The battery LED is controlled by circuitry inside the cathode driver chip and a voltage set resistor connected to pin #15 to ground.

This is a measurement I took on a 97, although this may vary a bit between calculators.

The low battery LED begins lighting at 4.77 volts and is fully lit at 4.6 volts

The battery voltage for this circuit comes from pin #4 of the 9 way connector on main circuit board just below the CRC chip. There is another unused 3 pin connector to the right of this connector. (Be careful playing around here with probes if the calculator is turned on or off as any inadvertent short could kill the calculator). Looking left to right, pins 7 and 8 are live battery connections.

It is difficult to fault find the 97, but with the rear cover removed, you can put a heavy elastic band (or similar idea) around the battery to hold it in place when the calculator is upside down. You could also construct some clips and leads to an external battery.

There is some bare circuit board space just to the left of the 9 pin connector, I would try putting light pressure, on and off, here with a finger to see if the LED changes. If so it might just be a poor connection and requires some cleaning. Do not try to bend the connecting pins to get a better connection, they are brittle and will snap apart.

The voltage set resistor that feeds the cathode driver battery sense pin may have poor connections or has changed its resistance. It is listed as 200K or 330K depending on what was installed.

If these check out ok, then the fault may lie in the cathode driver. Even so, you might be able to fiddle with the resistor value to get it to work. It is difficult to get to it though, requiring removing the main circuit board and underlying plastic panel.

cheers

Tony