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

[PANAMATIK]

I made some measurements with my HP-25C.

The white circular trimmer below the LED display near the anode driver adjusts the "Low Batt" threshold voltage. I never have seen a Woodstock calculator without this trimmer. Its middle pin is connected to pin 4 of the anode driver, which contains a voltage comparator. It compares battery voltage at pin 17 and outputs a high signal at pin 7 if battery is above the threshold. Pin 4 should be adjusted to 1.20 Volt, which sets the threshold to approx 2.35V in my calculator. If turned to the far left (anti clockwise) pin 4 shows 1,27 V and the threshold is below 2.0V, the HP-25 calculator never shows "Low Batt" dots. If turned to the far right (clockwise) pin4 shows 1,17V and the threshold is at about 2.8V, which shows the dots all the time. Pin 7 is connected to digital input pin3 of the ACT chip. The HP-29C firmware shows a blinking dot instead of all dots on, when ACT pin 3 is at low level.

If your HP-29C board has a Power On Reset chip, I assume it has nothing to do with measuring the battery voltage.

In your case you should adjust the trimmer a little bit anticlockwise.




Bernhard