HP-25 Co-luminous segments

[Tom Flatterhand]

Hello Tony,

there were traces of a leaking battery or accumulator on the battery contact on the right in the picture. I cleaned them with isopropyl alcohol.

(04-08-2022 12:01 AM)teenix Wrote:  As mentioned, because the display/keyboard operate in a multiplexed manner, an oscilloscope would help diagnose the problem.

I have a Fluke scopemeter. Could it be used to make the measurements? If you write me where to measure, I could try that.

Addendum: I read a bit in the beginning of the document 'Notes on HP's Classic Calculators'. There is a lot of knowledge in there! This is from you, isn't it?

Does this document also apply to the 'Woodstock' series?

Anyway, I think I have a rough understanding of how the LED display works and that it requires an interaction of different chips to display the characters correctly.

Nevertheless, the display is normally fine, if no key of the middle row is pressed. That means that the chips are all working properly! Only if this one line carries current, it comes to the weak shining of the upper segments in the display. This indicates that here the multiplexed signal for the LEDs contains an error.

The document says that there are 5 lines from the arithmetic chip to the anode driver chip. The signal is coded at this point and if the code is not correct here, it could cause such an error, right?

Possibly there is a short circuit or 'overvoltage' here that causes the code to be incorrect?

Regards

Thomas